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The genetic analysis of factors influencing the lethality associated with the scute inversions of Drosophila… Johnson, Carey 1981

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THE GENETIC ANALYSIS OF FACTORS INFLUENCING THE LETHALITY ASSOCIATED WITH THE SCUTE INVERSIONS OF DROSOPHILA MELANOGASTER. by CAREY JOHNSON B S c , U n i v e r s i t y o f B r i t i s h C o l u m b i a , 1978 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE I n t h e Department o f Zoo l o g y ' We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA May, 1981 © Carey Johnson, 1981 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by h i s or her representatives. I t i s understood that copying or pu b l i c a t i o n of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of 3 s The University of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date i ABSTRACT I t has been p r e v i o u s l y e s t a b l i s h e d t h a t i n v i a b i l i t y i s a s s o c i a t e d w i t h X/0 males c a r r y i n g a s c u t e i n v e r s i o n X c h r o -mosome. E x p l a n a t i o n s o f t h i s phenomenon suggest t h a t the l e t h a l i t y may "be due t o a p o s i t i o n e f f e c t s u p p r e s s i o n o f the rRNA c i s t r o n s . T h i s t h e s i s p r o v i d e s an a l t e r n a t i v e e x p l a n a t i o n o f the s c u t e X/0 l e t h a l i t y and p r o v i d e s s u p p o r t f o r t h e p r i m a r y f u n c t i o n o f t h e s c u t e l o c u s . These s t u d i e s i n d i c a t e t h a t t h e l e t h a l i t y a s s o c i a t e d w i t h t h e s c u t e X/0 genotype i s c o n d i t i o n a l and dependent upon t h e p a r e n t a l o r i g i n o f t h e X chromosome. Numerous e x p e r i m e n t s i n d i c a t e t h a t t h e l e t h a l i t y r e s u l t s from a d i s f u n c t i o n o f t h e s c u t e l o c u s r a t h e r t h a n the rRNA c i s -t r o n s . i i TABLE OF CONTENTS Page ABSTRACT i TABLE OF CONTENTS i i LIST OF TABLES i i i LIST OF FIGURES i v ACKNOWLEDGEMENT '." v i GENERAL INTRODUCTION 1 . MATERIALS AND METHODS 5 ^ -RESULTS Ch a p t e r 1. P a r e n t a l Source E f f e c t : I . P a t e r n a l l y d e r i v e d X 62. I I . M a t e r n a l l y d e r i v e d X 78. C h a p t e r 2. X/O V i a b i l i t y o f Recombinant S c u t e I n v e r s i o n s 88. C h a p t e r 3* M a t e r n a l E f f e c t s o f I n v e r s i o n H e t e r o z y g o t e s 100. C h a p t e r 4. M o d i f i c a t i o n o f X/0 V i a b i l i t y by X-chromosome H e t e r o c h r o m a t i c D u p l i c a t i o n and A l t e r e d Y-Chromosomes 108. C h a p t e r 5- The Compound X E f f e c t 121. C h a p t e r 6. The Temperature E f f e c t 127. C h a p t e r 7- The Dev e l o p m e n t a l D e l a y 136. C h a p t e r 8. The B r i s t l e Phenotype 1-55• DISCUSSION 165. BIBLIOGRAPHY 183-i i i L IST OF TABLES TABLE PAGE 1. M u t a t i o n s u s e d as g e n e t i c markers 55' 2. P a t e r n a l l y d e r i v e d X-chromosome: S c u t e X/0 l e t h a l i t y "." 65 • 3 . The s c u t e m a t e r n a l e f f e c t 79-^« X/0 male v i a b i l i t y i n s c u t e X-chromosome r e c o m b i n a n t s 89. 5. X/0 v i a b i l i t y from h e t e r o z y g o u s s c u t e f e m a l e s . 101. 6. A l t e r a t i o n o f p a t e r n a l e f f e c t by d u p l i c a t i o n s and a l t e r e d Y's 109 • ( s c x A x C(1)RM yw/Dp ) 7. A l t e r a t i o n o f p a t e r n a l e f f e c t by d u p l i c a t i o n s and a l t e r e d Y's 110., ( C(1)RM yvpn/Dp x s c x / Y ) 8. A l t e r a t i o n o f m a t e r n a l e f f e c t by d u p l i c a t i o n s and a l t e r e d Y's 11?. 9. The compound-X e f f e c t . . . . 122. 10. The p a t e r n a l t e m p e r a t u r e e f f e c t 128. 11. The m a t e r n a l t e m p e r a t u r e e f f e c t 133* 12. The d e v e l o p m e n t a l d e l a y 138. 13. M i c r o c h a e t a e f r e q u e n c y 156. 1^. S c u t e l l a r b r i s t l e f r e q u e n c y l 6 l . i v LIST OF FIGURES FIGURE PAGE 1. D i a g r a m a t i c r e p r e s e n t a t i o n o f t h e s c u t e i n -v e r s i o n chromosomes 56. 2. D e s c r i p t i o n and sou r c e o f s p e c i a l chromosomes... 58. 3 . M a t i n g scheme f o r g e n e r a t i n g s c u t e X / 0 males w i t h d i f f e r e n t p a r e n t a l o r i g i n s o f t h e X-chromosome 63-^' X / 0 v i a b i l i t y a s s o c i a t e d w i t h d i f f e r e n t p a r e n -t a l i n h e r i t a n c e o f t h e s c u t e X-chromosomes 81. 5- X / 0 v i a b i l i t y o f s c u t e recombinant X-chromo-somes o f d i f f e r e n t p a r e n t a l o r i g i n 91-6. V i a b i l i t y o f s c u t e i n v e r s i o n X / 0 males d e r i -v i n g t h e i r X-chromosome from h e t e r o z y g o u s and homozygous f e m a l e s o r hemizygous males 10k. 7. E f f e c t o f Ybb", an a d d i t i o n a l s c u t e l o c u s , and h e t e r o c h r o m a t i c d u p l i c a t i o n s on t h e v i a -b i l i t y o f s c u t e X / 0 males 111. I : P a t e r n a l i n h e r i t a n c e o f s c u t e i n v e r -s i o n 8. E f f e c t o f Ybb~, an a d d i t i o n a l s c u t e l o c u s , and h e t e r o c h r o m a t i c d u p l i c a t i o n s on t h e v i a -b i l i t y o f s c u t e X / 0 males 118. I I : M a t e r n a l i n h e r i t a n c e o f s c u t e i n v e r -s i o n 9. The compound-X e f f e c t 12^. 10. The t e m p e r a t u r e e f f e c t 130. I P a t e r n a l l y i n h e r i t e d X-chromosome 11. The t e m p e r a t u r e e f f e c t I3A-. I I : M a t e r n a l l y i n h e r i t e d X-chromosome V FIGURE . PAGE 1 2 . . The d e v e l o p m e n t a l d e l a y 1 3 9 -I : Raw d a t a 1 3 . The d e v e l o p m e n t a l d e l a y . I I : C u m u l a t i v e p e r c e n t a g e r e c o v e r y p l o t s ' a) s c 8 / Y x C (1)RM yw/O 1 ^ 2 . b) s c 8 A x C (1)RM yw/Y 1^3-c) s c 8 / s c 8 x X-Y/O 1 ^ . d) s c 8 / s c 8 x X-Y/Y 1 ^ 5 . lk. The d e v e l o p m e n t a l d e l a y . I l l s P r o b i t s a n a l y s i s o f d a t a a) s c 8 / Y x C (1)RM yw/O ^ ' b) s c 8 / Y x C (1)RM yw/Y 1^8. c) s c 8 / s c 8 x X-Y/O 1^9-d) s c 8 / s c 8 x X-Y/Y 1 5 0 . 1 5 - B r i s t l e f r e q u e n c y i n a s e r i e s o f s c u t e i n v e r -s i o n X/O males. I : The m i c r o c h a e t a e 1 5 8 . 1 6 . B r i s t l e f r e q u e n c y i n a s e r i e s o f s c u t e i n v e r -s i o n X/0 males. I I : The s c u t e l l a r s 1 6 3 . v i ACKNOWLEDGMENT I would l i k e t o thank Dr. D a v i d G. Holm f o r h i s sup-p o r t and c o n t i n u i n g i n t e r e s t i n t h e s e p r o j e c t s . I must e x p r e s s my s i n c e r e g r a t i t u d e t o Bob D e v l i n , who i n s p i r e d many o f t h e r e s e a r c h s t r a t e g i e s and p r o v i d e d v a l u a b l e t e c h n i c a l a s s i s t a n c e . A l s o , I would l i k e t o extend my th a n k s t o a l l t h o s e p e o p l e a s s o c i a t e d w i t h t h e f l y l a b s who s t i m u l a t e d d i s c u s s i o n s a t our c o n f e r e n c e s e s s i o n s . F i n a l l y , I would l i k e t o thank Suzanne N o b l e , who c o n t r i -b u t e d g r e a t l y t o t h e p r e p a r a t i o n o f t h i s m a n u s c r i p t . 1 . INTRODUCTION I t has l o n g "been e s t a b l i s h e d t h a t a gene's e x p r e s s i o n i s p r o f o u n d l y i n f l u e n c e d by i t s n e i g h b o r i n g g e n e t i c environment. I n p r o k a r y o t e s , the p o l a r i t y o f t h e g e n e t i c i n f o r m a t i o n i n f e r s t h a t a s p e c i f i c s p a c i a l arrangement m a i n t a i n s maximum f u n c t i o n a l e f f i c i e n c y . I n the more complex e u k a r y o t i c genome, the a r r a n g e -ment o f a d j a c e n t genes i s n o t so o b v i o u s l y p o l a r and r e s e a r c h e r s have been c o m p e l l e d t o s t u d y g e n e t i c i n t e r a c t i o n s a t a l e s s r e f i n e d l e v e l . U n t i l r e c e n t advances i n b i o c h e m i c a l g e n e t i c s , t h e s t u d i e s o f e u k a r y o t i c gene e x p r e s s i o n have i n v o l v e d f u n d a -m e n t a l problems i n t h e o r g a n i z a t i o n o f the h e r e d i t a r y m a t e r i a l . I n D r o s o p h i l a m e l a n o g a s t e r , the phenomenon o f p o s i t i o n e f f e c t has p r o v i d e d numerous r e s e a r c h s t r a t e g i e s i n v o l v e d w i t h gene e x p r e s s i o n . P o s i t i o n e f f e c t s a r e d e f i n e d as a l l those owing t o changes i n chromosomal arrangement. S t u r t e v a n t ( 1 9 2 5 ) f i r s t c o i n e d t h e p h r a s e " p o s i t i o n e f f e c t " i n r e f e r e n c e t o a s t a b l e phenomenon i n v o l v i n g t h e Bar (B) mutant l o c u s . H i s s t u d i e s i n d i c a t e d t h a t f o r two d i f f e r e n t a l l e l e s o f B, the p h e n o t y p i c e f f e c t was f a r g r e a t e r when the two a l l e l e s were on t h e same chromosome ( c i s ) as opposed t o b e i n g on s e p a r a t e X-chromosomes ( t r a n s ) . T h i s c i s - d o m i n a n t phenomenon was termed £3 o r s t a b l e - t y p e p o s i t i o n e f f e c t as the mutant pheno-t y p e was s t a b l e o r c o n s i s t e n t t h r o u g h o u t the a f f e c t e d i n d i -s i t u a t i o n s where s t a n d a r d g e n e t i c e x p r e s s i o n i s d i s t u r b e d 2. v i c t u a l . A second t y p e o f p o s i t i o n e f f e c t t h a t has been i d e n t i -f i e d i s termed V-type p o s i t i o n e f f e c t . T h i s phenomenon i s a s s o c i a t e d w i t h a v a r i e g a t e d (V) o r mosaic e x p r e s s i o n o f a mutant gene owing t o s p e c i f i c t y p e s o f chromosomal r e a r r a n g e -ments. T h i s anomaly was f i r s t r e p o r t e d by B r i d g e s and Morgan (1923). who d e s c r i b e d a weak e x p r e s s i o n o f the p l e x u s (px) phenotype a s s o c i a t e d w i t h a chromosome rearrangement. S t u r t e v a n t (1925) commented t h a t the i n c o m p l e t e e x p r e s s i o n o f the p_x m u t a t i o n c o u l d be a r e s u l t o f the p r o x i m i t y o f the p x + l o c u s t o the b r e a k p o i n t s o f t h e rearrangement. He f u r t h e r s p e c u l a t e d t h a t t h e p a r t i a l e x p r e s s i o n o f the l o c u s was a f u n c t i o n o f the a l t e r e d g e n e t i c environment i n the immediate v i c i n i t y o f the b r e a k p o i n t . Thus, S t u r t e v a n t f i r s t proposed t h e a s s o c i a t i o n o f chromosomal r e a r r a n g e m e n t s w i t h p o s i t i o n e f f e c t v a r i e g a t i o n . The d i s c o v e r y t h a t X - r a d i a t i o n i n d u c e d chromosomal r e a r -rangements p r o v e d t o be a major impetus t o the s t u d i e s o f p o s i t i o n e f f e c t ( M u l l e r , 1930a). M u l l e r (1930b) r e p o r t e d t he g e n e r a t i o n o f " e v e r s p o r t i n g d i s p l a c e m e n t s " i n s t o c k s exposed t o X - r a y s . He observ e d the i n c o m p l e t e e x p r e s s i o n o f y e l l o w (y;) and achaete (ac) and a t t r i b u t e d t he r e s u l t t o a somatic gene i n s t a b i l i t y . P a t t e r s o n (1932c) a l s o used X - r a y s t o gen-e r a t e a s t o c k v a r i e g a t i n g f o r numerous genes n e a r the t i p o f th e X-chromosome. He e x p l a i n e d t h i s m u l t i p l e - m u t a n t c o n d i -t i o n as a r e s u l t o f a s p e c i f i c s o m a t i c l o s s o f " u n s t a b l e " 3 -g e n e t i c m a t e r i a l . Moreover, he observed t h a t the phenotype was a s s o c i a t e d w i t h a t r a n s l o c a t i o n o f X-chromosomal m a t e r i a l t o t h e f o u r t h chromosome. P a t t e r s o n suggested t h a t the t r a n s -l o c a t e d fragment may be l o s t d u r i n g somatogenesis and as a consequence, a l l descendant c e l l s would be d e f i c i e n t f o r the s m a l l X-chromosome r e g i o n . The c o e x i s t e n c e o f the d e f i c i e n t c e l l s w i t h t h e normal c e l l s would r e s u l t i n a v a r i e g a t e d o r mosaic phenotype. Gowan and Gay ( 1 9 3 3 a ) f i r s t engaged i n s t u d i e s on the m o d i f y i n g f a c t o r s i n v o l v e d w i t h v a r i e g a t i n g phenotypes. These o r i g i n a l e x p e r i m e n t s suggested t h a t t h e phenomenon was s e n s i t i v e t o t e m p e r a t u r e changes and t o Y-chromosomal m a t e r i a l (Gowan and Gay, 1 9 3 3 a , b ) . The w h i t e - m o t t l e d s t o c k s g e n e r a t e d by t h e s e a u t h o r s were c y t o l o g i c a l l y c h a r a c t e r i z e d as c a r r y i n g X-chromosomal i n v e r s i o n s w i t h b r e a k p o i n t s i n t h e p r o x i m i t y o f t h e w h i t e (w) l o c u s . T h i s f a c t g r e a t l y j e o p a r d i z e d t h e v a l i d i t y o f P a t t e r s o n ' s t h e o r y ( 1 9 3 2 c ) as v i a b l e a n e u p l o i d s o m a t i c c e l l l i n e s c o u l d n o t be g e n e r a t e d from th e i n v e r s i o n b e a r i n g c e l l s . V i e w i n g t h i s e v i d e n c e , Gowan and Gay t h e o r i z e d t h a t t h e mechanism o f v a r i e g a t i o n must i n v o l v e the d i s r u p t i o n o f the h e r e d i t a r y m a t e r i a l i n the v i c i n i t y o f the gene and t h i s d i s t u r b a n c e somehow i n f l u e n c e s normal gene e x p r e s s i o n (Gowan and Gay, 1 9 3 ^ ) • A f t e r t h e s e s t u d i e s , d a t a were accumulated r a p i d l y con-c e r n i n g t h e f a c t o r s i n v o l v e d w i t h p o s i t i o n e f f e c t v a r i e g a t i o n . S h o r t l y , an abundance o f v a r i e g a t i n g genes were i d e n t i f i e d k. and, i n a l l c a s e s , t h e r e l a t i v e p o s i t i o n o f the gene had "been a l t e r e d w i t h r e s p e c t t o n e i g h b o r i n g l o c i . The phenomenon was obs e r v e d i n many t y p e s o f chromosomal rearrangements such as i n v e r s i o n s , d u p l i c a t i o n s and t r a n s l o c a t i o n s ( D u b i n i n and S i d e r o v , 1935; P a n s h i n , 1935; G o l d s c h m i d t e t a l . , 1939). T h i s d i s c o v e r y l e d t o many new t h e o r i e s concerned w i t h t h e i n t e r p r e t a t i o n o f t h e phenomenon o f p o s i t i o n e f f e c t . P a t t e r s o n (1931c), S t e r n (1935) and S c h u l t z (1936) main-t a i n e d t he s u g g e s t i o n t h a t l o c i i n the p r o x i m i t y o f r e a r r a n g e -ment b r e a k p o i n t s were somehow s o m a t i c a l l y u n s t a b l e . C i t i n g o b v i o u s problems w i t h t h i s t h e o r y , many o t h e r a u t h o r s reasoned t h a t t h e m o s a i c i s m was a r e s u l t o f t h e new environment i n t o w h i c h t h e normal gene had been p l a c e d (Gowan and Gay, 193^; Demerec and S l i z y n s k a , 1937; Gruneberg, 1937; S u r r a r r e r , 1938) . These two a l t e r n a t i v e e x p l a n a t i o n s remained w e l l s u p p o r t e d f o r many y e a r s as i n v e s t i g a t o r s c o n t i n u e d t o produce e v i d e n c e s u p p o r t i n g b o t h s c h o o l s o f thought ( S t e r n e t a l . , 19^5; E p h r u s s i and S u t t o n , 19^ ]4; Gersh and E p h r u s s i , 19^6). F i n a l l y , a number o f a u t h o r s sought t o show t h a t t h e v a r i e g a t i n g pheno-t y p e s c o u l d be r e v e r s e d by r e i n v e r t i n g t he w h i t e - m o t t l e d o r r o u g h e s t ( r s t ) i n v e r s i o n s . T h i s would s t r o n g l y i m p l y t h a t t h e gene i t s e l f had n o t been p e r m a n e n t l y a l t e r e d and the mu-t a n t phenotype r e s u l t e d from t h e d i s t u r b e d environment. P a n s h i n (1938) r e p o r t e d t h e "complete r e v e r s a l " o f t h e w h i t e -m o t t l e d phenotype and t h i s o b s e r v a t i o n was s u p p o r t e d by the c y t o l o g i c a l i d e n t i f i c a t i o n o f t h e r e i n v e r s i o n . S i m i l a r r e -5-s u i t s f o r t h e r s t i n v e r s i o n s were r e p o r t e d by Kaufmann (19^2). G r a d u a l l y , e v i d e n c e mounted s u p p o r t i n g th e somatic gene s t a -b i l i t y h y p o t h e s i s and i t has now become an a c c e p t e d component o f t h e p o s i t i o n - e f f e c t v a r i e g a t i o n model ( L e w i s , 1950; G e r s h , 1957)• P r e v i o u s t o t h i s c o n c l u s i o n , many a u t h o r s had commented on the a s s o c i a t i o n o f the v a r i e g a t e d gene e x p r e s s i o n w i t h r e a r r a n g e m e n t s i n v o l v i n g the " i n e r t " o r h e t e r o c h r o m a t i c p o r -t i o n s o f chromosomes (Dobzhansky, 1936; D u b i n i n , 1936; S c h u l t z , 1936; Demerec and S l i z y n s k a , 1937). I t soon became app a r e n t t h a t t h e v a r i e g a t i n g p o s i t i o n e f f e c t s were p r i m a r i l y a r e s u l t o f th e r e l o c a t i o n o f euc h r o m a t i c genes i n t o the p r o x -i m i t y o f a h e t e r o c h r o m a t i c r e g i o n . T h i s e v i d e n c e prompted many s t u d i e s i n t o t h e p r o c e s s e s i n v o l v e d w i t h the compaction o r h e t e r o c h r o m a t i z a t i o n o f the DNA w i t h r e s p e c t t o p o s i t i o n -e f f e c t v a r i e g a t i o n ( P r o k o f y e v a - B e l g o v s k a y a , 19^7; Gersh, 19^9; S p o f f o r d , 1959)• A c i s - t r a n s r e l a t i o n s h i p can be e s t a b l i s h e d f o r a l l t r u e v a r i e g a t i n g p o s i t i o n e f f e c t s . The rearrangement and the w i l d t y p e gene must c o e x i s t on the same chromosome ( c i s ) as the p r o d u c t i o n o f mosaic t i s s u e r e q u i r e s b o t h normal and mutant c l o n e s o f somatic c e l l s . I n c o n t r a s t , t h e . t r a n s arrangement i n v o l v e s t h e mutant gene l i n k e d t o the r e a r r a n g e d chromosome. T h i s s i t u a t i o n does n o t a l l o w f o r the p r o d u c t i o n o f bo t h n o r -mal and mutant c e l l l i n e s . The c i s - t r a n s t e s t i n v o l v e s a w i l d t ype a l l e l e , a^, a mutant a l l e l e , a, a b r e a k p o i n t , bp, 6. and a. normal chromosome, hp . The a hp/a hp i n d i v i d u a l w i l l e x h i b i t v a r i e g a t i o n f o r the a l o c u s as the w i l d t ype a l l e l e and t he b r e a k p o i n t a r e c i s t o each o t h e r . An a bp / a bp i n -d i v i d u a l has t h e t r a n s arrangement and normal complementation r e s u l t s i n a w i l d phenotype. E x p e r i m e n t s i n v o l v e d w i t h r e v e r t i n g t h e mosaic phenotype s t r o n g l y i n d i c a t e t h a t t h e change i s b r o u g h t about o n l y by f u r t h e r chromosome breakage. Gruneberg (1937)> Kaufmann (19^2) and N o v i t s k i (1961) have a l l r e c o v e r e d r e v e r s i o n s o f the r s t phenotype by i r r a d i a t i n g r s t h e t e r o z y g o t e s . These a u t h o r s have shown .that t h e r e v e r s i o n s from the mutant phenotype were a l l accompanied by new rearrang e m e n t s w h i c h t r a n s f e r r e d a g r e a t d e a l o f the d i s p l a c e d h e t e r o c h r o m a t i n t o a new p o s i t i o n . O t h e r a u t h o r s have shown t h a t t h e i n c o m p l e t e c y t o l o g i c a l r e -v e r s i o n can r e s u l t i n a change i n e x p r e s s i o n o f the v a r i e g a t e d phenotype i n t h e d i r e c t i o n o f w i l d t ype ( H i n t o n and Goodsmith, 1950)• A t t e m p t s t o produce r e v e r t a n t s o f some v a r i e g a t i n g systems such as s c u t e (.sc) have been u n s u c c e s s f u l ( R a f f e l and M u l l e r , 19^0). Numerous f a c t o r s a r e known t o mo d i f y the e x p r e s s i o n o f p o s i t i o n - e f f e c t v a r i e g a t i o n . Gowan and Gay (1933a) f i r s t r e p o r t e d t h a t v a r i e g a t i o n f o r t h e w h i t e gene was suppressed by an e x t r a Y-chromosome. A v a r i e t y o f s t u d i e s i n v e s t i g a t i n g t h e u b i q u i t y o f t h i s phenomenon have r e v e a l e d t h a t v a r i e g a -t i o n i s d e c r e a s e d as the number o f Y-chromosomes i s i n c r e a s e d . E x p e r i m e n t s g e n e r a t i n g X/0 males have suggested t h a t the mu-7-t a n t p r o g r e s s i o n ( X / 0 > X / Y > X / Y / Y ) i s a d i r e c t r e s u l t o f a m o d i f y i n g f e a t u r e o f the Y-chromosome (Gowan and Gay, 1933a; S c h u l t z , 1936; N o u j d i n , 1936). -L i n d s l e y _et a l . (i960) X - r a y i n d u c e d a l a r g e number o f s e x - l i n k e d r e c e s s i v e mutants t h a t were v i a b l e as X / Y males but l e t h a l as X / 0 i n d i v i d u a l s . E x c l u d i n g t h e i n d u c t i o n o f bobbed l e t h a l s , a l l o f t h e s e mutants were a s s o c i a t e d w i t h e u c h r o m a t i c - h e t e r o c h r o m a t i c r e a r r a n g e m e n t s , t h u s f u l f i l l i n g one o f t h e c r i t e r i a f o r p o s i t i o n e f f e c t . L i n d s l e y s u g g e s t s t h a t t h e s e mutants a r e v a r i e g a t i n g f o r l e t h a l m u t a t i o n s a l -though no som a t i c phenotype i s v i s i b l e . More r e c e n t s t u d i e s o f v a r i e g a t i n g gene p r o d u c t s have shown s i g n i f i c a n t i n c r e a s e s i n v a r i e g a t i n g enzyme a c t i v i t y c o r r e l a t e d w i t h i n c r e a s e s i n the number o f Y-chromosome w i t h i n a sex (Bahn, 1971; Gerazimova e t a l . , 1972). I n many v a r i e g a t i n g systems, t h e m o s a i c i s m i s so extreme t h a t normal t i s s u e can be observ e d o n l y when e x t r a Y-chromosomal m a t e r i a l i s p r e s e n t ( G e r s h , I963)• T h i s i s w e l l i l l u s t r a t e d by c e r -t a i n mutants t h a t can s u r v i v e o n l y i n the p r e s e n c e o f a c c e s -s o r y Y-chrdmosomes ( K e r s c h n e r , 19^ +9; L i n d s l e y _et a l . , i960; Ben Zeev and F a l k , 1966). Thus, i t appears f o r the v a s t ma-j o r i t y o f v a r i e g a t i n g systems t h a t t h e e f f e c t o f a d d i t i o n a l ' Y-chromosomal m a t e r i a l i s a p o t e n t s u p p r e s s i o n o f the mutant phenotype. Temperature i s a l s o known t o be a m o d i f i e r o f p o s i t i o n -e f f e c t v a r i e g a t i o n i n somati c t i s s u e . I n g e n e r a l , mosaic 8. s t o c k s r a i s e d a t l o w e r t e m p e r a t u r e s e x h i b i t a l a r g e r p r o p o r -t i o n o f mutant t i s s u e t h a n t h o s e r a i s e d a t h i g h e r t e m p e r a t u r e s . Thus, the t y p i c a l p h e n o t y p i c r e s p o n s e t o low tempe r a t u r e i s i n c r e a s e d mutant e x p r e s s i o n . The phenotype i s m o d i f i e d t o -wards w i l d t y p e as tem p e r a t u r e i s i n c r e a s e d . The m o d i f y i n g a b i l i t y o f te m p e r a t u r e i s a phenomenon t h a t has been e x t e n s i v e l y s t u d i e d f o r many v a r i e g a t i n g mutants (Gowan and Gay, 193^» Kaufmann, 19^2; S t e r n and K o d a n i , 1955; S c h a l e t , 1969). T h i s e f f e c t has a l s o been extended t o th e enzymatic l e v e l (Bahn, 1971) and most o f th e v a r i e g a t i n g l e t h a l systems (Ben Zeev and F a l k , 1966) . The e x p l a n a t i o n o f the te m p e r a t u r e e f f e c t has been ap-pro a c h e d by many a u t h o r s ( P r o k o f y e v a - B e l g o v s k y a , 19^5; S t e r n and K o d a n i , 1955)> and a common d e d u c t i o n i s made by a l l . They suggest t h a t l a r g e r a r e a s o f g e n e t i c m a t e r i a l a r e s u b j e c t t o i n a c t i v a t i o n a t l o w e r t e m p e r a t u r e s . T h i s i s s t r o n g l y sup-p o r t e d by ex p e r i m e n t s i n v o l v i n g mutants t h a t v a r i e g a t e f o r numerous l o c i . A t l o w e r t e m p e r a t u r e s , t h e s e v e r i t y o f the phenotypes i n c r e a s e s i n r e l a t i o n t o t h e i r p r o x i m i t y t o t h e h e t e r o c h r o m a t i c b r e a k p o i n t . A l s o , a t reduced t e m p e r a t u r e s , v a r i e g a t i n g l o c i can be i d e n t i f i e d t h a t have p r e v i o u s l y ex-h i b i t e d a w i l d phenotype. These e x p e r i m e n t s s t r o n g l y suggest t h a t t h e te m p e r a t u r e e f f e c t i n v o l v e s a mutant g r a d a t i o n de-pendent upon t h e p r o x i m i t y o f th e w i l d t y p e gene t o t h e break-p o i n t o f th e rearrangement. One would n o t ex p e c t t e m p e r a t u r e t o be as c o n s i s t e n t as 9 . o t h e r m o d i f i e r s o f v a r i e g a t i o n - a s many mutants d e r i v e t h e i r phenotype from t h e r m a l i n s t a b i l i t y o f abnormal p r o t e i n forms. S h o u l d a mutant enzyme be heat l a b i l e , one would expect a more mutant e x p r e s s i o n a t t h e r e s t r i c t i v e t e m p e r a t u r e . T h i s type o f e x p r e s s i o n has been obse r v e d f o r the mutant l i g h t ( I t ) and f o r b r i s t l e number i n I n ( l ) s c u t e - 4 ( G e r s h , 19^9; Mampell, 1956b) . As v a r i e g a t i o n i s a phenomenon a s s o c i a t e d w i t h p r o x i m i t y t o a e u c h r o m a t i c - h e t e r o c h r o m a t i c j u n c t i o n , one e x p e c t s l o c i adjacent'.to t h e b r e a k p o i n t s t o v a r i e g a t e . However, owing t o t h e chromosomal breakage and rearrangement, the normal i n t e r -g e n i c r e l a t i o n s h i p may be r a d i c a l l y a l t e r e d and the r e s u l t i n g phenotype may r e a c t u n c o n v e n t i o n a l l y t o tem p e r a t u r e changes. T h i s i s e x e m p l i f i e d by t h e complex l o c u s s c u t e (_sc) , which i s i m p a i r e d by t h e s c u t e - 8 i n v e r s i o n . T h i s rearrangement r e s u l t s i n a phenotype mosaic f o r s c u t e (_sc) , achaete (ac) and H a i r y w i n g (Hw). The v a r i e g a t i o n o f t h e s c u t e and achaete l o c u s r e s u l t i n a r e d u c t i o n o f b r i s t l e number a t l o w e r t e m p e r a t u r e s . The mutant form o f Hw, however, r e s u l t s i n a c c e s s o r y b r i s t l e s a t l o w e r t e m p e r a t u r e s (Crew and Lamy, 19^0; Sutton,.1 9 4 3 b ) . I t i s o b v i o u s t h a t t h e compound e f f e c t o f numerous v a r i e g a t i n g genes may produce a phenotype d i f f e r i n g s i g n i f i c a n t l y from t h e e x p e c t e d form. Temperature appears t o be an i m p o r t a n t m o d i f i e r o f v a r i e g a t e d e x p r e s s i o n but one must e x e r c i s e c a u t i o n when i n t e r p r e t i n g d a t a d e r i v e d from complex mosaic systems. The tem p e r a t u r e s e n s i t i v e p e r i o d has been s t u d i e d f o r a 10 . number o f v a r i e g a t i n g mutants. The mosaic b r i s t l e phenotype a s s o c i a t e d w i t h t h e s c u t e - 8 , the s c u t e - ^ and the s c u t e - S l i n -v e r s i o n s i s s e n s i t i v e t o t e m p e r a t u r e changes between l a t e t h i r d i n s t a r l a r v a e and two days a f t e r puparium f o r m a t i o n (Crew and Lamy, 19^-0; S u t t o n , 19^3b; Gersh, 19^9)- The o b s e r v a t i o n i s s i m i l a r t o t h a t seen w i t h t h e w h i t e - m o t t l e d mutants (Chen, 19^8; S c h u l t z , 1956; H a r t m a n n - G o i d s t e i n , 1967)• The tempera-t u r e s e n s i t i v e p e r i o d appears t o precede t h e time a t which t h e a f f e c t e d l o c u s would be e x p e c t e d t o be a c t i v e . D a t a a s s o -c i a t e d w i t h t h i s phenomenon a r e l a c k i n g and t h e s e s t u d i e s have f a i l e d t o e x p l a i n t h e d e v e l o p m e n t a l o r m o l e c u l a r b a s i s o f po-s i t i o n - e f f e c t v a r i e g a t i o n . F u t u r e i n v e s t i g a t i o n s must v a r y t h e r e s e a r c h s t r a t e g i e s t o p r o v i d e f u r t h e r v a l u a b l e i n f o r m a -t i o n c o n c e r n i n g the r e g u l a t i o n o f v a r i e g a t i n g genes t h r o u g h development. The most p r o f o u n d m o d i f i e r o f p o s i t i o n e f f e c t i s genomic h e t e r o c h r o m a t i n c o n t e n t . The most o b v i o u s example o f t h i s e f f e c t i s the v a r i e g a t i o n - s u p p r e s s i n g a c t i v i t y o f t h e Y-chromosome w h i c h has been d i s c u s s e d p r e v i o u s l y . I n D. m e l a n o g a s t e r , many e x p e r i m e n t s have been u n d e r t a k e n t o l o c a l i z e s p e c i f i c r e g i o n s o f the Y-chromosome which mediate t h e mosaic s u p p r e s s i o n (Baker and S p o f f o r d , 1959; Baker and R e i n , 1962; B r o s s e a u , 196^). These s t u d i e s have been hampered by problems i n v o l v e d w i t h d e f i n i n g h e t e r o c h r o m a t i c c o n t e n t as t h e Y-chromosome has o n l y t h e f e r t i l i t y f a c t o r s and the nu-c l e o l u s o r g a n i z e r as r e f e r e n c e p o i n t s . The a u t h o r s c o n c l u d e d t h a t the a b i l i t y o f a Y-chromosomal fragment t o mediate mosaic s u p p r e s s i o n c o u l d n o t be c o r r e l a t e d w i t h i t s s i z e . I n R' hydei> Hess (1970) r e p o r t s t h a t two v a r i e g a t i o n suppres-s i n g r e g i o n s e x i s t a t v e r y s p e c i f i c s i t e s on the Y-chromosome. A s i m i l a r s i t u a t i o n has been i m p l i e d f o r D. m e l a n o g a s t e r (Baker and S p o f f o r d , 1959)• The Y-chromosome i s o f s p e c i a l i n t e r e s t t o t h i s t h e s i s as i t appears t o be a f a c t o r t h a t g r e a t l y i n f l u e n c e s t h e ex-p e r i m e n t a l r e s u l t s . There are.numerous p r o p e r t i e s o f t h i s chromosome w h i c h may i n f l u e n c e t h e d a t a and t h e r e f o r e I s h a l l d i g r e s s a t ..this p o i n t t o r e v i e w t h e c h a r a c t e r i s t i c f e a t u r e s o f t h e Y-chromosome. O r i g i n a l l y , B r i d g e s (1913) d i s c o v e r e d t h a t the Y-chromo-some was r e s p o n s i b l e f o r male f e r t i l i t y i n D. m e l a n o g a s t e r . I n h i s s t u d i e s on sex chromosome n o n - d i s j u n c t i o n , B r i d g e s d i s c o v e r e d t h a t b o t h X/0 and X/Y f l i e s were p h e n o t y p i c a l l y male, but t h e f o r m e r c l a s s was s t e r i l e . S a f i r (1920) was the f i r s t t o e s t a b l i s h t h a t X/0 males do n o t produce m o t i l e sperm, a l t h o u g h t h e i r g e n i t a l i a appear t o be n o r m a l . L a t e r work r e -v e a l e d .that the s t e r i l i t y o b served i n X/0 males i s a r e s u l t o f a d e f i c i e n c y o f a group o f f e r t i l i t y f a c t o r s p r e s e n t on b o t h th e s h o r t arm (K ) and the l o n g arm (K-, ) o f t h e Y-chro-S -L mosome ( B r o s s e a u , 1960b). I n a d d i t i o n t o t h e f e r t i l i t y f a c t o r s , t h e Y-chromosome has been shown t o c a r r y a bobbed l o c u s ( S t e r n , 1927), t h a t i s t h e s i t e o f t h e rRNA genes ( R i t o s s a and Spiegelman, 1965). The bobbed s i t e has been i d e n t i f i e d on t h e s h o r t arm o f t h e Y-chromosome and i s homologous t o t h e bobbed l o c u s s i t u a t e d i n t h e p r o x i m a l h e t e r o c h r o m a t i n o f t h e X-chromosome. I t has a l s o been d e t e r m i n e d t h a t the bobbed phenotype ( d e l a y e d d e v e l -opment, s h o r t , t h i n b r i s t l e s and abdominal e t c h i n g ) i s caused by a r e d u c t i o n i n t h e numbers o f rRNA genes a t e i t h e r the X o r Y bobbed l o c u s ( R i t o s s a e_t a l . , 1966) . The bobbed l o c u s has a l s o been i d e n t i f i e d c y t o l o g i c a l l y and b i o c h e m i c a l l y as t h e s i t e o f t h e n u c l e o l u s o r g a n i z e r r e g i o n ( R i t o s s a and Spiegelman, 1965; P e r r y , 1967) . Owing t o the redundancy a t t h e Y bobbed l o c u s and i t s s i m i l a r i t y t o t h e homologous X-chromosome s i t e , t h e Y bobbed l o c u s mutants have been ex-t e n s i v e l y s t u d i e d and t h i s has p r o v i d e d much i n f o r m a t i o n p e r -t a i n i n g t o t h e r e g u l a t i o n o f rRNA s y n t h e s i s i n D r o s o p h i l a . I n i t i a l l y , t h e Y-chromosome and the c e n t r o m e r i c r e g i o n s o f t h e o t h e r chromosomes were d i s t i n g u i s h e d by t h e i r s t a i n i n g p r o p e r t i e s . I t was apparent t h a t t h e s e r e g i o n s s t a i n e d d a r k l y and appeared condensed t h r o u g h o u t t h e m a j o r i t y o f m i t o s i s ( H e i t z , 1933)* These a r e a s were termed h e t e r o c h r o m a t i c and soon i t was demonstrated t h a t t h e s e r e g i o n s c o r r e s p o n d e d to the s o - c a l l e d " g e n e t i c a l l y i n e r t " r e g i o n s p r e v i o u s l y i d e n t i -f i e d i n D. m e l a n o g a s t e r chromosomes ( M u l l e r and P a i n t e r , 1932) . E v i d e n c e q u i c k l y mounted t o suggest t h a t t h e p r o p e r t i e s o f h e t e r o c h r o m a t i n were much d i f f e r e n t t h a n th o s e o f the e u c h r o m a t i n . I n e a r l y s t u d i e s , t h e Y-chromosome was.used ex-t e n s i v e l y t o m o d i f y genomic h e t e r o c h r o m a t i n c o n t e n t . These works i d e n t i f i e d a h o s t o f p r o p e r t i e s a t t r i b u t a b l e t o h e t e r o -c h r o m a t i c c o n t e n t : m o d i f i c a t i o n o f s p e c i f i c gene a c t i o n ( D a r l i n g t o n , 19^7; Hannah, 1951)> v a r i e g a t i o n c o n t r o l (Gowan and Gay, 1933; S c h u l t z , 1939; Baker and S p o f f o r d , 1959; G r e l l , 1959)> and s u p p r e s s i o n o f l e t h a l m u t a t i o n s ( L i n d s l e y e t a l . , I 9 6 0 ) . A l t h o u g h most s t u d i e s on h e t e r o c h r o m a t i c c o n t e n t i n v o l v e Y-chromosome m o d i f i c a t i o n s , t h e r e a re many examples o f s i m i l a r f e a t u r e s a s s o c i a t e d w i t h o t h e r h e t e r o c h r o m a t i c r e g i o n s . The b a s a l h e t e r o c h r o m a t i n o f the X-chromosome a l s o has been shown t o be a p o t e n t s u p p r e s s o r o f v a r i e g a t i o n . N o u j d i n (1938) r e -p o r t e d t h a t a d d i t i o n o f X - h e t e r o c h r o m a t i n s u c c e s s f u l l y sup-p r e s s e d v a r i e g a t i o n o f y e l l o w (v;) , and l a t e r d e t e r m i n e d t h a t I n ( l ) scute-8 i n d i v i d u a l s c a r r y i n g s m a l l X - h e t e r o c h r o m a t i c d u p l i c a t i o n s were r a r e l y mosaic f o r y_ o r ac w h i l e i n d i v i d u a l s l a c k i n g t h e d u p l i c a t i o n commonly were v a r i e g a t e d f o r t h e s e genes ( N o u j d i n , 19^ *0 • D e f i c i e n c i e s o f the b a s a l X - h e t e r o -c h r o m a t i n have a l s o been r e p o r t e d t o enhance v a r i e g a t i o n ( P a n s h i n , 1938; Baker and S p o f f o r d , 1959). I t i s a l s o e v i d e n t t h a t a f u l l complement o f X - h e t e r o c h r o m a t i n has a s i m i l a r e f -f e c t as a s i n g l e Y-chromosome. T h i s i s apparent v i e w i n g the many s t u d i e s i n v o l v i n g c omparisons o f v a r i e g a t i o n i n X/X f e -males and X/Y males ( G r e l l , 1958; Bahn, 1971). The c e n t r o m e r i c r e g i o n s o f t h e autosomes a l s o e x h i b i t a v a r i e g a t i o n s u p p r e s s i n g f u n c t i o n . The b u l k o f t h e work i n 10 t h i s a r e a i n v o l v e s D f ( 2 R ) M -S2 w h i c h i s a l a r g e d e l e t i o n o f lk. c e n t r o m e r i c h e t e r o c h r o m a t i n on t h e r i g h t arm o f chromosome 2 ( H i l l i k e r and Holm, 1 9 7 5 ) . S c h u l t z ( 1 9 3 9 ) observed t h a t the p r e s e n c e o f t h i s d e f i c i e n c y g r e a t l y enhanced v a r i e g a t i o n f o r t h e w h i t e (w) , y e l l o w (y_) and brown (bw) mutants. More r e -c e n t s t u d i e s i n v o l v i n g o t h e r mosaic systems have c o r r o b o r a t e d t h i s phenomenon ( B r o s s e a u , I960; L i n d s l e y _et a l . , i960) . Numerous e x p e r i m e n t s now e x i s t s u g g e s t i n g t h a t an u n d e r s t a n d -i n g o f t h e f u n d a m e n t a l b a s i s o f h e t e r o c h r o m a t i z a t i o n w i l l u l t i m a t e l y l e a d t o a comprehension o f t h e phenomenon o f p o s i -t i o n - e f f e c t v a r i e g a t i o n . The a s s o c i a t i o n between p o s i t i o n e f f e c t and h e t e r o c h r o -m a t i z a t i o n i s s t r o n g l y i m p l i e d by two g e n e t i c o b s e r v a t i o n s . The g r e a t m a j o r i t y o f v a r i e g a t i n g mutants a r e found t o i n v o l v e t h e rearrangement o f t h e e u c h r o m a t i c gene i n t o the p r o x i m i t y o f a h e t e r o c h r o m a t i c segment. A l s o , the amount o f h e t e r o c h r o -m a t i n w i t h i n t h e genome has a p r o f o u n d e f f e c t on t h e e x p r e s -s i o n o f t h e v a r i e g a t i n g l o c u s . The f o l l o w i n g i s a s y n o p s i s o f t h e s u b j e c t o f h e t e r o c h r o m a t i n w i t h p a r t i c u l a r r e f e r e n c e t o t h o s e f a c e t s r e l e v a n t t o p o s i t i o n e f f e c t . H e t e r o c h r o m a t i n i s d e f i n e d c y t o l o g i c a l l y as d e n s e l y s t a i n i n g , F e u l g e n p o s i t i v e m a t e r i a l t h a t remains v i s i b l e t h r o u g h o u t th e m a j o r i t y o f t h e c e l l c y c l e ( H e i t z , 1 9 3 3 ) -T h i s r e g i o n i s c o n t r a s t e d w i t h e u c h r o m a t i n w h i c h becomes c y t o l o g i c a l l y v i s i b l e i n l a t e prophase and o n l y s t a i n s w e l l a t metaphase. The d i s t i n c t i o n o f t h e elements o f h e t e r o -c h r o m a t i n and e u c h r o m a t i n on t h e b a s i s o f c y t o l o g i c a l e v i -15-dence does n o t s e r v e t o a d e q u a t e l y d e f i n e t h e s e elements g e n e t i c a l l y . The v a r i a b i l i t y o f r e g i o n a l s t a i n i n g t h r o u g h o u t development s u g g e s t s s t r o n g l y t h a t t h e c y t o l o g i c a l d e f i n i t i o n w i l l n o t c o r r e l a t e d i r e c t l y w i t h t h e g e n e t i c d e f i n i t i o n . T h i s p o i n t i s w e l l i l l u s t r a t e d i n t h e d i s c u s s i o n o f h e t e r o c h r o m a t i n v a r i a t i o n . I n mammalian f e m a l e s , the p r o c e s s o f L y o n i z a t i o n i n v o l v e s t h e random i n a c t i v a t i o n o f a s i n g l e X-chromosome t h r o u g h o u t t h e s o m a t i c t i s s u e (Lyon, 1962). T h i s system a c h i e v e s dosage compensation f o r f e m a l e s as h a l f o f t h e i r c e l l s have one X-chromosome i n a c t i v a t e d w h i l e t h e o t h e r h a l f have th e homologous X i n e r t . C y t o l o g i c a l l y , t h i s pheno-menon i s r e c o g n i z e d by t h e p r e s e n c e o f h i g h l y condensed X - c h r o -mosomal m a t e r i a l known as B a r r b o d i e s w h i c h are p e c u l i a r t o female somatic c e l l s . The h e t e r o c h r o m a t i z a t i o n o f the e n t i r e X e u c h r o m a t i n i n t o an i n e r t s t r u c t u r e i s termed f a c u l t a t i v e h e t e r o c h r o m a t i z a t i o n and t h i s phenomenon c h a r a c t e r i s t i c a l l y i n v o l v e s the i n a c t i v a t i o n o f n o r m a l l y a c t i v e c h r o m a t i n . Fac-u l t a t i v e h e t e r o c h r o m a t i n can be c o n t r a s t e d w i t h c o n s t i t u t i v e h e t e r o c h r o m a t i n w h i c h makes up t h e r e m a i n d e r o f the condensed chromatin.. C o n s t i t u t i v e h e t e r o c h r o m a t i n i s t h a t element commonly obs e r v e d i n t h e c e n t r o m e r i c r e g i o n s , a t chromosome t i p s o r i n t h e v i c i n i t y o f t h e n u c l e o l u s o r g a n i z e r r e g i o n . There are a l s o i s o l a t e d r e g i o n s o f i n e r t c h r o m a t i n d i s p e r s e d t h r o u g h o u t t h e e u c h r o m a t i n w h i c h a r e termed i n t e r c a l a r y h e t e r o c h r o m a t i n . C o n s t i t u t i v e h e t e r o c h r o m a t i n i s t h e m a t e r i a l c u s t o m a r i l y a s s o -16. d a t e d w i t h p o s i t i o n - e f f e c t v a r i e g a t i o n . I n D r o s o p h i l a m e l a n o g a s t e r , the genomic c o n t e n t o f h e t -e r o c h r o m a t i n has been e s t i m a t e d t o be 25% (Peacock et a l . , 1973). The m a j o r i t y o f t h i s m a t e r i a l i s found on t h e X o r Y-chromosomes but t h e r e a r e s i g n i f i c a n t p o r t i o n s found on t h e autosomes as w e l l . E u c h r o m a t i c i n t e r c h a n g e s w i t h any o f t h e s e r e g i o n s has been o b s e r v e d t o e l i c i t v a r i e g a t e d gene e x p r e s s i o n . Much e f f o r t has been a p p l i e d t o t h e d i s t i n c t i o n o f h e t -e r o c h r o m a t i n from e u c h r o m a t i n . There .are c y t o l o g i c a l t e c h -n i q u e s t h a t s t a i n t h e s e • r e g i o n s d i f f e r e n t i a l l y and t h e C-band-i n g t e c h n i q u e i s s p e c i f i c f o r c o n s t i t u t i v e h e t e r o c h r o m a t i n . S e v e r a l a u t h o r s suggest t h a t t h e DNA a s s o c i a t e d p r o t e i n compo-s i t i o n i n h e t e r o c h r o m a t i n i s f u n d a m e n t a l l y d i f f e r e n t t h a n t h a t o b s e r v e d i n e u c h r o m a t i n ( B e r l o w i t z , 1965; H s i e n . a n d B r u t l a g , 1979; B l u m e n f e l d , 1979) . The DNA c o n s t i t u t i o n o f h e t e r o c h r o m a t i c r e g i o n s has been s t u d i e d t h o r o u g h l y and ap-p e a r s t o be composed p r i m a r i l y o f r e p e t i t i v e s a t e l l i t e DNA (Yasmineh and Y u n i s , 1969; B l u m e n f e l d and F o r r e s t , 1971; Kram e t a l . 1 1972; Peacock e t a l . , 1973) . The s t u d i e s o f t h e s e d i f f e r e n c e s s h o u l d produce i n s i g h t i n t o the mechanisms o f chromosome c o n d e n s a t i o n and gene r e g u l a t i o n . The most p r o f o u n d b i o c h e m i c a l d i f f e r e n c e between euchro-m a t i n and h e t e r o c h r o m a t i n appears t o be t h e e x i s t e n c e o f t h e s a t e l l i t e sequences. T h i s t y p e o f DNA i s composed o f s h o r t , h i g h l y r e p e t i t i v e sequences and are found almost e x c l u s i v e l y i n c o n s t i t u t i v e h e t e r o c h r o m a t i n . I n D. m e l a n o g a s t e r , Peacock 17-e t a l . (1973) have i d e n t i f i e d f o u r major s a t e l l i t e sequences t h a t make up t h e v a s t m a j o r i t y o f the h i g h l y r e p e t i t i v e DNA. These a u t h o r s a l s o l o c a l i z e d t h e d i s t r i b u t i o n o f t h e s a t e l l i t e sequences by h y b r i d i z i n g l a b e l l e d cDNA t o d e n a t u r e d p o l y t e n e chromosomes o r m i t o t i c p r e p a r a t i o n s from b r a i n c e l l s . V i r t u -a l l y a l l o f t h e s e sequences were i d e n t i f i e d i n the chromo-c e n t r a l r e g i o n i n t h e p o l y t e n e chromosomes w h i l e v e r y s p e c i f i c h e t e r o c h r o m a t i c s i t e s a r e d e f i n e d by the m i t o t i c chromosomes ( P e r r a u l t e t a l . , 1978) . There i s good e v i d e n c e t h a t each o f t h e major s a t e l l i t e sequences have a h i g h l y s p e c i f i c d i s t r i b u t i o n i n the genome. P e r r a u l t e t a l . (1978) found d i f f e r e n c e s between X/Y and X/X t e m p l a t e DNA and G o l d r i n g e_t a l . (1975) suggest t h a t each sequence i s s i t e - s p e c i f i c . Peacock e t - a l . (1973) a l s o e s t i -mate t h a t 80fo o f t h e h e t e r o c h r o m a t i n i s composed of t h e f o u r major sequences. T h i s o b s e r v a t i o n has promoted i n v e s t i g a t i o n s i n v o l v e d w i t h d e f i n i n g p o s s i b l e c o d i n g r e g i o n s w i t h i n the h e t e r o c h r o m a t i n . The g e n e t i c i n e r t n e s s o f h e t e r o c h r o m a t i n has l o n g been q u e s t i o n e d . I n r e p o r t i n g the f i r s t l i n k a g e map e v er con-s t r u c t e d , S t u r t e v a n t (1913) remarked t h a t " . . . t h e r e i s no way o f knowing whether or n o t t h e s e d i s t a n c e s as drawn r e p r e -s e n t t h e a c t u a l r e l a t i v e s p a c i a l d i s t a n c e s a p a r t o f t h e s e f a c t o r s . " He o b s e r v e d i n h i s work t h a t t h e X - l i r i k e d genes were crowded t o g e t h e r a t one end o f the chromosome. The X - r a y i n d u c t i o n o f i n v e r s i o n s w i t h h e t e r o c h r o m a t i c b r e a k s was 18. found t o t r a n s f e r l a r g e b l o c k s o f c h r o m a t i n i n t o new e n v i r o n -ments ( M u l l e r and P a i n t e r , 1929)• Re c o m b i n a t i o n d a t a show t h a t l i t t l e exchange o c c u r s i n t h e c e n t r o m e r i c r e g i o n o r h e t e r o c h r o -m a t i c r e g i o n s t h a t have been t r a n s f e r r e d i n t o t h e euchromatin (Offerman e t a l . , 1931)' The i n d u c t i o n o f mutants mapping i n the h e t e r o c h r o m a t i c r e g i o n s has pr o v e n t o be v e r y d i f f i c u l t . A number o f genes have been a s s o c i a t e d w i t h h e t e r o c h r o m a t i c r e g i o n s b u t t h e s e s e r v e t o i n d i c a t e t h e low g e n e t i c d e n s i t y i n such a r e a s . D a t a such as t h e s e s t r o n g l y s u p p o r t t h e n o t i o n o f g e n e t i c i n e r t n e s s o f h e t e r o c h r o m a t i c r e g i o n s . F i n e s t r u c t u r a l a n a l y s i s o f th e b a s a l h e t e r o c h r o m a t i n o f the X-chromosome s u g g e s t s t h a t t h e t r a n s i t i o n from e u c h r o m a t i n t o h e t e r o c h r o m a t i n i s g r a d u a l r a t h e r t h a n a b r u p t ( L i f s c h y t z , 1978). Numerous complementation groups can be i d e n t i f i e d w i t h i n t h e t r a n s i t i o n zone and i t i s b e l i e v e d t h a t t h e s e c o d i n g s i t e s a r e s e p a r a t e d by i n t e r c a l a r y h e t e r o c h r o m a t i n . I n t h e a r e a p r o x i m a l t o the bobbed l o c u s , t h e r e a re o n l y l o n g h e t e r o -c h r o m a t i c sequences a p p a r e n t l y u n i n t e r r u p t e d by c o d i n g r e -g i o n s . Other a u t h o r s have suggested t h a t t h e c y t o l o g i c a l o b s e r v a t i o n o f " d i f f u s e " h e t e r o c h r o m a t i n i n p o l y t e n e chromo-somes r e p r e s e n t s a v i s u a l i z a t i o n o f th e t r a n s i t i o n a l r e g i o n between t r u e e u c h r o m a t i n and t r u e h e t e r o c h r o m a t i n ( G a l l , 1973; L a k h o t i a and Jacob, 197*0. A t t h i s t i m e , the d i f f u s e h e t e r o -c h r o m a t i n was termed B w h i l e t h e more condensed form was termed A. A v a r i e t y o f ex p e r i m e n t s have s i n c e enhanced t h i s a s s o c i a t i o n and i t seems l i k e l y t h a t t h e B h e t e r o c h r o m a t i n a c t u a l l y r e p r e s e n t s the t r a n s i t i o n zone. The p r o x i m a l h e t e r o c h r o m a t i n o f t h e major autosomes are a l m o s t e n t i r e l y v o i d o f mutable l o c i . The detachment of com-pound autosomes and subsequent a n a l y s i s has c o n f i r m e d the l o c a t i o n o f t h e genes l i g h t ( i t ) and r o l l e d ( r l ) i n the p r o x -i m a l h e t e r o c h r o m a t i n o f chromosome 2 ( H i l l i k e r and Holm, 1975)' F u r t h e r s t u d i e s o f chromosome 2 h e t e r o c h r o m a t i n i n v o l v e d the i n d u c t i o n o f p o i n t m u t a t i o n s w i t h i n h e t e r o c h r o m a t i n u s i n g the mutagen e t h y l methane s u l f o n a t e (EMS). Numerous l o c i were i d e n t i f i e d on e i t h e r s i d e o f the centromere but t h e g e n e t i c d e n s i t y was s t i l l e s t i m a t e d t o be l e s s t h a n 1% o f t h a t o b s e r -ved i n e u c h r o m a t i n ( H i l l i k e r , 1976). A number o f complement-a t i o n groups have been r e c o v e r e d from t h e c e n t r o m e r i c r e g i o n s o f chromosome but u n f o r t u n a t e l y t h e i r h e t e r o c h r o m a t i c assignment i s s p e c u l a t i v e ( B a l d w i n and S u z u k i , 1971)- A l t h o u g h some l o c i appear t o be p r e s e n t i n the l a r g e b l o c k s o f a u t o -somal h e t e r o c h r o m a t i n , i t seems t h a t t h e m a j o r i t y o f t h i s c h r o -m a t i n i s i n e r t and s e r v e s a n o n - c o d i n g f u n c t i o n . The e x i s t e n c e o f s p e c i f i c X-chromosome l o c i w i t h i n h e t e r o -c h r o m a t i n has been i m p l i c a t e d i n s t u d i e s i n v o l v e d w i t h t h e h e t e r o c h r o m a t i c i n f l u e n c e on c e r t a i n autosomal l o c i . S a n d l e r (197^) r e p o r t s t h a t t h e m a t e r n a l e f f e c t mutants, abnormal o o c y t e (abo) and d a u g h t e r l e s s ( d a ) , a r e p a r t i a l l y r e v e r t e d i n t h e p r e s e n c e o f e x t r a c o p i e s o f p a r t i c u l a r X h e t e r o c h r o m a t i c f r a g m e n t s . He s u g g e s t s t h a t t h e s e l o c i may r e g u l a t e t h e syn-t h e s i s o f a h e t e r o c h r o m a t i c gene p r o d u c t ( P a r r y and S a n d l e r , 20. 1975)• R e c e n t l y , S a n d l e r (1977) has o u t l i n e d a group o f t i g h t l y l i n k e d autosomal markers t h a t i n t e r a c t w i t h the X h e t e r o c h r o m a t i n . He a l l u d e s t o a more g e n e r a l i n t e r a c t i o n between t h e s e r e g i o n s as t h e mutants are s e n s i t i v e t o a l a r g e a r e a o f p r o x i m a l X h e t e r o c h r o m a t i n . P r o c u n i e r and T a r t o f (1978) r e p o r t e v i d e n c e t h a t a l o c u s w i t h i n t h e same r e g i o n may c o n t r o l the m u l t i p l i c i t y o f t h e rRNA genes i n t h e n u c l e o l u s o r g a n i z e r r e g i o n . The compensa-t o r y r e s p o n s e l o c u s ( c r ) , i s r e p o r t e d t o c o n t r o l t h e compen-s a t i o n o f rRNA when an i n d i v i d u a l i s d e f i c i e n t o f t h e rDNA c i s t r o n s . -The i n t e r a c t i o n between s p e c i f i c l o c i and c e r t a i n r e g i o n s o f h e t e r o c h r o m a t i n suggest t h a t t h e " i n e r t r e g i o n s " may s e r v e r e g u l a t o r y f u n c t i o n s w h i c h , as y e t , a r e u n d e f i n e d . The a s s o c i a t i o n between p o s i t i o n - e f f e c t v a r i e g a t i o n and h e t e r o c h r o m a t i n must i n v o l v e t h e p e c u l i a r c h e m i s t r y o f the compacted c h r o m a t i n and t h e mechanisms i n v o l v e d i n the main-tenance o f t h e i r s t r u c t u r e . The s a t e l l i t e sequences compose up t o 80% o f the h e t e r o c h r o m a t i n and a r e a r r a n g e d i n homoge-neous b l o c k s o f 600 k i l o b a s e s , r e p r e s e n t i n g thousands o f t a n -dem r e p e a t s o f a f i v e t o t w e l v e base p a i r u n i t (Peacock _et a l . , 1973; Endow e t a l . , 1975)- I t seems l i k e l y t h a t t h e p r o c e s -s i n g o f t h i s d i s t i n c t i v e r e g i o n i n t o condensed b l o c k s i n v o l -v e s the compaction o f t h e c h r o m a t i n i n t o an i n a c t i v e s t a t e . Numerous t h e o r i e s have been advanced as t o t h e n a t u r e o f t h i s p r o c e s s and, a t t h e moment, the s p e c i f i c b i n d i n g o f m o d i f i e d h i s t o n e components appears t o mediate t h e compaction o f c h r o -21 . m a t i n ( B l u m e n f e l d jet a l . , 1978) . Numerous n o n - h i s t o n e p r o -t e i n s a r e "believed a l s o t o be i n v o l v e d w i t h the d e t e r m i n a t i o n o f i n a c t i v e c h r o m a t i n s t r u c t u r e d u r i n g development ( P a u l , 1970)-The r e g i o n a l c ompaction o f DNA mediated by s p e c i f i c b i n d -i n g p r o t e i n s may p r o v i d e a t e s t a b l e e x p l a n a t i o n o f t h e mech-anism o f p o s i t i o n e f f e c t . I t has been p r e v i o u s l y observed t h a t t h e d e r i v a t i o n o f a v a r i e g a t e d phenotype i n v o l v e d t h e i n a c t i v a t i o n o f e u c h r o m a t i c genes i n t r o d u c e d i n t o the p r o x i m i t y o f h e t e r o c h r o m a t i n . S p o f f o r d (1976) comments t h a t t h e v a r i e -g a t i n g gene appears t o be i n a c t i v a t e d by a " s p r e a d i n g e f f e c t " a s s o c i a t e d - w i t h i t s p r o x i m i t y t o the h e t e r o c h r o m a t i n . Z u c k e r k a n d l (197*0 has a l s o suggested t h a t t h e s p r e a d i n g e f -f e c t may be e x p l a i n e d by a l o c a l i z e d i n a c t i v a t i o n o f t h e gene by d i f f u s i o n o f a s u b s t a n c e n a t i v e t o t h e condensed c h r o m a t i n . S p e c i f i c h i s t o n e s o r n o n - h i s t o n e p r o t e i n s may r e p r e s e n t t h i s f a c t o r as e u c h r o m a t i c genes t r a n s f e r r e d i n t o t h e v i c i n i t y o f h e t e r o c h r o m a t i n may be i n f l u e n c e d by t h e DNA-binding p r o t e i n s t h a t a c t t o i n a c t i v a t e c h r o m a t i n . E v i d e n c e s u p p o r t i n g t h i s h y p o t h e s i s i s r e p o r t e d by Moore e t a l . , (1979) who e x p l o r e d t h e e f f e c t o f h i s t o n e gene d e f i c i e n c i e s on t h e e x p r e s s i o n o f v a r i e g a t i n g genes. These a u t h o r s p r e d i c t e d t h a t a r e d u c t i o n o f c e l l u l a r amounts o f h i s t o n e would reduce th e e x t e n t o f m o s a i c i s m i n v a r i e g a t i n g mutants. The.data s u p p o r t t h e i r h y p o t h e s i s and p r o v i d e f u r t h e r e v i d e n c e t h a t h i s t o n e s may be t h e v e h i c l e o f c h r o m a t i n c o n d e n s a t i o n o r h e t e r o c h r o m a t i z a t i o n . The d e v e l o p m e n t a l s t a g e a t which h e t e r o c h r o m a t i z a t i o n 22. o c c u r s i s o f i n t e r e s t as t h i s may determine t h e a b i l i t y o f a l o c u s t o v a r i e g a t e . C h r o m a t i n c o n d e n s a t i o n i s f i r s t d e t e c t -a b l e a t t h e b l a s t o d e r m s t a g e ( H u e t t n e r , 1933; Mahowald, 1 9 6 8 ) . P r i o r t o t h i s p e r i o d , gene a c t i v i t y and chromosome d i f f e r e n t -i a t i o n have n o t been i n i t i a t e d ( I l l m e n s e e , 1973) . I+- i s ap-p a r e n t t h a t most genes i n i t i a t e t h e i r f u n c t i o n a f t e r t h e b l a s -toderm s t a g e has been completed. As t h e f i r s t heterochroma-t i z a t i o n c o i n c i d e s w i t h t h i s s t a g e , most genes would be sub-j e c t t o h e t e r o c h r o m a t i n - i n d u c e d v a r i e g a t i o n . I n r e v i e w i n g t h e many examples o f p o s i t i o n e f f e c t mosa-i c i s m , i t i s ap p a r e n t t h a t t h e e x t e n t o f mutant t i s s u e i s d i f f e r e n t f o r s e p a r a t e m u t a t i o n s . I n many c a s e s , the mutant t i s s u e appears as l a r g e , homogeneous c l o n e s o f c e l l s w h i l e a n o t h e r mutant may p r e s e n t a v e r y f i n e - g r a i n e d m o s a i c i s m . T h i s d i f f e r e n c e i s a t t r i b u t e d t o t h e t i m i n g o f t h e o r i g i n a l i n a c t i v a t i o n e v e nt. An e a r l y d e c i s i o n a l l o w s numerous m i t o -t i c d i v i s i o n s and s u b s e q u e n t l y , a l a r g e c l o n e o f mutant t i s -sue i s formed. L a t e d e c i s i o n s r e s u l t i n much s m a l l e r c l o n e s and t h e s e a re seen as " s a l t and pepper" m o s a i c i s m . From t w i n spot e x p e r i m e n t s i n v o l v i n g m i t o t i c recombina-t i o n , many a u t h o r s have a t t e m p t e d t o e s t i m a t e the number o f p r o g e n i t o r c e l l s p r e s e n t a t v a r i o u s d e v e l o p m e n t a l s t a g e s . F o r the mesonotum, t h e r e seems t o be l e s s t h a n 10 p r o g e n i t o r c e l l s i n newly h a t c h e d l a r v a e and t h i s e s t i m a t e i s a l s o ob-s e r v e d a t t h e b l a s t o d e r m n u c l e i s t a g e ( G a r c i a - B e l l i d o and Merriam, 1969, 1971)- E x p e r i m e n t s such as t h e s e suggest t h a t 23-t h e d e t e r m i n a t i o n pathways a r e f i x e d i n numerous c e l l s from t h e b l a s t o d e r m s t a g e onward and t h a t i n a c t i v a t i o n can be a s -s o c i a t e d w i t h any c e l l d i v i s i o n a f t e r . From t h i s d i s c u s s i o n o f h e t e r o c h r o m a t i n and i t s a s s o c i -a t i o n w i t h p o s i t i o n - e f f e c t v a r i e g a t i o n , one can see t h a t t h e c o m p l e x i t i e s i n v o l v e d i n m a i n t a i n i n g c h r o m a t i n s t r u c t u r e t h r o u g h o u t development l i k e l y i n f l u e n c e gene e x p r e s s i o n . I t i s a p p a r e n t t h a t s p e c i a l i z e d , n o n - c o d i n g f u n c t i o n s c h a r a c t e r -i s t i c o f h e t e r o c h r o m a t i n do n o t p r o v i d e a s u i t a b l e e n v i r o n -ment f o r c l a s s i c gene f u n c t i o n . There are. a number o f genes t h a t can t o l e r a t e t h e h e t e r o c h r o m a t i c s i t u a t i o n but t h e y may be v e r y s p e c i a l i z e d i n f u n c t i o n . D r o s o p h i l a m e l a n o g a s t e r has e v o l v e d two s e p a r a t e t y p e s o f c h r o m a t i n t h a t p r o v i d e n e c e s s a r y c e l l u l a r f u n c t i o n s . The e u c h r o m a t i n p r o v i d e s most o f the c o d i n g r e g i o n s w h i l e t h e h e t e r o c h r o m a t i n p r o v i d e s s t r u c t u r a l and o t h e r n o n - c o d i n g f u n c t i o n s . These o p e r a t i o n s have been s t r u c t u r a l l y s e p a r a t e d and t h e r e would be o b v i o u s s e l e c t i o n p r o m o t i n g such c o m p a r t m e n t a l i z a t i o n . Owing t o t h e s t r i c t c o n s e r v a t i o n o f t h e e u c h r o m a t i c and h e t e r o c h r o m a t i c r e g i o n s , one would expect a b e r r a n t phenomena t o be a s s o c i a t e d w i t h t h e r e o r g a n i z a t i o n o f the c h r o m a t i n . P o s i t i o n - e f f e c t v a r i e g a t i o n r e p r e s e n t s an example o f such an anomaly and c o n s e q u e n t l y , t h i s phenomenon has a t t r a c t e d much a t t e n t i o n r e c e n t l y . One o f the i m p o r t a n t c o n s i d e r a t i o n s i n t h i s t h e s i s must be t h e mechanisms o f b r i s t l e development. The phenotype t h a t i s a s s o c i a t e d w i t h the s c u t e i n v e r s i o n s i n v o l v e s t h e improper 2k. e x p r e s s i o n o f genes t h a t c o n t r o l t h e development and d i s t r i -b u t i o n o f b r i s t l e s on the e p i d e r m a l s u r f a c e o f the f l y . The l e t h a l i t y a s s o c i a t e d w i t h t h e s e s c u t e i n v e r s i o n s may be c o r -r e l a t e d w i t h the b r i s t l e development d e f e c t and, t h e r e f o r e , i t i s advantageous t o examine the pathways by w h i c h th e b r i s -t l e s a r e formed. Normal b r i s t l e development i n v o l v e s t h e d i f f e r e n t i a t i o n o f t h e b r i s t l e f o r m i n g c e l l s a t about 15 hours a f t e r puparium f o r m a t i o n (APF) (Lees and Waddington, 1 9 ^ 2 ) . A p a i r o f c e l l s m i g r a t e t o a s p e c i f i c p o i n t on t h e e p i d e r m i s and form a r e c -o g n i z a b l e complex a t 25 t o 30 h o u r s APF. The c e l l l y i n g n e a r e s t t o t h e s u r f a c e forms th e s o c k e t o f the b r i s t l e and i s known as t h e tormogen ( W i g g l e s w o r t h , 1 9 3 3 ) - Beneath th e tormogen l i e s a second c e l l termed th e t r i c h o g e n w h i c h se-c r e t e s t h e b r i s t l e m a t e r i a l . Between 30 and kO h o u r s , t h e c e l l s e n l a r g e t h e i r l i n e a r d i m e n s i o n s by t e n t i m e s and have gone t h r o u g h numerous e n d o m i t o t i c d i v i s i o n s t o produce p o l y -p l o i d n u c l e i . A s s o c i a t e d w i t h t h e p o l y p l o i d y i s t h e forma-t i o n o f prominent n u c l e o l i w h i c h presumably f u n c t i o n t o i n -c r e a s e rRNA p r o d u c t i o n f o r b r i s t l e f o r m a t i o n . I n i t i a l l y (30 h o u r s ) , the t r i c h o g e n s e c r e t e s a b r i s t l e m a t e r i a l w h i c h i s s o l i d and c o l o r l e s s . S i m u l t a n e o u s w i t h b r i s t l e p r o d u c t i o n i s t h e development o f t h e b r i s t l e s o c k e t . The tormogen b e g i n s s e c r e t i o n o f the s o c k e t m a t e r i a l s l i g h t l y l a t e r t h a n t h e i n i t i a l t r i c h o g e n a c t i v i t y but b o t h r e a c h maximum a c t i v i t y a t about kO h o u r s . The t r i c h o g e n and t o r -25. mogen a r e a c t i v e l y g r o w i n g and s e c r e t i n g m a t e r i a l s u n t i l s l i g h t l y a f t e r 4-0 h o u r s . A f t e r t h i s t i m e , t h e l a r g e volume o f c y t o p l a s m e s t a b l i s h e d by t h e s e c e l l s i s d i m i n i s h e d as the b r i s t l e and s o c k e t s t r u c t u r e s become more complete. The s o c k e t s t r u c t u r e i s c o m p l e t e l y formed a t 48 hours w h i l e t h e b r i s t l e i s n o t completed u n t i l 60 h o u r s . A l l s e c r e t i o n s from t h e tormogen and t r i c h o g e n have ceased by 70 hours and t h e s e c e l l s have now become i n d i s t i n g u i s h a b l e from t h e o t h e r e p i d e r m a l c e l l s . The m a t u r a t i o n o f t h e b r i s t l e i n v o l v e s p i g m e n t a t i o n and a h o l l o w i n g o f t h e immature s o l i d s t r u c t u r e . I t i s suggested t h a t t h e h o l l o w i n g o f the b r i s t l e s r e s u l t s from t h e h a r d e n i n g and c o n t r a c t i o n o f t h e c h i t i n o f which t h e b r i s t l e i s made (Lees and Waddington, 194-2) . A f t e r a p e r i o d o f a p p r o x i m a t e l y 10 h o u r s , the b r i s t l e s have a t t a i n e d t h e i r f u l l y mature s t r u c t u r e and a r e p r e p a r e d f o r the a d u l t environment. I t i s app a r e n t t h a t b r i s t l e development i n v o l v e s a complex c h a i n o f e v e n t s t h a t i s d e t e r m i n e d by s t r i c t g e n e t i c c o n t r o l s . I n v e s t i g a t i o n s o f t h e mutants o f t h e b r i s t l e development pathway p r o v i d e s f u r t h e r i n s i g h t s i n t o t h e mechanisms o f g e n e t i c r e g u l a t i o n . A c l a s s i c paper c o n c e r n i n g the d i s s e c t i o n o f t h e b r i s t l e d e v e l o p m e n t a l pathways was w r i t t e n by Lees and Waddington (194-2). I n t h i s s t u d y , t h e a u t h o r s i n v e s t i g a t e d a v a r i e t y o f mutants t h a t were known t o e f f e c t b r i s t l e e x p r e s s i o n and de t e r m i n e d t h e d e v e l o p m e n t a l s t a g e a t w h i c h the m u t a t i o n a c t e d . 2 6 . They d e f i n e d t h e s e d e v e l o p m e n t a l s t a g e s and a s s o c i a t e d mu-t a n t s : d e t e r m i n a t i o n o f b r i s t l e c e l l - s c u t e (ssc) ; b r i s t l e c e l l d i v i s i o n - s p l i t ( s p _ l ) ; a r r i v a l a t e p i d e r m a l s u r f a c e -D i c h a e t e (D); arrangement o f tormogen and t r i c h o g e n - S t u b b l e (Sb) j growth o f c e l l s - s p i n e l e s s ( J E S S ) ; c h i t i n s e c r e t i o n -s i n g e d ( s n ) , f o r k e d ( f ) ; d a r k e n i n g and h a r d e n i n g - t h e "body c o l o r " genes. The a u t h o r s completed t h e s t u d y w i t h t w e l v e d i f f e r e n t mutants and t h e y a l l f i t i n t o t h e s e seven c a t e g o r i e s . T h e i r d a t a p r o v i d e s t r o n g e v i d e n c e s u p p o r t i n g the- e x i s t e n c e o f a complex pathway c o n t r o l l i n g b r i s t l e d e t e r m i n a t i o n . Of p a r t i c u l a r i n t e r e s t t o t h i s s t u d y i s the development o f t h e jsc phenotype. I t has been found t h a t s c u t e mutants a r e d e f i c i e n t o f h a i r c e l l s from the e a r l i e s t p o s s i b l e p u p a l s t a g e (Lees and Waddington, 1 9 ^ 2 ) . T h i s s u g g e s t s t h a t t h e -i n h e r e n t d e f e c t i n t h e s c u t e mutants i n v o l v e s t h e d e t e r m i n a -t i o n and/or m i g r a t i o n o f the b r i s t l e c e l l s . F u r t h e r i n v e s -t i g a t i o n s o f the s c u t e d e v e l o p m e n t a l p r o f i l e i n d i c a t e t h a t t h e t e m p e r a t u r e - e f f e c t i v e p e r i o d o c c u r s a t the end o f t h e l a r v a l p e r i o d . Thus, t h e a c t i o n o f t h e w i l d t y p e gene must o c c u r between t h e end o f the l a r v a l p e r i o d and 1 6 t o 2 0 h o u r s a f t e r puparium f o r m a t i o n . The a s s o c i a t i o n o f t h e b r i s t l e s w i t h t h e n e r v e network was f i r s t p o i n t e d out by S t e r n ( 1 9 3 8 ) . He o b s e r v e d t h a t t h e macro- and m i c r o c h a e t a e on t h e t h o r a x were i n t i m a t e l y a s s o -c i a t e d w i t h a network o f p e r i p h e r a l n e r v e c e l l s . T h i s i s c o n s i s t e n t w i t h t h e s e n s o r y f u n c t i o n o f t h e b r i s t l e s and 27. p o i n t s to a p o s s i b l e a s s o c i a t i o n "between nervous d i f f e r e n t -i a t i o n and the scute mutation. Clever (I960) found that when p e r i p h e r a l nerves were poisoned by methylene blue, the tormogen and t r i c h o g e n c e l l s f a i l e d to d i f f e r e n t i a t e . I t i s p o s s i b l e t h a t the f a i l u r e of nervous growth may i n f l u e n c e the normal generation of the b r i s t l e forming complex. Other data suggest that the a c t i o n of the scute-achaete system i s even e a r l i e r than t h i s stage. C e l l s d e f i c i e n t f o r the scute l o c u s can be produced at d i f f e r e n t times of development by induced m i t o t i c exchange. I t has been observed that these scute d e f i c i e n t c e l l s are able to d i f f e r e n t i a t e the b r i s t l e forming complex i f the i n -d u c t i o n occurs l a t e r than 4-0 to 4-8 hours (macrochaetae) or 16 to 24- hours (microchaetae) before puparium formation ( G a r c i a - B e l l i d o and Santamaria, 1978). There i s some e v i -dence which suggests that the mother epidermal c e l l i n i t i a t e s d i f f e r e n t i a l d i v i s i o n s g i v i n g r i s e to nervous and epidermal pathways at t h i s s p e c i f i c time ( G a r c i a - B e l l i d o and Merriam, 1971a; G a r c i a - B e l l i d o and Santamaria, 1978). There are a number of mutants which map to the scute r e g i o n t h a t are defined as embryonic l e t h a l s . H i s t o l o g i c a l d e s c r i p t i o n of these mutants revealed a b n o r m a l i t i e s i n gas-t r u l a t i o n w i t h anomalies i n segregation of nervous t i s s u e and hypodermis (Ede, 1956b). Gynandromorph s t u d i e s c a r r i e d out by G a r c i a - B e l l i d o and Santamaria (1978) i n d i c a t e that v i a b l e gynanders only occur when _sc d e f i c i e n c y t i s s u e does 28. n o t e x t e n d a c r o s s t h e v e n t r a l m i d l i n e o f the embryo from w h i c h , presumably, t h e n e r v o u s system i s d e r i v e d . I t i s t h i s e v i d e n c e t h a t s t r o n g l y s u g g e s t s t h a t t h e r e g u l a r f u n c t i o n o f t h e a c h a e t e - s c u t e l o c u s i n v o l v e s t h e d i f f e r e n t i a t i o n o f n e r v e e l e m e n t s . I n a r e c e n t s e r i e s o f s t u d i e s , t h e g e n e t i c and d e v e l o p -m e n t a l c h a r a c t e r i s t i c s o f the a c h a e t e - s c u t e system have been i n v e s t i g a t e d ( G a r c i a - B e l l i d o and S a n t a m a r i a , 1978; G a r c i a -B e l l i d o , 1979)- I n t h e d e v e l o p m e n t a l s t u d i e s , t h e a u t h o r s d e f i n e d t h e p h e n o e f f e c t i v e phase and d e f i n e d s p e c i f i c pheno-t y p e s i n gynandromorphs and m i t o t i c r e c o m b i n a t i o n c l o n e s . The d e v e l o p m e n t a l r e s u l t s suggest t h a t t h e normal f u n c t i o n o f t h e a c h a e t e - s c u t e complex i n v o l v e s two f u n c t i o n s ; one i n -v o l v e d w i t h t h e d i f f e r e n t i a t i o n o f t h e embryonic c e n t r a l n e r v o u s system and t h e o t h e r i n v o l v e d w i t h d i f f e r e n t i a t i o n o f a d u l t p e r i p h e r a l n e r v o u s elements. The l e t h a l phase a s s o c i a t e d w i t h a c - s c d e f i c i e n c i e s were d e t e r m i n e d by e x a m i n i n g v i a b i l i t y a t t h e egg, embryo, l a r v a l , p u p a l and a d u l t s t a g e s . I t i s e v i d e n t t h a t a l l o f the d e f i -c i e n c i e s a r e hemizygous l e t h a l s b u t t h e r e appears t o be two s e p a r a t e l e t h a l i t y p r o f i l e s d i s t i n g u i s h i n g t h e mutants. One group has been shown t o be d e f i c i e n t f o r a r e g i o n known as l e t h a l o f s c u t e ( 1 ' s c ) which maps p r o x i m a l l y from the euchro-m a t i c b r e a k s o f s c u t e - 4 and s c u t e - S i . These i n d i v i d u a l s are i n v a r i a b l y embryonic l e t h a l s and a r e e n t i r e l y s c u t e mutants. The o t h e r group shows d e f i c i e n c i e s f o r b o t h _sc and ac but not 29. 1'sc and d i e i n advanced s t a g e s o r even emerge as d e b i l i t a t e d a d u l t s . These a u t h o r s a l s o n o t e d t h e c h a r a c t e r i s t i c motor p r o b -lems a s s o c i a t e d w i t h a d u l t s u r v i v o r s c a r r y i n g the s c u t e d e f i -c i e n c i e s . I t was o b s e r v e d t h a t I n ( l ) y ^ ^ s c 8 ^ males are f u l l y m o t i l e and f e r t i l e w h i l e sc sc and D f ( l ) sc are n o n - m o t i l e and s t r o n g l y ac and sc. The n o n - m o t i l e i n d i v i d u a l s were o b s e r v e d t o emerge and r e m a i n immobile on t h e f o o d , o r f r e q u e n t l y , were found t o d i e when a t t e m p t i n g t o emerge from t h e p u p a r i a l c a s e . T h i s abnormal d e v e l o p m e n t a l phenotype has a l s o been o b s e r v e d i n many o t h e r r e c o m b i n a n t s c o n s t r u c t e d from v a r i o u s s c u t e i n v e r s i o n s ( i n p r e p a r a t i o n ) . D i f f e r e n t l e t h a l d e f i c i e n c i e s were s t u d i e d f o r t h e i r v i a b i l i t y i n male s p o t s i n gynanders. D e f i c i e n c y male t i s s u e was o b s e r v e d o v e r a l l a r e a s o f t h e e p i d e r m i s and c r o s s e d the d o r s a l m e d i a l l i n e f r e q u e n t l y . However, t h e r e were no v i a b l e gynanders o b s e r v e d t h a t had male d e f i c i e n c y t i s s u e c r o s s i n g t h e v e n t r a l m e d i a l l i n e , s u g g e s t i n g t h a t gynanders w i t h n e r -vous t i s s u e o f t h e d e f i c i e n c y t y p e a r e i n v i a b l e ( P o u l s o n , 1 9 5 0 ) . I n t h e s e s t u d i e s , t h e r e were a l s o a number o f f l i e s o b s e r v e d w i t h m o b i l i t y p roblems. Of s p e c i a l i n t e r e s t t o t h i s s t u d y are t h e e x p e r i m e n t s i n v o l v i n g the D f ( l ) s c u t e - 8 . G a r c i a - B e l l i d o and S a n t a m a r i a (1978) d e t e r m i n e d t h a t t h e l e t h a l i t y a s s o c i a t e d w i t h s c u t e - 8 / 0 t i s s u e was n o t due t o v a r i e g a t i o n o f t h e rRNA genes b u t , r a t h e r v a r i e g a t i o n o f a v i a b i l i t y gene n e a r t h e b r e a k p o i n t 30. o f s c u t e - 8 . These a u t h o r s a l s o e s t a b l i s h e d t h a t none o f the genes d i s t a l t o ac a r e r e q u i r e d f o r c h a e t e d i f f e r e n t i a t i o n . S t u d i e s i n v o l v i n g m i t o t i c r e c o m b i n a t i o n p e r m i t t h e exam-i n a t i o n o f t i s s u e c l o n e s n o r m a l l y l e t h a l i n t h e gynandro-morph s t u d i e s . R e s u l t s from t h e s e e x p e r i m e n t s c o n f i r m e d much o f t h e d a t a produced i n t h e p r e v i o u s e x p e r i m e n t s and a l s o showed t h a t much l a r g e r d e f i c i e n c i e s c o u l d be s t u d i e d . T h i s work a l s o e s t a b l i s h e d t h e absence o f b r i s t l e - a f f e c t i n g genes d i s t a l t o s c u t e ( G a r c i a - B e l l i d o , 1979)-The r e m o v a l , by m i t o t i c r e c o m b i n a t i o n , o f a w i l d t y p e a l l e l e o f some genes a f t e r a g i v e n p o i n t i n development may have no e f f e c t o f t h e phenotype o f t h e r e s u l t i n g c l o n e s as t h e major f u n c t i o n o f t h e l o c u s may be complete ( G a r c i a -B e l l i d o and M e r r i a m , 1971c)• The phenomenon i n v o l v e d w i t h t h e s p e c i f i c d e v e l o p m e n t a l p r o f i l e o f t h e s e e x p e r i m e n t s i s c a l l e d p e r d u r a n c e . From t h e i r s t u d i e s , t h e a u t h o r s e s t i m a t e t h a t t h e i n i t i a l f u n c t i o n o f t h e a c - s c complex o c c u r s p r i o r t o 4-0 hours p r e p u p a r i u m f o r m a t i o n f o r the macrochaetae and p r i o r t o 16 h o u r s f o r t h e m i c r o c h a e t a e . These works have uncovered a number o f i n t e r e s t i n g o b s e r v a t i o n s r e l a t i n g t o t h e a c h a e t e - s c u t e r e g i o n . A r e l a -t i o n s h i p has been e s t a b l i s h e d between the s c u t e mutants and n e r v o u s t i s s u e d i s f u n c t i o n . T h i s o b s e r v a t i o n i s s t r o n g l y s u p p o r t e d by m i t o t i c r e c o m b i n a t i o n e x p e r i m e n t s and t h e e x i s -t e n c e o f s u r v i v i n g mutant a d u l t s w i t h s e v e r e l y abnormal mo-t o r f u n c t i o n s . The o r i g i n a l f u n c t i o n o f t h e a c - s c l o c u s 31-appears t o "be e x e r t e d p r i o r t o puparium f o r m a t i o n w i t h se-condary f u n c t i o n s a p p e a r i n g somewhat l a t e r i n development. The d e v e l o p m e n t a l a n a l y s i s o f t h e s c u t e m u t a t i o n has produced many i n t e r e s t i n g r e s u l t s but i t has been t h e fun d a -m e n t a l g e n e t i c a n a l y s i s o f t h i s l o c u s t h a t has g e n e r a t e d the m a j o r i t y o f r e s e a r c h i n t e r e s t . The o r i g i n a l d i s c o v e r y o f t h e s c u t e gene o c c u r r e d i n 1 9 1 6 " when B r i d g e s s t u d i e d t h e o c c u r r e n c e o f spontaneous mutants i n D r o s o p h i l a . I n s t u d i e s on t h e e f f e c t s o f X - r a y s t e n y e a r s l a t e r , M u l l e r i s o l a t e d a c o l l e c t i o n o f b r i s t l e v a r i a n t s w h i c h mapped t o t h e same l o c u s . From t h e s e v a r i a n t s , a group o f S o v i e t g e n e t i c i s t s s e p a r a t e d a s p e c i f i c s e t o f a l l e l e s which t h e y termed s c u t e ( s c ) and mapped t h e s e mutants t o t h e t i p o f t h e X-chromosome. I n v e s t i g a t i o n s o f t h e s e s c u t e mutants r e v e a l e d t h a t t h e y e x i s t e d as a heterogeneous group w i t h the common phenotype o f r e d u c e d b r i s t l e number. A l t h o u g h a l l t h e a l l e l e s mapped t o i d e n t i c a l p o s i t i o n s , each a l l e l e p r o v i d e d a c h a r a c t e r i s t i c phenotype. T h i s i n f o r m a t i o n p r o v i d e d t h e f i r s t i n d i c a t i o n t h a t t h e component d i s s e c t i o n o f a gene c o u l d be p o s s i b l e . The s t u d y o f p a r t i a l c o m plementation c o u l d p r o v i d e e v i d e n c e o f t h e c o m p l e x i t y o f the s c u t e l o c u s . The S o v i e t group was t h u s prompted t o a c c e l e r a t e t h e i r r e s e a r c h i n t o t h e s c u t e " a l l e l o m o r p h s " and g e n e r a t e d f u r t h e r a l l e l e s by X - i r r a d i a t i o n o f s t a n d a r d s t o c k s ( S e r e b r o v s k y e t a l . , 1 9 2 8 ) . Subsequent r e s e a r c h i n t o t h e phenomenon o f s t e p - a l l e l o m o r p h i s m o r p a r t i a l - c o m p l e m e n t a t i o n p r o v i d e d the 32. f i r s t e v i d e n c e f o r t h e e x i s t e n c e o f "complex" genes and p r o -duced the f i r s t l i n e a r gene map c o r r e l a t i n g an i n t r a - g e n e p o s i t i o n w i t h a s p e c i f i c f u n c t i o n . The predominant r e s u l t s ' o f t h e i r s t u d i e s can he summarized as f o l l o w s : 1. a l a r g e number o f s c u t e a l l e l e s can be i d e n t i f i e d , each showing b r i s t l e r e d u c t i o n but d i f f e r i n g i n th e s p e c i f i c l o c a t i o n o f phenotype. 2. s t u d i e s o f h e t e r o z y g o t e s i n d i c a t e t h a t p h e n o t y p i c c h a r a c t e r i s t i c s common t o s c u t e a l l e l e s w i l l not complement, w h i l e d i s t i n g u i s h i n g c h a r a c t e r i s t i c s - r e v e r t t o w i l d t y p e . 3- t h e r e e x i s t s a s e r i e s o f a l l e l o m o r p h s t h a t r e p r e -s e n t a g r a d i e n t w i t h i n t h e s c u t e gene. There i s a t r a n s i t i o n from v e r y s i m i l a r a l l e l e s t o thos e t h a t show few s i m i l a r c h a r a c t e r i s t i c s . k. comparison o f t h e v a r i o u s a l l e l o m o r p h s f a c i l i t a t e s t h e d i s s e c t i o n o f t h e s c u t e gene i n t o complement-a t i o n u n i t s o r "gene c e n t r e s . " The arrangement o f t h e s e gene c e n t r e s w i l l s u b s e q u e n t l y r e s u l t i n t he c o n s t r u c t i o n o f a l i n e a r map o f t h e s c u t e gene. ( D u b i n i n , 1929b, 1930b, 1930d; A g o l , 1929; S h a p i r o , 1930; S e r e b r o v s k y , 193°a). The a c c u m u l a t i o n o f t h e s e d a t a r e p r e s e n t e d an enormous body o f work and when t h r e e p r o m i n e n t g e n e t i c i s t s c r i t i c i z e d t h e work, c o n f i d e n c e i n t h e p r o j e c t was d r a m a t i c a l l y under-33-mined and i n v e s t i g a t o r s were a v e r s e t o f u r t h e r s t u d i e s on t h e complex s c u t e l o c u s . The c r i t i c s o f the "complex" gene t h e o r y c i t e d t h e sen-s i t i v i t y o f t h e s c u t e mutants t o m o d i f y i n g f a c t o r s as e v i d e n c e o f t h e inadequacy o f t h e sub-gene h y p o t h e s i s . S t u r t e v a n t and S c h u l t z (1931) u t i l i z e d X - d u p l i c a t i o n s o f v a r i o u s s i z e s t o c o v e r t h e s c u t e - 1 a l l e l o m o r p h . The a u t h o r s observed v a r i a n t b r i s t l e phenotypes w i t h each d u p l i c a t i o n d e s p i t e t h e f a c t t h a t t h e d u p l i c a t i o n s were a p p a r e n t l y c a r r y i n g t h e w i l d t y p e s c u t e a l l e l e . I t seems p o s s i b l e t h a t t h e w i l d t y p e s c u t e gene p r e s -ent on the - d u p l i c a t i o n may be s u b j e c t t o p o s i t i o n - e f f e c t v a r i e g a t i o n owing t o t h e p r o x i m i t y o f the gene t o h e t e r o c h r o -m a t i n . T h i s seems more l i k e l y i n l i g h t o f the f a c t t h a t t h e s e s t u d i e s were based on d a t a o b t a i n e d e n t i r e l y from males and t h a t no such "dominant" phenotype has been r e p o r t e d f o r s c u t e s i n c e t h i s s t u d y . These a u t h o r s a l s o s t u d i e d t h e e f f e c t o f a chromosome _2 dominant gene, H a i r l e s s (H), w i t h r e s p e c t t o t h e s c u t e - 1 a l l e l e . They obser v e d a r e d u c t i o n o f b r i s t l e s n o t p r e d i c t e d by t h e "sub-gene" t h e o r y and c o n c l u d e d t h a t t h e s e r e s u l t s undermined th e whole l o g i c a l b a s i s o f t h e h y p o t h e s i s . G o l d s c h m i d t (1930) and C h i l d (1935) a l s o c r i t i c i z e d the work on t h e b a s i s o f the o b s e r v e d e f f e c t o f a number o f mod-i f y i n g f a c t o r s . C h i l d d e t e r m i n e d t h a t t h e b r i s t l e s i n the s c u t e - 1 i n d i v i d u a l were te m p e r a t u r e s e n s i t i v e . A t d i f f e r e n t t e m p e r a t u r e s , d i f f e r e n t groups o f b r i s t l e s were a f f e c t e d . 34. I t was n o t mentioned t h a t a t a l l t e m p e r a t u r e s , t h e major a f f e c t e d b r i s t l e s p o s t u l a t e d f o r s c u t e - 1 remained a f f e c t e d . A n o t h e r segment o f the c r i t i q u e i n v o l v e d a s e l e c t i o n from th e s c u t e - 1 s t o c k o f a s e r i e s o f mutants i n v o l v i n g r e d u c t i o n o f s i n g l e b r i s t l e s w i t h o u t a f f e c t t o o t h e r b r i s t l e groups. The a u t h o r s s u g g ested t h a t t h e s e r e s u l t s r e v e a l e d t h e p r e s -ence of m o d i f i e r s elsewhere i n the genome and showed t h a t the development o f t h e b r i s t l e s i s dependent upon the g e n e t i c system as a whole r a t h e r t h a n m e r e l y th e s c u t e l o c u s as i n -f e r r e d by t h e S o v i e t g e n e t i c i s t s . C h i l d d i s m i s s e s t h e S o v i e t work by c o n c l u d i n g " . . . t h e s e d a t a show t h a t t h e concept o f p a t t e r n o f d i s t r i b u t i o n i n s c u t e i s based on n o t h i n g more t h a n d i f f e r e n c e s i n t h e mean numbers, i n a group o f f l i e s , o f d i f f e r e n t b r i s t l e s and does n o t c o r r e s p o n d t o any d e v e l -opmental p r o c e s s e s i n t h e i n d i v i d u a l f l y . " A l t h o u g h C h i l d d emonstrated t h a t a v a r i e t y o f f a c t o r s were i n v o l v e d i n t h e development o f t h e b r i s t l e s , t h e o t h e r c r i t i c i s m s were l e s s e r and o n l y c o n s t i t u t e d grounds f o r m o d i f i c a t i o n o f t h e o r i g i n a l h y p o t h e s i s . U n f o r t u n a t e l y , as a consequence o f t h e admonishment o f t h e s e prominent g e n e t i c i s t s , f u r t h e r s t u d i e s o f s t e p - a l l e l i s m were d i s c o n t i n u e d and t h i s mass o f d a t a has been n e g l e c t e d f o r over f o r t y y e a r s w i t h o u t subsequent c o n s i d e r a t i o n . Recent p r o g r e s s i n t h e f i e l d o f m o l e c u l a r g e n e t i c s has e l u c i d a t e d a number o f complex g e n e t i c l o c i (Jacob and Monod, 1964; R a t n e r , 197^)• Methods o f complementation, recombin-35-a t i o n and gene s e q u e n c i n g have been u t i l i z e d i n t h e d i s s e c -t i o n o f g e n e t i c t o pography and c o n t r o l mechanisms i n p r o -k a r y o t e s . S i m i l a r t e c h n i q u e s have o n l y v e r y r e c e n t l y been used t o s t u d y e u k a r y o t i c genomes but the s t u d y o f c o n t r o l mechanisms and p r o p e r t i e s o f complex l o c i remains somewhat r e f r a c t o r y . The r a p i d p r o g r e s s i n the a n a l y s i s o f complex l o c i i n the p r o k a r y o t i c genome has r e g e n e r a t e d i n t e r e s t i n t h e phenomenon o f s t e p - a l l e l i s m a t t h e s c u t e l o c u s . Recent s t u d i e s by a S o v i e t r e s e a r c h group i n d i c a t e t h a t t h e p r e v i o u s r e s u l t s o b t a i n e d by D u b i n i n ejb a l . , (1929) are i n a c c o r d w i t h an o p e r o n - l i k e system, composed o f t h r e e o r f o u r c i s t r o n s , each r e s p o n s i b l e f o r b r i s t l e development on a s p e c i f i c domain o f t h e f l y (Furman et a l . , 1979)- O r i g i n a l l y , t h e s e a u t h o r s s t u d i e d a group o f r e c e n t l y i s o l a t e d s c u t e mutants f o r c o n f i r m a t i o n o f t h e o r i g i n a l work by D u b i n i n est a l . The r e s u l t s a r i s i n g from t h e newly d e r i v e d l i n e s d i f f e r e d a p p r e c i a b l y from t h e i n i t i a l s t u d i e s but subsequent comple-m e n t a t i o n t e s t s r e s u l t e d i n an i d e n t i c a l map f o r t h e s c u t e l o c u s ( R a t n e r : e t a l . , I969). The a u t h o r s p r oposed two o p e r o n - l i k e models t o e x p l a i n t h e complex n a t u r e o f t h e s c u t e l o c u s . One model r e c o g n i z e s each b r i s t l e g r o u p i n g as r e p r e s e n t a t i v e o f a s i n g l e c i s t r o n w i t h i n t h e s c u t e operon. The second model c o r r e s p o n d s the b r i s t l e g r o u p i n g s w i t h o v e r l a p p i n g r e c o g n i t i o n s i t e s w i t h i n t h e o p e r a t o r o f t h e s c u t e operon. I n o r d e r t o d i s t i n g u i s h between t h e s e two t h e s e s , t h e a u t h o r s found i t n e c e s s a r y t o 3 6 . a s s i g n t h e s c u t e mutants t o a number o f c a t e g o r i e s i n v a r i -a n t w i t h r e s p e c t t o known m o d i f y i n g f a c t o r s o f t h e s c u t e phenotype (Furman and R a t n e r , 1977)- T h i s approach s u p p l i e d f o u r complementation groups, each h a v i n g u n i q u e r e s p o n s e s t o t h e m o d i f y i n g f a c t o r s . T h i s i s t h e b a s i c d a t a upon which t h e operon models a r e t e s t e d . The a u t h o r s t h e n i n v e s t i g a t e d t h e e x p l i c i t b o u n d a r i e s o f t h e complementation groups by examining t y p i c a l r e s p o n s e s t o m o d i f y i n g f a c t o r s and f o c u s s i n g a t t e n t i o n on t h e b r i s t l e s deemed p r o x i m a l t o t h e l i m i t s o f each group (Furman e t a l . , 1 9 7 7 a , b , c ) . U s i n g a computer a n a l y s i s i n v o l v i n g o n l y the s i m i l a r i t y o r d i f f e r e n c e o f t h e m a n i f e s t a t i o n s o f t h e muta-t i o n s on each s p e c i f i c b r i s t l e t y p e , a m a t r i x can be c o n s t r u c -t e d w h i c h i n c l u d e s a l l a v a i l a b l e i n f o r m a t i o n . T h i s m u l t i f a c -t o r i a l a n a l y s i s has p r o v i d e d c o m p e l l i n g e v i d e n c e s u g g e s t i n g a p o l a r s t r u c t u r e o f t h e s c u t e l o c u s . E v i d e n c e a l s o suggests t h a t t h e r e a r e a t l e a s t t h r e e , p o s s i b l y f o u r , i n v a r i a n t b l o c k s o f b r i s t l e i n d i c e s w i t h i n t h e m a t r i x and t h e a u t h o r s have found a s t r o n g c o r r e l a t i o n between s c u t e f u n c t i o n a l u n i t s and t h e t o p o g r a p h i c a l arrangement o f t h e b r i s t l e s on t h e f l y ( i . e . t h o r a x , head, s c u t e l l u m ) . The e s s e n t i a l p r o d u c t s o f t h i s r e s e a r c h are t h e o b s e r -v a t i o n o f map l i n e a r i t y , t h e i n v a r i a n c e o f t h e c l u s t e r s w i t h r e s p e c t t o t e m p e r a t u r e and s c u t e gene dosage and an e x p l a n a -t i o n o f t h e v a r i a b i l i t y o f e x p r e s s i o n i n s c u t e gene h e t e r o -z y g o t e s . T h i s s t u d y o f m a p p i n g - i n v a r i a n t s o f s c u t e has 37-g r e a t l y expanded the e v i d e n c e s u p p o r t i n g a complex gene a t t h i s l o c u s . The r e s e a r c h has a l s o f u r n i s h e d a s i g n i f i c a n t d a t a hase and a t e s t a b l e h y p o t h e s i s t h a t p r o v i d e s t h e ground-work f o r f i n e s t r u c t u r a l a n a l y s i s . The use o f t h i s d a t a base and t h e f u r t h e r t e s t i n g o f t h e topography o f t h e s c u t e gene w i l l u l t i m a t e l y l e a d t o a knowledge o f t h e m o l e c u l a r and f i n e s t r u c t u r a l o r g a n i z a t i o n w i t h i n t h i s l o c u s . More r e c e n t g e n e t i c a n a l y s i s o f the a c h a e t e - s c u t e com-p l e x l o c u s has r e v e a l e d r e s u l t s v e r y s i m i l a r t o t h o s e r e p o r t e d by t h e e a r l y S o v i e t w o r k e r s and t h e i r s u c c e s s o r s ( G a r c i a -B e l l i d o , 1979) . The f i n d i n g s from t h e s e s t u d i e s i n d i c a t e t h a t t h e r e i s a t l e a s t t h r e e f u n c t i o n a l c i s t r o n s w i t h i n t h e a c - s c r e g i o n which i s e n t i r e l y c o n s i s t e n t w i t h p r e v i o u s d i s c u s s i o n s o f t h e s u b j e c t ( D u b i n i n , 1930d; M u l l e r , 1955; Furman e t a l . , 1979) . These s t u d i e s have a l s o a s s i g n e d t h e p o s i t i o n o f t h e H a i r y wing (Hw) l o c u s t o an a r e a v e r y c l o s e t o t h e s c u t e - 8 e u c h r o m a t i c b r e a k . A second l o c u s , l e t h a l o f s c u t e ( 1 ' s c ) , has been a s s i g n e d t o t h e r e g i o n d e l i m i t e d by t h e d i s -9 4-t a l b r e a k p o i n t s o f t h e i n v e r s i o n s s c ^ and sc . The o r g a n i z a t i o n o f t h e d a t a c o m p i l e d i n t h e s e e x p e r i -ments s u g g e s t s t h a t t h e s c u t e system i s c o n s t r u c t e d as a m i r r o r image d u p l i c a t i o n a t b o t h s i d e s o f t h e l e t h a l o f s c u t e l o c u s ( G a r c i a - B e l l i d o , 1979) . Those rearra n g e m e n t s h a v i n g b r e a k s i n t h e p r o x i m i t y o f t h e 1'sc l o c u s have a se-v e r e s c u t e phenotype w h i l e t h o s e b r o k e n more d i s t a l l y are l e s s mutant. These p r o p o s a l s o f g e n e t i c o r g a n i z a t i o n and f u n c t i o n 38. a r e s t i l l h i g h l y s p e c u l a t i v e and a w a i t f u r t h e r i n v e s t i g a t i o n . V e r y e a r l y i n t h e s t u d i e s o f t h e s c u t e a l l e l o m o r p h s e r i e s , i t became v e r y o b v i o u s t o w o r k e r s t h a t o f t e n the s c u t e pheno-t y p e was a s s o c i a t e d w i t h a rearrangement o f the g e n e t i c ma-t e r i a l ( S e r e b r o v s k y , 1930; M u l l e r , 1930a,b, 1932; P a t t e r s o n , 1932b; M u l l e r and P r o k o f y e v a , 1934; M u l l e r , P r o k o f y e v a and R a f f e l , 1935)- Upon a n a l y s i s o f t h e a v a i l a b l e s c u t e mutants, i t became e v i d e n t t h a t most o f 'the s c u t e a l l e l e s owed t h e i r phenotype t o chromosome breakage and r e a t t a c h m e n t i n t h e v i -c i n i t y o f t h e s c u t e gene l o c u s ( R a f f e l a n d . M u l l e r , 1 9 4 0 ) . T h i s o b s e r v a t i o n f o s t e r e d t h e f i r s t i n t e r e s t i n d e f i n i n g a d i f f e r e n c e between t r u e "gene m u t a t i o n s " and " p o s i t i o n e f f e c t m u t a t i o n s " w i t h i n t h e s c u t e a l l e l e s e r i e s . Of p a r t i c u l a r i n t e r e s t a r e t h e o r i g i n a l s t u d i e s e x amining th e p h e n o t y p i c changes a s s o c i a t e d w i t h a number o f X i n v e r s i o n s w i t h b r e a k s n e a r th e s c u t e l o c u s ( R a f f e l and M u l l e r , 1940; G e r s h , 1949; M u l l e r and V a l e n c i a , 1947) . E v o l v i n g from t h e s e s t u d i e s were the mutants c e n t r a l t o t h i s thesis... S c u t e - 4 and s c u t e - 8 were o r i g i n a l l y r e p o r t e d by t h e S o v i e t group and c h a r a c t e r i z e d as i n v e r s i o n s by M u l l e r (1930b) . S c u t e - L 8 was f o u n d by H. Levy i n Texas i n 1932 and s c u t e - S i was found by T.G. S i n i t s k a y a i n L e n i n g r a d i n 1934. The r e a r r a n g e m e n t s were c o n f i r m e d i n t h e s e chromosomes d u r i n g a s t u d y i n v o l v i n g t h e o r g a n i z a t i o n o f t h e X - h e t e r o c h r o m a t i n ( M u l l e r e t a l . , 1937)• S c u t e - V 2 was d i s c o v e r e d by V a l e n c i a i n 1946 and was c h a r a c t e r i z e d as a s i m p l e i n v e r s i o n i n 39-M u l l e r ' s l a b i n 1947 ( M u l l e r and V a l e n c i a , 194-7). The r e p o r t o f t h e s e s p e c i f i c chromosome rea r r a n g e m e n t s prompted a number o f s t u d i e s i n v e s t i g a t i n g t h e r e l a t i o n s h i p between th e map po-s i t i o n o f t h e d i f f e r e n t chromosome b r e a k s and the k i n d s o f s c u t e phenotype a s s o c i a t e d w i t h them. A l t h o u g h many o f t h e p r e v i o u s l y s t u d i e d s c u t e a l l e l e s showed g r e a t d i v e r s i t y i n p h e n o t y p i c e x p r e s s i o n , t h e r e was a s t r o n g c o r r e l a t i o n o b s e r v e d between t h r e e p a r t i c u l a r s c u t e i n v e r s i o n s ( R a f f e l and M u l l e r , 1940). These a u t h o r s observed t h a t t h e p h e n o t y p i c e x p r e s s i o n s o f s c u t e - 4 , s c u t e - S i and s c u t e - L 8 were more comparable t h a n any o f t h e numerous o t h e r s c u t e m u t a t i o n s . The a u t h o r s a l s o a l l u d e d t o t h e a s s o c i a t i o n between t h e s t r i k i n g l y s i m i l a r i n v e r s i o n s and t h e c o r r e s p o n d -i n g phenotypes. A t t h i s t i m e , such o b s e r v a t i o n s were i n t e r -p r e t e d as s t r o n g e v i d e n c e o f a n o v e l p h e n o t y p i c change known as p o s i t i o n - e f f e c t v a r i e g a t i o n ( M u l l e r and P r o k o f y e v a , 1934, 1935a) . F u r t h e r i n v e s t i g a t i o n o f t h e s e t h r e e s i m i l a r s c u t e s i n v o l v e d t h e s t u d y o f t h e p h e n o t y p i c e x p r e s s i o n o b s e r v e d i n r e c o m b i n a n t s o f t h e s e chromosomes. R e s u l t s o f t h i s t e c h n i q u e s u g g e s t e d t h a t t h e phenotypes were much a l i k e , but s u f f i c i e n t l y d i f f e r e n t t o unambiguously c h a r a c t e r i z e each chromosome. The e v i d e n c e accumulated s t r o n g l y i n d i c a t e s t h a t b o t h b r e a k p o i n t s o f t h e i n v e r s i o n a f f e c t t h e development o f t h e b r i s t l e s a l -though the d a t a demonstrates t h a t t h e i n f l u e n c e s o f t h e l e f t r e g i o n s are c l e a r l y t h e p r e v a l e n t source o f t h e s c u t e pheno-4 0 . t y p e ( R a f f e l and M u l l e r , 194-0). A n o t h e r i m p o r t a n t r e s u l t o f t h i s r e s e a r c h was t h e d e t e r -m i n a t i o n o f a p h e n o t y p i c " g r a d i e n t . " Wherever s i g n i f i c a n t d i f f e r e n c e s were observed between t h e s e s t o c k s , the most s e v e r e phenotype was always o b s e r v e d i n t h e s c u t e - L 8 s t o c k w h i l e t h e scute - 4 - s t o c k was l e a s t a f f e c t e d and s c u t e - S i was i n t e r m e d i a t e i n phenotype. I t was observ e d t h a t t h e s e mu-t a n t s p r i m a r i l y a f f e c t e d t h o s e b r i s t l e s t h a t p r e v i o u s l y had been a s s o c i a t e d w i t h t h e s c u t e l o c u s ( D u b i n i n , 1930d). Those b r i s t l e s a s s o c i a t e d w i t h t h e ac h a e t e l o c u s a r e l a r g e l y u n a f-f e c t e d w i t h one major e x c e p t i o n . The v e n t r a l and d o r s a l s t e r n i t e s were s u b s t a n t i a l l y a f f e c t e d and showed the d i s t i n c -t i v e p h e n o t y p i c s e r i e s mentioned above. T h i s v e r y o b v i o u s d i f f e r e n c e between t h e s t o c k s p r o v i d e d a c o n v e n i e n t p h e n o t y p i c c h a r a c t e r w i t h w h i c h t h e s t o c k s c o u l d be d i f f e r e n t i a t e d . I t has p r e v i o u s l y been d e t e r m i n e d t h a t the rearrangements i n t h e s e t h r e e mutants do n o t a c t u a l l y a l t e r t h e p o s i t i o n o f th e s c u t e l o c u s , t h e r e f o r e , the euc h r o m a t i c b r e a k s a re a l l p r o x i m a l w i t h r e s p e c t t o t h e centromere ( M u l l e r and P r o k o f y e v a , 1934, 1935a). I t seems r e a s o n a b l e t o presume t h a t the s c u t e phenotype i s a r e s u l t o f a d i s r u p t i o n o f t h e f u n c t i o n o f the s c u t e gene and t h i s must a r i s e as a consequence o f t h e a l t e r e d chromosomal c o n s t i t u t i o n i n the p r o x i m i t y o f the s c u t e l o c u s . T h i s c o n c l u s i o n was a l s o r e a c h e d by M u l l e r and P r o k o f y e v a (1934) who f i r s t s u ggested t h a t t h e phenotypes e x h i b i t e d by s c u t e - 4 and s c u t e - L 8 were a r e s u l t o f " p o s i t i o n e f f e c t . " The 41. e u c h r o m a t i c "breakpoint o f t h e s e t h r e e i n v e r s i o n s must he al m o s t i d e n t i c a l as a l l o f the r e c o m b i n a n t s c o n s t r u c t e d from t h e s t o c k s a r e v i a b l e and f e r t i l e . S h o u l d t h e b r e a k p o i n t s d i f f e r w i t h r e s p e c t t o any e s s e n t i a l gene, one o f t h e recom-b i n a n t s would be s t e r i l e o r l e t h a l . A u t h o r s have t h u s sug-g e s t e d t h a t t h e p h e n o t y p i c d i f f e r e n c e s observed i n t h e s e s t o c k s a r e a r e s u l t o f t h e d i f f e r i n g b r e a k p o i n t s i n t h e p r o x -i m a l h e t e r o c h r o m a t i n ( R a f f e l and M u l l e r , 1940) . D i s c u s s i o n o f t h e e f f e c t s o f d i f f e r e n t a r e a s o f h e t e r o c h r o m a t i n and t h e i r r e l a t i o n s h i p t o p o s i t i o n - e f f e c t v a r i e g a t i o n a re p r e -s e n t e d elsewhere i n the t e x t . The two o t h e r i n v e r s i o n s s i g n i f i c a n t t o t h i s s t u d y are s c u t e - V 2 and s c u t e - 8 . F a r l e s s e f f o r t has been expended t o s t u d y t h e s c u t e phenotype i n t h e s e i n d i v i d u a l s as a r e s u l t o f t h e o r i g i n a l r e p o r t s w h i c h suggested t h a t the b r i s t l e phenotype was i r r e g u l a r and u n p r e d i c t a b l e ( S i d o r o v , 1931; P r o k o f y e v a - B e l g o v s k a y a , 1947; G e r s h , 1949) . The i n s t a b i l i t y o f t h e phenotypes has been r e c e n t l y c o r r o b o r a t e d i n a s t u d y i n v o l v i n g t h e complementation groups o f t h e s c u t e l o c u s (Furman jet a l . , 1979)- I n many s i t u a t i o n s , f l i e s c a r r y i n g t h e s e i n v e r s i o n s produce a c c e s s o r y macrochaetae a t some s i t e s w h i l e r e d u c e d number o f major b r i s t l e s o c c u r a t o t h e r s i t e s . There i s a l s o p r o f o u n d v a r i a t i o n o f the m i c r o c h a e t a e which appears t o be a t t r i b u t a b l e t o a p o s i t i o n e f f e c t on t h e achaete l o c u s . The o r i g i n a l s t u d i e s o f u s i n g s c u t e - 8 i n v o l v e d a s t u d y 42. o f t h e e x a c t l i m i t s o f t h e i n v e r s i o n ( P a t t e r s o n , 1932b). The f a c t t h a t t h e h e t e r o c h r o m a t i c p o r t i o n o f t h e i n v e r s i o n c a r r i e d t h e bobbed r e g i o n and t h a t a l l e u c hromatic genes p r o x i m a l t o t h e s c u t e l o c u s were i n v e r t e d was e s t a b l i s h e d by recombinant a n a l y s i s ( P a t t e r s o n , 1932b). I t was a l s o shown t h a t the e u c h r o m a t i c break i n s c u t e - 8 o c c u r r e d between the l o c i s c u t e and a c h a e t e , w i t h the s c u t e l o c u s b e i n g r e l o c a t e d t o t h e c e n t r o m e r i c end o f t h e X-chromosome ( P a t t e r s o n and Stone, 1934) . I n t he s t u d y o f mosaic f o r m a t i o n ( P a t t e r s o n , 1933)> s c u t e - 8 males were X - r a y e d and i t was found t h a t t h e r e g i o n e x t e n d i n g from t h e l e f t b r eak t o t h e t i p c o u l d be e l i m i n a t e d . T h i s c r e a t e d a d e f i c i e n c y f o r t h o s e l o c i between achaete and t h e t i p . These a b e r r a t i o n s were i d e n t i f i e d as y e l l o w hypo-p l o i d f e m a l e s and were t h e n mated t o bobbed males i n an e f -f o r t t o demonstrate the p o s i t i o n o f the bobbed l o c u s i n the i n v e r s i o n . Of p a r t i c u l a r i n t e r e s t t o t h i s paper i s t h e ob-s e r v a t i o n t h a t t h e g r e a t m a j o r i t y o f b r e a k s o c c u r r e d between t h e l o c u s o f y e l l o w and bobbed which i n d i c a t e s t h a t t h e r e must be a c o n s i d e r a b l e p i e c e o f p r o x i m a l h e t e r o c h r o m a t i n r e -l o c a t e d t o t h e t i p o f t h e X-chromosome. The use o f a s i m i l a r t e c h n i q u e t o c h a r a c t e r i z e t h e h e t e r o c h r o m a t i c b r e a k p o i n t s o f t h e v a r i o u s s c u t e i n v e r s i o n s i s p r e s e n t l y i n p r o g r e s s . The i r r a d i a t i o n o f the s c u t e - 8 males a l s o produces s m a l l h e t e r o c h r o m a t i c d u p l i c a t i o n s c a r r y i n g t h e w i l d t y p e a l l e l e o f y e l l o w and a s m a l l number o f o t h e r e u c h r o m a t i c l o c i . These ^3 . r e s u l t from two h e t e r o c h r o m a t i c b r e a k s and. e l i m i n a t i o n o f th e m a j o r i t y o f t h e X-eu c h r o m a t i n . Based upon the p r e v i o u s e x p e r i m e n t , one would expect most b r e a k s t o o c c u r between y e l l o w and t h e bobbed l o c u s . The r e a s s o c i a t i o n o f t h e c h r o -mosome p i e c e s s h o u l d produce d u p l i c a t i o n s c a r r y i n g no bobbed l o c u s . R a r e l y , one s h o u l d r e c o v e r a d u p l i c a t i o n c o n t a i n i n g a normal bobbed l o c u s w h i c h r e s u l t e d from a h e t e r o c h r o m a t i c b r eak t o t h e r i g h t o f t h e bobbed l o c u s and a n o t h e r b reak i n t h e c e n t r o m e r i c r e g i o n . The r e s u l t s o f P a t t e r s o n and Stone (1935) a r e i n d i r e c t c o n f l i c t w i t h t h i s r e a s o n i n g . A l t h o u g h 87$ o f a l l .breaks o c c u r r e d between t h e y e l l o w l o c u s and the bobbed l o c u s i n t h e f i r s t e x p e r i m e n t , the d u p l i c a t i o n exper-i m e n t s i n d i c a t e t h a t o n l y 8% o f t h e b r e a k s o c c u r i n t h i s r e g i o n . The a u t h o r s d i d n o t comment on t h i s d i s c r e p a n c y . The d u p l i c a t i o n e x p e r i m e n t s a l s o e s t a b l i s h e d t h a t t h e eu c h r o m a t i c b r e a k p o i n t i n scute-8 was l o c a t e d between the s c u t e and achaete l o c i . T h i s r e s u l t i s based upon t h e ob-s e r v a t i o n t h a t t h e s c u t e l o c u s i s always a s s o c i a t e d w i t h the r e a r r a n g e d genes i n t h e c e n t r o m e r i c r e g i o n . I n c o n t r a s t , t h e achaete l o c u s i s always a s s o c i a t e d w i t h t h e genes r e t a i n -i n g t h e i r n o r m a l sequence n e a r t h e t i p o f t h e chromosome. T h i s work f i r s t d e monstrated t h e d i v i s i b i l i t y o f the ac h a e t e -s c u t e complex and t h a t t h e a c t i o n o f t h e s e genes were n o t c i s - c o n t i g u o u s . The e x p r e s s i o n o f p o s i t i o n e f f e c t s has a l s o been s t u d i e d i n scute-8 w i t h r e l a t i o n s h i p t o t h e h e t e r o c h r o m a t i z a t i o n o f 44. e u c h r o m a t i c l o c i ( P r o k o f y e v a - B e l g o v s k a y a , 194?; Gersh, 1949) . Many a u t h o r s have suggested t h a t t h e p o s i t i o n e f f e c t s o f e u c h r o m a t i c genes a r e a r e s u l t o f a " s p r e a d i n g e f f e c t " o f the o p p o s i n g h e t e r o c h r o m a t i n ( S c h u l t z , 1 9 4 l ; S p o f f o r d , 1959. 1976) . C y t o l o g i c a l s t u d i e s o f p o l y t e n e chromosomes suggest t h a t h e t e r o c h r o m a t i z a t i o n o f t h e e u c h r o m a t i c l o c i a s s o c i a t e d w i t h t h e s c u t e - 8 rearrangement i s tem p e r a t u r e s e n s i t i v e . The e f -f e c t i s maximized a t 25°C w h i l e s i g n i f i c a n t l y reduced a t JO°C and l4°C ( P r o k o f y e v a - B e l g o v s k a y a , 1947)• T h i s s t r a t e g y was p u r s u e d f u r t h e r by c o r r e l a t i n g t h e c y t o l o g i c a l o b s e r v a t i o n s w i t h t h e p h e n o t y p i c e x p r e s s i o n o f v a r i e g a t i n g genes known t o r e s i d e i n t h e v a r i a b l y condensed r e g i o n s a d j a c e n t t o t h e b r e a k p o i n t s ( G e r s h , 1949) . The s t u d y i n v e s t i g a t e d t h e e f f e c t o f t e m p e r a t u r e on t h e v a r i e g a t i n g e x p r e s s i o n o f the H a i r y w i n g and s c u t e m u t a t i o n s i n the s c u t e - 8 i n v e r s i o n . The genes r e s i d e i n p o l y t e n e bands 1B1 .2 t o 1 B 3 . 4 ( L i n d s l e y and G r e l l , 1968) which a r e v a r i a b l y h e t e r o c h r o m a t i z e d i n s t o c k s o f s c u t e - 8 grown a t d i f f e r e n t t e m p e r a t u r e s . Gersh (1949) t h e o r -i z e d t h a t i f t h e r e e x i s t s a d i r e c t r e l a t i o n s h i p between p o s i -t i o n e f f e c t and h e t e r o c h r o m a t i z a t i o n , one s h o u l d be a b l e to r e g u l a t e t h e e x p r e s s i o n o f a v a r i e g a t i n g gene by a l t e r i n g t h e e x t e n t o f h e t e r o c h r o m a t i z a t i o n w i t h t e m p e r a t u r e change. P r o k o f y e v a - B e l g o v s k a y a (1945) o b s e r v e d a maximum o f h e t e r o -c h r o m a t i z a t i o n a t 25° and a r e d u c e d e f f e c t a t t h e more ex-treme t e m p e r a t u r e s . One would ex p e c t t h e p h e n o t y p i c e x p r e s -s i o n o f the mutants t o be most s e v e r e a t 25° and l e s s drama-45-t i c a t o t h e r t e m p e r a t u r e s . The d a t a , however, suggest t h a t an i n v e r s e r e l a t i o n s h i p e x i s t s between b r i s t l e e x p r e s s i o n and t e m p e r a t u r e , t h e r e f o r e i m p l y i n g t h a t t h e b r i s t l e v a r i e g a -t o r s a r e not c l a s s i c a l l y a f f e c t e d o r t h a t t h e r e l a t i o n s h i p between h e t e r o c h r o m a t i z a t i o n and p h e n o t y p i c e x p r e s s i o n i n -v o l v e s more t h a n m e r e l y t h e c o n d e n s a t i o n o f p r e v i o u s l y euchro-m a t i c s i t e s . S i m i l a r r e s u l t s a r e r e p o r t e d by C h i l d (1935) who s t u d i e d t h e b r i s t l e e x p r e s s i o n i n a n o n - v a r i e g a t i n g s y s -tem. I t seems l i k e l y t h a t t h e m a n i f e s t a t i o n o f b r i s t l e pheno-t y p e s as a r e s u l t o f p o s i t i o n e f f e c t may i n v o l v e many s t e p s o f b r i s t l e -development. Such a complex system may n o t r e a c t as e x p e c t e d when s u b j e c t e d t o c l a s s i c a l t e s t s o f p o s i t i o n -e f f e c t v a r i e g a t i o n . A n o t h e r o f t h e s i g n i f i c a n t phenomena a s s o c i a t e d w i t h t h e s c u t e i n v e r s i o n s i s the m a t e r n a l e f f e c t w h i c h r e s u l t s i n g r e a t l y i n c r e a s e d v i a b i l i t y o f s c u t e X/0 males (Johnson, 1979)-G e n e r a l l y , t h e v i a b i l i t y o f s c u t e X/0 i n d i v i d u a l s i s a p p r o x i -m a t e l y 10% o f c o n t r o l s when t h e s c u t e i n v e r s i o n i s i n h e r i t e d from t h e male p a r e n t . When t h e same s c u t e X-chromosome i s i n h e r i t e d from t h e female p a r e n t , t h e v i a b i l i t y i s enhanced t e n - f o l d . The b a s i s o f a l l m a t e r n a l e f f e c t s i s the d i f f e r e n c e i n phenotype o f g e n e t i c a l l y i d e n t i c a l i n d i v i d u a l s as a r e s u l t o f t h e p a r e n t a l source o f t h e chromosomes. R e c i p r o c a l c r o s s e s have o f t e n been obs e r v e d t o produce d i f f e r e n t r e s u l t s and c l a s s i c a l a n a l y s t s have quoted such r e s u l t s as e v i d e n c e s u p p o r t i n g c y t o p l a s m i c i n h e r i t a n c e . 4 6 . There a r e o b v i o u s f a l l a c i e s w i t h t h i s t h e o r y t h a t have been a d e q u a t e l y d e a l t w i t h s i n c e the chromosomal t h e o r y o f i n h e r i -t a n c e has been adopted. I n more r e c e n t y e a r s , g e n e t i c i s t s have been more i n t e r e s t e d i n t h e d i f f e r e n c e s i n phenotype a c r u e d by t r a n s m i s s i o n o f i d e n t i c a l h e r e d i t a r y m a t e r i a l v i a t h e egg o r sperm. The f i r s t i n d i c a t i o n s o f p a r e n t a l m o d i f i c a t i o n s o f pheno-t y p e t h a t were a s s o c i a t e d w i t h p o s i t i o n e f f e c t can be t r a c e d t o i n v e s t i g a t i o n s o f the fundamental b a s i s o f mosaic forma-t i o n . I n 1937 i Demerec and S l i z y n s k a n o t e d t h a t t h e I n (1) w h i t e - m o t t l e d (w_^) homozygotes had l i g h t e r eyes t h a n th e wm/w h e t e r o z y g o t e s . T h i s was a l s o t h e case observed by S c h u l t z (194-1) who remarked t h a t t h e homozygotes always show more v a r i e g a t i o n t h a n the h e t e r o z y g o t e s . N o u j d i n (1941) s t u d i e d v a r i e g a t i o n o f t h e y e l l o w and achaete l o c i a s s o c i a t e d w i t h t h e s c u t e - 8 i n v e r s i o n and e s t a b l i s h e d s p e c i f i c m a t e r n a l and p a t e r n a l e f f e c t s f o r b o t h l o c i . N o u j d i n d e t e r m i n e d t h a t v a r i e g a t i o n o f b o t h x. a n d w a s r e d u c e d when the s c u t e - 8 i n -v e r s i o n was t r a n s m i t t e d - f r o m the mother but i n c r e a s e d when t r a n s m i t t e d t h r o u g h the f a t h e r . He a l s o d e t e r m i n e d t h a t v a r i e g a t i o n was more extreme i n male progeny from h e t e r o z y g o u s f e m a l e s . To d a t e , t h e most e x t e n s i v e l y s t u d i e d m a t e r n a l e f f e c t s have been t h o s e a s s o c i a t e d w i t h t h e tumerous head ( t u h ) , m a r o o n l i k e (mal) and the w h i t e - m o t t l e d (w^) mutants. There have been many o t h e r s d e s c r i b e d i n t h e l i t e r a t u r e as f o l l o w s : 47-cinnamon ( c i n ) - (Browder and W i l l i a m s o n , 1976); d a u g h t e r l e s s (da) - ( C l i n e , 1976); deep orange (dor) - (Marsh e t a l . , 1977); m a t e r n a l e f f e c t l e t h a l (mat(3) 3) - ( R i c e and Garen, 1975); abnormal o o c y t e (abo) - ( S a n d l e r , 1970) and many o t h e r unnamed mutants ( M o h l e r , 1976) . The most o b v i o u s d i f f e r e n c e between p a r e n t a l s o u r c e s o f genes i s the environment o f t h e gamete. P h y s i c a l l y , t h e s i z e o f t h e egg c e l l i s many t i m e s t h a t o f t h e sperm and t h e con-t e n t s a r e f a r more complex. E a r l y embryonic development i s m a i n t a i n e d by t h e l a r g e amount o f c y t o p l a s m which c o n t a i n s p r o d u c t s o f t h e m a t e r n a l genome. Thus, e a r l y embryogenesis i s p r o f o u n d l y i n f l u e n c e d by t h e c h a r a c t e r o f t h e m a t e r n a l genome and c y t o p l a s m ( K h e s i n , 194-7; D a v i d s o n , 19 7 6 ) . More r e c e n t i n v e s t i g a t i o n s o f t h e s e t y p e s o f m a t e r n a l . e f f e c t s i n d i c a t e t h a t t h e phenomenon may be due t o p r o t e i n s o r mRNAs r e s u l t a n t from t h e o o c y t e m a t u r a t i o n p r o c e s s (Wright and Shaw, 1970; Hough-Evans e t a l . , 1977) . E x c l u s i v e p r o t e i n s y n t h e s i s from m a t e r n a l t e m p l a t e has been shown f o r some o f t h e p r o d u c t s e s s e n t i a l i n e a r l y embryogenesis ( R a f f e_t a l . , 1972; L i f t o n and Kedes, 1976) . L o n g - l i v e d , s t a b l e mRNA mole-c u l e s c o d i n g f o r a number o f m a t e r n a l enzymes have a l s o been d i s c o v e r e d (Gerasimova and Smirnova, 1980). Thus, th e d e v e l -opmental phenomenon o f m a t e r n a l e f f e c t i s based upon th e un-e q u a l c o n t r i b u t i o n o f m a t e r i a l t o t h e zygote by the egg and sperm. The g e n e t i c phenomenon o f m a t e r n a l e f f e c t i s l e s s w e l l 4-8. d e f i n e d but may w e l l be i n t i m a t e l y a s s o c i a t e d w i t h t h e d e v e l -opmental p r o c e s s e s p r e v i o u s l y d i s c u s s e d . Most e x p e r i m e n t s have s t u d i e d t h e d i f f e r e n t l e v e l s o f v a r i e g a t i n g phenotype r e s u l t i n g from a d i f f e r e n t p a r e n t a l source o f the r e a r r a n g e -ment ( L u n i n g , 1954; S p o f f o r d , 1958; H e s s l e r , 1961). These e x p e r i m e n t s were u n d e r t a k e n t o determine whether the i n f l u -e n t i a l f a c t o r i n v o l v e d w i t h g e n e t i c m a t e r n a l e f f e c t s was the chromosomal c o n s t i t u t i o n o r t h e mechanism o f t r a n s m i s s i o n v i a egg o r v i a sperm. S p o f f o r d (1961) o b s e r v e d s t r o n g p a r e n t a l e f f e c t s i n the s t u d y o f w h i t e - m o t t l e d v a r i e g a t i o n . The e x p e r i m e n t s i n v o l v e d a s m a l l X-chromosome d u p l i c a t i o n t h a t c a r r i e d a v a r i e g a t i n g w h i t e l o c u s . The f r e e d u p l i c a t i o n y i e l d e d a s i g n i f i c a n t l y l e s s mutant phenotype when t r a n s m i t t e d t h r o u g h the egg. When the d u p l i c a t i o n was t r a n s l o c a t e d i n t o an autosomal e n v i r o n -ment, however, t h e o p p o s i t e r e s u l t was observed: i . e . the most w i l d t y pe phenotype was obser v e d when t h e d u p l i c a t i o n was t r a n s m i t t e d v i a t h e sperm. I t was d i f f i c u l t t o i n t e r p r e t t h e s e r e s u l t s c o n s i d e r i n g t h e number o f f a c t o r s t h a t a r e known t o a f f e c t p o s i t i o n - e f f e c t v a r i e g a t i o n . I t i s p o s s i b l e t h a t t h e s e d a t a a r e u n i q u e l y a f u n c t i o n o f t h e w h i t e l o c u s o r a p h y s i o l o g i c a l phenomenon t h a t i s a s s o c i a t e d w i t h t h e mechanism o f p o s i t i o n e f f e c t . These e x p e r i m e n t s do n o t de-t e r m i n e a g e n e r a l r e l a t i o n s h i p between m a t e r n a l e f f e c t s and p o s i t i o n - e f f e c t v a r i e g a t i o n and i t w i l l be n e c e s s a r y t o d e f i n e new v a r i e g a t i n g m a t e r n a l e f f e c t mutants t o e s t a b l i s h t h i s 49. a s s o c i a t i o n . The o r i g i n a l purpose o f t h i s s t u d y was t o i n v e s t i g a t e t h e p o s i t i o n e f f e c t l e t h a l i t y t h a t was a s s o c i a t e d w i t h the s c u t e i n v e r s i o n X/0 c o n d i t i o n . I t was d e t e r m i n e d "by Hess (1962) t h a t s c u t e - 8 and Base males e x h i b i t e d g r e a t l y reduced v i a b i l i t y when l a c k i n g a Y-chromosome. I n p a r a l l e l e x p e r i -ments, Hess showed t h a t t h e s c u t e males were r e s c u e d i n t h e p r e s e n c e o f a Y-chromosome or a number o f Y-chromosome f r a g -ments. Hess p o s t u l a t e d t h a t the l e t h a l i t y was a s s o c i a t e d w i t h a p o s i t i o n e f f e c t o f the genes i n t h e p r o x i m i t y o f t h e e u c h r o m a t i c b r e a k p o i n t o f t h e s c u t e i n v e r s i o n s . From the r e s u l t s o f a l a t e r s t u d y , Baker (1971) o f f e r e d an a l t e r n a t i v e i n t e r p r e t a t i o n . He s u g g ested t h a t t h e l e t h a l -i t y o b s e r v e d i n s c u t e X/0 males was due t o a p o s i t i o n e f f e c t s u p p r e s s i o n o f t h e r i b o s o m a l RNA genes r a t h e r t h a n a d i s t u r -bance o f the genes i n t h e p r o x i m i t y o f t h e e u c hromatic break-p o i n t . B a k e r ' s f i n d i n g c o r r o b o r a t e d t h e o r i g i n a l o b s e r v a t i o n s made by Hess and extended t h e s t u d y t o i n c l u d e o t h e r s c u t e i n v e r s i o n s . I n an e f f o r t t o s t u d y the c o n t r i b u t i o n o f v a r i e g a t i n g genes i n t h e v i c i n i t y o f t h e b r e a k p o i n t s , B a ker u t i l i z e d s m a l l d u p l i c a t i o n s w h i c h were g e n e r a t e d from th e t i p o f the X -chro-mosome. Baker o b s e r v e d t h a t a l l o f the s c u t e i n d i v i d u a l s had i n c r e a s e d v i a b i l i t y i n t h e p r e s e n c e o f t h e d u p l i c a t i o n s . The i n v e r s i o n s s c u t e - 8 and scute-V2 were e x c l u d e d from c o n s i d e r -50. a t i o n a f t e r t h i s p o i n t as t h e i r v i a b i l i t y was g r e a t l y en-hanced by t h e d u p l i c a t i o n s . However, th e v i a b i l i t y o f the s c u t e - 8 / 0 and s c u t e - V 2 / 0 males n e v e r r e t u r n s t o c o n t r o l l e v e l s i n t h e p r e s e n c e o f t h e d u p l i c a t i o n s and t h e a u t h o r does n o t comment on t h i s d i s c r e p a n c y . The d a t a prompted Baker t o con-8 V 2 e l u d e t h a t t h e l e t h a l i t y a s s o c i a t e d w i t h sc / 0 and sc / 0 i n d i v i d u a l s was a r e s u l t o f a l e t h a l s u p p r e s s i o n o f genes l o c a t e d a t t h e t i p o f t h e X-chromosome. The r e m a i n i n g s c u t e i n v e r s i o n s , s c u t e - S i and s c u t e - L 8 , showed o n l y a modest i n c r e a s e i n v i a b i l i t y i n a s s o c i a t i o n w i t h t h e d u p l i c a t i o n s . The i n s e n s i t i v i t y o f t h e s e two geno-t y p e s t o t h e d u p l i c a t i o n s p r o v i d e d e v i d e n c e s u g g e s t i n g t h a t t h e l e t h a l i t y o b s e r v e d w i t h t h e s e X / 0 males must be caused by genes l o c a t e d a t t h e base o f t h e X-chromosome. The r e -c o v e r y o f t h e s e i n d i v i d u a l s i n t h e p r e s e n c e o f a n o r m a l Y-chromosome prompts two l i n e s o f t h o u g h t . The r e s c u e d v i a -b i l i t y may r e s u l t from t h e v a r i e g a t i o n s u p p r e s s i n g a c t i o n o f t h e Y-chromosome o r complementation o f t h e v a r i e g a t i n g bobbed l o c u s by the s t a n d a r d Y-chromosome bobbed l o c u s . I n an e f f o r t t o d i s c r i m i n a t e between t h e s e two h y p o t h e s e s , B a k e r used a number o f Y-chromosomal fragments which c a r r i e d v a r y i n g amounts o f h e t e r o c h r o m a t i n and rDNA. O "I B a k e r ' s r e s u l t s suggest t h a t t h e v i a b i l i t y o f sc / 0 o r L 8 sc / 0 d i d n o t r e t u r n t o c o n t r o l v a l u e s i n t h e p r e s e n c e o f many Y-chromosomal f r a g m e n t s w h i c h l a c k a f u l l complement o f t h e r i b o s o m a l genes. Baker i n t e r p r e t s t h e s e d a t a as suppor-51. t i v e o f a h y p o t h e s i s o f s u p p r e s s i o n o f t h e r i b o s o m a l c i s t r o n s . There a r e many f l a w s a s s o c i a t e d w i t h t h e e x p e r i m e n t a l methods i n t h e s e s t u d i e s . The d a t a were a l l c o l l e c t e d from one-way c r o s s e s and a number o f d i f f e r e n t compound X-chromo-somes were used t h r o u g h o u t t h e s t u d i e s . The Y-chromosomal fragm e n t s have not been a c c u r a t e l y d e f i n e d as t o t h e i r h e t -e r o c h r o m a t i c c o n t e n t and t h e rDNA r e d u n d a n c i e s were n o t de-t e r m i n e d p r i o r t o t h e e x p e r i m e n t s . I t i s apparent t h a t l a r g e i n c r e a s e s i n X/0 v i a b i l i t y a r e a s s o c i a t e d w i t h t h e h e t e r o c h r o -m a t i c d u p l i c a t i o n s and one must c o n s i d e r o t h e r s o u r c e s o f l e t h a l i t y t o acc o u n t f o r t h e d i s c r e p a n c i e s i n t h e s e exper-i m e n t s . The t h i r d p a r t o f B a k e r ' s s t u d y i n v o l v e d a r e p o r t o f the v i a b i l i t y a s s o c i a t e d w i t h r e c o m b i n a n t s o f the v a r i o u s s c u t e i n v e r s i o n s . The i n v e s t i g a t i o n f o c u s s e d upon the v i a b i l i t y o f t h e s c u t e X/0 r e c o m b i n a n t s w i t h p a r t i c u l a r r e f e r e n c e t o t h e e x t e n t o f h e t e r o c h r o m a t i c a n e u p l o i d y r e s u l t i n g from recombin-a t i o n . The i n t e r p r e t a t i o n o f t h e s e r e s u l t s i s n e c e s s a r i l y s p e c u l a t i v e as t h e d e f i n i t i o n o f the h e t e r o c h r o m a t i c b r e a k s i n t h e s c u t e i n v e r s i o n s has been somewhat s u b j e c t i v e . The major o b j e c t i v e o f t h e p r e s e n t s t u d i e s has been t o i l l u s t r a t e numerous s h o r t c o m i n g s i n t h e "unambiguous" e v i d e n c e p r e s e n t e d by Baker and t o p r e s e n t d a t a s u p p o r t i n g an a l t e r n a t i v e h y p o t h -e s i s . A number o f a u t h o r s p r o v i d e d b i o c h e m i c a l e v i d e n c e sup-p o r t i n g the s u p p r e s s i o n o f rRNA i n s c u t e - S i i n d i v i d u a l s . 52. N i x (1973) i n v e s t i g a t e d t h e rRNA q u a n t i t i e s p r e s e n t i n S1 sc /O l a r v a e . N i x r e p o r t e d a c o n s i s t e n t 15$ r e d u c t i o n o f SI t h e 18s and 28s rRNA i n sc /0 t h i r d i n s t a r l a r v a e . The a u t h o r a l s o e s t a b l i s h e d t h a t t h e s c u t e - S i chromosome r e t a i n e d a f u l l complement o f rDNA by RNA:DNA h y b r i d i z a t i o n . N i x sug-g e s t e d t h a t t h e d e c r e a s e d s y n t h e s i s o f ribosomes c o u l d r e -s u l t from s u p p r e s s i o n o f t r a n s c r i p t i o n o f the rDNA o r d i s t u r b a n c e s o f o t h e r b i o c h e m i c a l p r o c e s s e s i n v o l v e d w i t h development. N i x mentions t h a t e x t r e m e l y bobbed f l i e s show a c o n s i s t e n t 15 - 20$ d e c r e a s e i n rRNA c o n t e n t but does n o t m e n t i o n t h e v i a b i l i t y a s s o c i a t e d w i t h t h e s e i n d i v i d u a l s o r t h e S1 f a c t t h a t t h e sc /0 i n d i v i d u a l s are n o t a s s o c i a t e d w i t h a d e t e c t a b l e bobbed phenotype. These d a t a do n o t produce con-v i n c i n g e v i d e n c e t h a t t h e r e d u c t i o n o f rRNA c o n t e n t i n SI sc /0 l a r v a e i s c a u s a l l y r e l a t e d t o t h e l e t h a l i t y o b s e r v e d i n a d u l t s . I n a s e r i e s o f s t u d i e s o f rRNA s y n t h e t i c r a t e , P u c k e t t and Snyder (1973) demonstrated a 10$ r e d u c t i o n o f rRNA i n XJ8 S1 mature o o c y t e s from homozygous sc or sc f e m a l e s . Owing t o t h e d e v e l o p m e n t a l d e l a y a s s o c i a t e d w i t h t h e s e f e m a l e s , i t i s e n t i r e l y l i k e l y t h a t t h e s i m u l t a n e o u s s a m p l i n g o f eggs from e x p e r i m e n t a l and c o n t r o l i n d i v i d u a l s would r e s u l t i n a b i a s . I t i s p o s s i b l e t h a t t h e c o n t r o l sample would c o n t a i n an e x c e s s o f stage 14 eggs which have been m a x i m a l l y s u p p l i e d SI w i t h rRNA. The sample o f eggs from the sc female c o u l d c o n t a i n l e s s s t a g e 14 eggs and c o n s e q u e n t l y , l e s s rRNA. 53-I n a subsequent paper, P u c k e t t and Snyder (1973a) r e p o r -S l t e d t h a t sc /0 male embryos had g r e a t l y reduced s y n t h e s i s o f rRNA d u r i n g t h e f i r s t f i v e h ours o f embryogenesis a f t e r w h i c h a normal r a t e was e s t a b l i s h e d . A g a i n , t h e s e e x p e r i m e n t s may be a l t e r e d by a s i m p l e sample b i a s . Eggs were s e l e c t e d e a r l y i n development and samples were removed a t d i f f e r e n t d e v e l o p -S1 m e n t a l s t a g e s up t o t h i r d i n s t a r . The sample o f sc / 0 embryos c o n t a i n e d a l l i n d i v i d u a l s t h a t were o f t h i s genotype. There i s a s i g n i f i c a n t l e t h a l i t y a s s o c i a t e d w i t h the embryonic phase SI and t h e sc /0 genotype and t h u s , samples t a k e n i n the l a r v a l s t a g e s r e p r e s e n t a h i g h l y s e l e c t e d sample. The comparison SI o f embryonic sc /0 and c o n t r o l i n d i v i d u a l s would be expected t o show such a d i f f e r e n c e s h o u l d t h e i n h e r e n t s c u t e d e f e c t r e s u l t i n r e d u c e d rRNA c o n t e n t i n the embryo. Thus, t h e s e r e s u l t s may be e x p l a i n e d i n terms o f a b a s i c d e f e c t a s s o c i -a t e d w i t h the s c u t e - S l i n v e r s i o n . W h i l e s t u d y i n g the s e g r e g a t i o n o f compound autosomes, H a r g e r (1974) o b s e r v e d a marked i n c r e a s e i n the v i a b i l i t y o f s c u t e X/0 i n d i v i d u a l s w h i c h was dependent upon the p a r e n t a l s o u r c e o f rearrangement. T h i s m a t e r n a l e f f e c t s u p p r e s s i o n o f the l e t h a l i t y p r o v i d e s an e n t i r e l y n o v e l approach t o the s t u d y o f the phenomena a s s o c i a t e d w i t h t h e s c u t e i n v e r s i o n s . These s t u d i e s may a l s o p r o v i d e f u r t h e r i n s i g h t i n t o the mechanisms o f p o s i t i o n e f f e c t and m a t e r n a l e f f e c t s . 54. MATERIALS AND METHODS M u t a t i o n s and Chromosome Rearrangements: A d e s c r i p t i o n o f m u t a t i o n s used i n t h i s s tudy i s p r o v i d e d i n T a b l e 1. The v a r i o u s i n v e r s i o n s and s p e c i a l chromosomes u t i l i z e d i n t h e e x p e r i m e n t s a r e d e p i c t e d i n F i g u r e s 1 and 2. F u r t h e r d e t a i l s o f t h e s e chromosomes may be o b t a i n e d from L i n d s l e y and G r e l l (1968) . G e n e r a l M a t i n g P r o c e d u r e s : C r o s s e s i n v o l v e d one female and two males mated i n a 25 mm x 95 mm s h e l l v i a l c o n t a i n i n g s t a n d a r d medium composed o f c o r n m e a l , s u g a r , y e a s t and agar w i t h p r o p i o n i c and phos-p h o r i c a c i d added as mold i n h i b i t o r s . F o r most e x p e r i m e n t s , p a r e n t s o f t h e d e s i r e d genotype were t r a n s f e r r e d t h r o u g h suc-c e s s i v e egg l a y i n g i n t e r v a l s o f 4-, 3 and 3 day p e r i o d s and t h e o f f s p r i n g o f each f e m a l e were m o n i t o r e d s e p a r a t e l y f o r each i n t e r v a l . The e x p e r i m e n t s were conducted a t 25°C f o r t h e d u r a t i o n o f e g g - l a y i n g and development, w i t h the e x c e p t i o n o f t h e t e m p e r a t u r e s e n s i t i v i t y s t u d i e s . F o l l o w i n g the i n i t i a l e c l o s i o n , a l l progeny were counted e v e r y second day ( i n some s t u d i e s , e v e r y day) and c l a s s i f i e d a c c o r d i n g t o phenotype. A l l v i a l s were s c o r e d t o e x t i n c t i o n and o b s e r v e d f o r a s h o r t p e r i o d t h e r e a f t e r . An i m p o r t a n t d e f i n i t i o n i n t h e s e s t u d i e s has been th e i n t e r p r e t a t i o n o f t h e term " s u r v i v i n g progeny." F o r t h e p u r -55. TABLE 1 MUTATIONS USED Symbol Name Map P o s i t i o n Phenotype y ac Hw sc pn a w cv sn v m f B c a r mal bb y e l l o w achaete H a i r y - w i n g e d s c u t e prune w h i t e - a p r i c o t c r o s s v e i n l e s s s i n g e d v e r m i l i o n m i n i a t u r e f o r k e d Bar c a r n a t i o n m a r o o n l i k e bobbed 0.0 0.0 0.0 0.0 0.8 1-5 13-7 21 .0 33-0 36.1 56.7 57-0 62 .5 64 . 8 70.0 body y e l l o w h a i r s m i s s i n g e x t r a b r i s t l e s b r i s t l e s m i s s i n g eyes p u r p l e - b r o w n eyes orange c r o s s v e i n s m i s s i n g b r i s t l e s t w i s t e d eyes s c a r l e t wings s m a l l b r i s t l e s f o r k e d eyes narrow eyes dark ruby eyes p u r p l e b r i s t l e s s h o r t , t h i n 5 6 . FIGURE 1 D i a g r a m a t i c r e p r e s e n t a t i o n o f the s c u t e i n v e r s i o n chromosomes. (C - centromere, NO - n u c l e o l u s o r g a n i z e r ) The h e t e r o c h r o m a t i c r e g i o n s are open and wide w h i l e euchro-m a t i n i s n arrow and c l o s e d . 57-wild type y s c w -H-r— su-f NO • SC .L8 + + N O su-f sc S1 + + ySC •H N O su -f • sc 8 N O su-f + + w sc -+4 • sc V2 + +• y N O su-f + + + w scNO +-+ • y sc su-r f SC w -4-N O 58. FIGURE 2 D e s c r i p t i o n and sou r c e o f  S p e c i a l Chromosomes. 59 . Chromosome A b b r e v i a t i o n Source I n ( l I n ( l I n ( l I n ( l I n ( l I n ( l I n ( l I n ( l I n ( l I n ( l I n ( l I n ( l I n ( l I n ( l s c S 1 + S y B s c S 1 + S y B J.B. S p o f f o r d s c S 1 + d l 49 B f v s c S 1 + d l 49 D.G. Holm j O O T O o sc sn- 3 sc sn- 5 B o w l i n g Green T O T Q sc m sc m B o w l i n g Green s c 8 s c 8 + D.G. Holm FM6 y 3 1 d s c 8 dm B sc 8- FM6 D.G. Holm s c 8 bb~ f v c v s c 8 f v cv J.B. S p o f f o r d s c 8 + d l 49 y B f v - s c 8 + d l 49 D.G. Holm V2 V2 sc sc B o w l i n g Green 4 4 sc y sc D.G. Holm s c S 1 L s c 8 R + S B w a Base D.G. Holm s c S 1 L s c 8 R + S w a asc D.G. Holm s c S 1 L s c 8 R + d l 49 B f v v i n s c y D.G. Holm s c S 1 L s c L 8 R S1L L8R Generated Df (Y) bb" Ybb" J.D. P r o c u n i e r Dp ( l ; f ) 1187 Dp 1187 J.D. P r o c u n i e r Dp ( l ; f ) 1337 Dp 1337 J.D. P r o c u n i e r Dp ( l ; f ) 856 Dp 856 . J.D. P r o c u n i e r Df (1) bb 1 - 4 5 2 b b 1 452 J.D. P r o c u n i e r Df (1) bb 2 r l " 3 bb 2 r l " 3 J.D. P r o c u n i e r y 2 su-w a w a Y s- Y V X-Y D.G. Holm C ( l ) RM y w C(1)RM y w D.G. Holm C ( l ) RM y v pn C(1)RM y v pn L a J o l l a 60. poses o f t h i s r e s e a r c h , an i n d i v i d u a l f l y w i t h f r e e m o b i l i t y o f l e g and wing p a r t s i s c o n s i d e r e d a s u r v i v o r . O f t e n d u r i n g t h e s e s t u d i e s , e x t r e m e l y f e e b l e f l i e s were observed m i r e d i n the medium. As t h e s e i n d i v i d u a l s l a c k e d f r e e m o b i l i t y , t h e y have n o t been i n c l u d e d i n t h e r e s u l t s and t h i s e x c l u s i o n may s l i g h t l y b i a s t h e d a t a . When t h e s e weak i n d i v i d u a l s were a prominent f e a t u r e o f an ex p e r i m e n t , an e f f o r t was made t o e s t i -mate t h e i r f r e q u e n c y and phenotype. I n v a r i a b l y i t was found t h a t t h e v a s t m a j o r i t y o f t h e s e i n v a l i d f l i e s were o f t h e s c u t e X / 0 genotype. N e g l e c t i n g t h e s e i n d i v i d u a l s may s l i g h t l y p r e j u d i c e the d a t a but a l l v a l u e s a r e t h u s c o n s e r v a t i v e i n d i -c a t i o n s o f v i a b i l i t y . I t s h o u l d be p o i n t e d out t h a t i n no case d i d t h e s e dead a d u l t s r e p r e s e n t more t h a n 1$ o f t h e t o t a l p rogeny i n s p e c t e d . I n a number o f the e x p e r i m e n t s r e p o r t e d , i t has been n e c e s s a r y t o v a r y the r e s e a r c h t e c h n i q u e s from the p r e c e d i n g p r o t o c o l . S p e c i f i c changes a r e n o t e d a t t h e s t a r t o f each c h a p t e r a l o n g w i t h an e x p l a n a t i o n f o r t h e d e v i a t i o n . An out -l i n e o f s p e c i f i c m a t i n g p r o t o c o l s i s a l s o p r o v i d e d w i t h each c h a p t e r . F o r t h e pu r p o s e s o f t h i s e n t i r e m a n u s c r i p t , the v i a b i l -i t y o f t h e s c u t e X/0 i n d i v i d u a l s w i l l be e x p r e s s e d as a sex r a t i o . T h i s number i s d e r i v e d by d i v i d i n g t h e number o f male progeny by t h e number o f female progeny. Thus, a sex r a t i o o f 1.00 r e p r e s e n t s an e q u a l f r e q u e n c y o f male and female p r o -geny w h i l e a sex r a t i o o f 0 .05 i n d i c a t e s a r a t i o o f one male 61. t o twenty f e m a l e s . I n t h e h e t e r o z y g o t e e x p e r i m e n t s , i t i s n e c e s s a r y t o compare t h e number o f X/0 males, w i t h t h e average r e c o v e r y o f one female genotype. The c o n t r o l s i t u a t i o n s sug-g e s t t h a t t h i s mechanism does n o t i n f l u e n c e the r e s u l t s t o a s i g n i f i c a n t e x t e n t . I n t h e s e e x p e r i m e n t s , t h e comparison o f v i a b i l i t i e s w i t h i n an experiment a r e more v a l i d t h a n compar-i s o n s between s t u d i e s as i t i s apparent t h a t many f a c t o r s may i n f l u e n c e the X/0 v i a b i l i t y . 62. RESULTS - C h a p t e r 1 -P a r e n t a l Source E f f e c t : I . P a t e r n a l l y D e r i v e d X I n t h e f i r s t p a r t o f t h i s s t u d y , male f l i e s c a r r y i n g one o f t h e s c u t e i n v e r s i o n X _ chromosomes and a marked Y-chromosome were mated t o compound-X f e m a l e s l a c k i n g a f r e e Y. The r e -s u l t i n g progeny were compound-X/Y fe m a l e s and s c u t e X/0 males ( F i g u r e 3)' The c o n t r o l , w hich was r u n s i m u l t a n e o u s l y , d i f -f e r e d o n l y . i n t h a t t h e p a r e n t a l female c a r r i e d a f r e e Y-chro-mosome. The d i f f e r e n c e s i n v i a b i l i t y i n t h i s experiment are presumably a r e s u l t o f t h e p r e s e n c e o r absence o f t h e Y-chro-mosome i n male progeny. The r e s u l t s o f t h i s s e c t i o n a r e p r e s e n t e d i n T a b l e 2 and w i l l be d e s i g n a t e d " the p a t e r n a l e f f e c t . " The most s t r i k i n g o b s e r v a t i o n i s t h e l e t h a l i t y a s s o c i a t e d w i t h t h e s c u t e X/0 c o n d i t i o n . N o r m a l l y , X/0 males a r e p h e n o t y p i c a l l y i d e n t i c a l t o t h e i r X/Y c o u n t e r p a r t s e xcept f o r s t e r i l i t y . I n t h e p a t e r -n a l e f f e c t e x p e r i m e n t s , X/0 males e x h i b i t a pronounced decrease i n v i a b i l i t y as compared t o t h e i r X/Y c o n t r o l s . The f i r s t s c u t e i n v e r s i o n s t u d i e d was scute-8, an i n v e r -s i o n w i t h break p o i n t s i n t h e h e t e r o c h r o m a t i n p r o x i m a l t o t h e n u c l e o l u s o r g a n i z e r and d i s t a l t o the s c u t e l o c u s i n t h e eu c h r o m a t i n . The scute-8 X/0 i n d i v i d u a l s showed a t e n - f o l d r e d u c t i o n i n v i a b i l i t y as compared t o X/Y c o n t r o l s . There 63-FIGURE 3 M a t i n g scheme f o r g e n e r a t i n g s c u t e X/0 males w i t h  d i f f e r e n t p a r e n t a l o r i g i n s o f the X-chromosome. 64. Paternal Inheritance scx/Y ® C(DRM/0 1 C(1)RM/ Y scx/ o + scx/ X-Y A scx/scx ® x - y / o Maternal Inheritance 65. TABLE 2 P a t e r n a l l y d e r i v e d X-chromosome: Sc u t e X/0 l e t h a l i t y . CONTROL EXPERIMENT Female M a l e Sex R a t i o Female M a l e Sex Rati< 8 ^ sc + 1138 1173 1.03 2413 193 0.08 s c 8 + dl49 1587 1682 1.06 2290 275 0.12 8 sc CV V f 981 962 O.98 3647 0 0.00 s c 8 (FM6) 1086 998 0.92 836 26 0.03 8 sc s u - f 739 790 1.07 1681 223 0.13 SI sc ym 734 682 0.93 1431 106 0.07 s c S 1 +S B 626 616 O.98 1340 54 0.04 sc 1 +S yB 1048 966 O.92 3812 198 0.05 SI 6lk sc y 617 551 0.89 2192 0 0.00 s c S 1 + dl49 834- 711 -0.85 2608 0 0 .00 SI sc y 1116 1072 0.96 1317 54 0 .04 L8 3 sc s n ^ 846 672 0.79 2509 77 0.03 L8 sc m 1010 84-9 0.84 1984 81 0 .04 s c L 8 f 3 6 a 94-1 673 O.72 2131 0 0.00 s c V 2 743 802 1.08 2371 671 0.26 4 sc 1236 1393 1.13 1964 929 0.87 + 649 664- 1.02 815 791 0-97 6 6 . were two p a r t i c u l a r scute-8 chromosomes t h a t v a r i e d g r e a t l y from t h i s observation. The i n v e r s i o n scute-8 f v cv was i d e n t i f i e d as a bobbed l e t h a l i n numerous experiments of t h i s paper. This chromo-some produces no v i a b i l i t y i n combination w i t h Ybb", Li, 8R -sc sc bb , bb-2rl-3 or d u p l i c a t i o n s t h a t are bb . Recom-binants of t h i s chromosome c a r r y i n g the l e f t end have a l l been shown to be s i m i l a r l y l e t h a l . One cannot expect to ob-serve standard scute X/0 l e t h a l i t y i f the X-chromosome i s c a r r y i n g an a d d i t i o n a l Y suppressed l e t h a l . The FM6-chromosome i s a standard, m u l t i p l y broken X-chro-mosome of the scute-8 type that i s commonly used as a balancer. One of the mutations c a r r i e d on t h i s chromosome (dm) i s asso-c i a t e d w i t h reduced v i a b i l i t y i n X/0 males and s t e r i l i t y i n females. The severe l e t h a l i t y observed w i t h FM6/0 males i s l i k e l y a r e s u l t of the combined scute and diminutive marker e f f e c t s . P h e n o t y p i c a l l y , the scute-8 X/0 males had a character-i s t i c appearance. The most obvious anomaly was the strong achaete (ac) v a r i e g a t i o n which removed a considerable per-centage of the microchaetae from the epidermal surfaces. T h i s conspicuous fe a t u r e i s d e a l t w i t h i n Chapter 8 of t h i s t e x t . Other v a r i e g a t i n g systems observed were scute ( s c ) , y e l l o w (v_) and Hairy-wing (Hw) . I n these s t u d i e s , i t has been very d i f f i c u l t to separate the component e f f e c t s of the sc and Hw v a r i e g a t i o n as these mutations evoke q u i t e 67. o p p o s i t e phenotypes on the f l y . Upon e c l o s i o n , t h e s e X/O f l i e s appeared somewhat imma-t u r e and most e x h i b i t e d s p a s t i c c o n t r o l o f l e g s and wings. A f t e r a day o f m a t u r i n g , t h e s e i n d i v i d u a l s became c o m p e t i t i v e w i t h t h e o t h e r progeny but t h i s d e v e l o p m e n t a l a b n o r m a l i t y i s a g e n e r a l phenomena a s s o c i a t e d w i t h t h e s c u t e X/0 c o n d i t i o n . T h i s i s i l l u s t r a t e d by t h e f a c t t h a t t h e v a s t m a j o r i t y o f i n d i v i d u a l s found dead i n the f o o d are i d e n t i f i e d as r e c e n t l y e c l o s e d X/0 males r a t h e r t h a n t h e o t h e r genotypes. A l l s u r v i v i n g X/0 males were c a r e f u l l y examined f o r the p r e s e n c e o f bobbed b r i s t l e s , i n d i c a t i v e o f reduced r i b o s o m a l RNA l e v e l s . Throughout t h e s e s t u d i e s , no e x c e s s bobbed b r i s -t l e s were a s s o c i a t e d w i t h the s c u t e X/0 males. A l t h o u g h many o f t h e v a r i e g a t i n g phenotypes a f f e c t e d b r i s t l e e x p r e s s i o n , when p r e s e n t , t h e b r i s t l e s were l o n g and t h i c k as opposed t o s h o r t and s l e n d e r , i n d i c a t i v e o f t h e bobbed c o n d i t i o n . An-o t h e r i n d i c a t i o n o f r e d u c e d rRNA c o n t e n t i s the p r e s e n c e o f abdominal e t c h i n g . T h i s p e c u l i a r i t y o c c u r s i n s e v e r e l y bobbed f l i e s but was absent t h r o u g h o u t t h e s e s t u d i e s . The b r i s t l e e x p r e s s i o n and arrangement was e x t r e m e l y v a r i a b l e i n t h e s c u t e - 8 X/0 males. The v a s t m a j o r i t y o f f l i e s l a c k e d one o r more s c u t e l l a r b r i s t l e s and most were d e f i c i e n t i n macrochaetae from t h e head, t h o r a x and abdomen. A phenomenon p e c u l i a r t o t h e s c u t e - 8 type i n v e r s i o n s was t h e f r e q u e n t p r e s e n c e o f a c c e s s o r y d o r s o c e n t r a l b r i s t l e s on t h e p o s t e r i o r r e g i o n o f t h e t h o r a x . These e x t r a b r i s t l e s 68. had a unique s o c k e t and were n o t a s s o c i a t e d w i t h t h e r e g u l a r d o r s o c e n t r a l s o c k e t s . The p o s i t i o n i n g o f the macrochaetae on a l l . b o d y s u r f a c e s was abnormal. O f t e n , b r i s t l e s appear t o be randomly d i s t r i b u t e d on t h e e p i d e r m i s r a t h e r t h a n c o n f o r m i n g t o t h e s p e c i f i c p a t t e r n s o f w i l d t y p e i n d i v i d u a l s . I t was a l s o observed t h a t c e r t a i n macrochaetae were e s p e c i a l l y sen-s i t i v e t o p a t t e r n d i s t u r b a n c e s w h i l e o t h e r s remained unaf-f e c t e d by t h e mutant c o n d i t i o n . These d i f f e r e n c e s w i l l be d e s c r i b e d i n a l a t e r c h a p t e r d e a l i n g w i t h v a r i e g a t i n g b r i s t l e phenotypes as a g e n e r a l phenomenon. The p a t t e r n d i s t u r b a n c e s were p a r t i c u l a r l y e v i d e n t when o b s e r v i n g t h e m i c r o c h a e t a e on t h e s u r f a c e o f the t h o r a x o f t h e X/0 i n d i v i d u a l s . There was g r e a t d i v e r s i t y o f t h e m i c r o -c h a etae p a t t e r n s and e x p r e s s i o n i n the s c u t e - 8 X/0 i n d i v i d -u a l s . Most commonly, t h e f l y e x h i b i t e d s m a l l b a l d p a t c h e s on t h e t h o r a x and d i s c o n t i n u i t y o f t h e l i n e a r b r i s t l e s e r i e s . L e s s e x t r e m e l y a f f e c t e d i n d i v i d u a l s d i s p l a y e d n e a r - n o r m a l b r i s t l e number but s t i l l showed abnormal b r i s t l e d i s t r i b u t i o n . F u r t h e r i n v e s t i g a t i o n d emonstrated t h a t t h e m i c r o c h a e t a e d i s t u r b a n c e s extended t o t h e abdominal s u r f a c e s where b o t h b a l d i n g and p a t t e r n a b n o r m a l i t i e s were obs e r v e d . The v a r i -e g a t i o n o f t h e b r i s t l e l o c i appears t o be a d e v e l o p m e n t a l problem i n v o l v i n g b o t h t h e b r i s t l e e x p r e s s i o n and p a t t e r n d e t e r m i n a t i o n . The s c u t e - 8 X/0 males were obser v e d t o e c l o s e much l a t e r t h a n t h e i r female s i b s . T h i s d e v e l o p m e n t a l d e l a y was s t r i k i n g 6 9 . and c o n s i s t e n t t h r o u g h o u t t h e s e e x p e r i m e n t s . G e n e r a l l y , the X / 0 males e c l o s e d 2 - 4 - days a f t e r the f e m a l e s w i t h the v a s t m a j o r i t y o f i n d i v i d u a l s e c l o s i n g a f t e r day 1 5 - A more c r i t i -c a l assessment o f t h i s d e v e l o p m e n t a l d e l a y i s p r o v i d e d i n C h a p t e r 7 « There appears t o he a r a t h e r s t r o n g c o r r e l a t i o n "between t h e l e n g t h o f development and t h e degree o f b r i s t l e v a r i e g a -t i o n i n t h e s c u t e - 8 X / 0 males. Those f l i e s e c l o s i n g a f t e r an a b n o r m a l l y extended development time a r e i n v a r i a b l y extreme mutants. O f t e n , l a t e e c l o s i n g f l i e s a re so s e v e r e l y a f f e c t e d t h a t t h e y drop d i r e c t l y i n t o t h e f o o d and are u n a b l e t o escape. I n a number o f t h e s e e x p e r i m e n t s , th e m i r e d i n d i v i d u a l s were r e s c u e d from t h e f o o d and b r i s t l e phenotypes were r e c o r d e d . G e n e r a l l y , t h e s e f l i e s r e p r e s e n t e d t h e most s e v e r e l y a f f e c t e d c l a s s o f progeny. These, d a t a a r e i n c l u d e d i n the d i s c u s s i o n o f t h e d e v e l o p m e n t a l d e l a y i n C h a p t e r 7 -The n e x t s c u t e i n v e r s i o n s t u d i e d was s c u t e - S i , a chromo-some t h a t i s b r o k e n i n t h e p r o x i m a l X - h e t e r o c h r o m a t i n and e u c h r o m a t i c a l l y j u s t p r o x i m a l t o t h e s c u t e l o c u s ( i . e . the i n v e r s i o n does n o t c o n t a i n the s c u t e l o c u s ) . Four o f the a v a i l a b l e s i x s c u t e - S i chromosomes a l s o c a r r y one o f two i n t e r -s t i t i a l i n v e r s i o n s , e i t h e r I n ( l ) d l 4 - 9 o r I n ( l ) S. I t has been o b s e r v e d p r e v i o u s l y t h a t m u l t i p l y b r o k e n chromosomes may have d e l e t e r i o u s p r o p e r t i e s a s s o c i a t e d w i t h t h e e x t r a b r e a k p o i n t s ( R a f f e l and M u l l e r , 1 9 4 - 0 ) . T h i s has a l s o been s p e c i f i c a l l y s u g g e s t e d f o r the f i r s t s c u t e - S i i n v e r s i o n s t h a t were r e c o v -70. e r e d by t h e S o v i e t group i n t h e e a r l y t h i r t i e s ( M u l l e r and S i n i t s k a y a , 1931)- E v i d e n c e f o r t h i s s u g g e s t i o n may be found i n t h e d a t a d i s p l a y e d i n T a b l e 2. I t appears t h a t t h e normal X/Y males c a r r y i n g a s c u t e - S i i n v e r t e d X-chromosome a r e some-what l e s s v i a b l e t h a n t h e i r f emale s i b s i n t h e c o n t r o l exper-i m e n t s . However, t h i s may a l s o be i n t e r p r e t e d as an e f f e c t o f t h e s c u t e i n v e r s i o n i t s e l f . As w i t h s c u t e - 8 , t h e r e i s a d r a m a t i c l e t h a l i t y a s s o c i -a t e d w i t h t h e s c u t e - S i X/0 c o n d i t i o n . . F o u r o f the mutants s t u d i e d showed a t w e n t y - f o l d r e d u c t i o n i n v i a b i l i t y when the Y-chromosome was a b s e n t . The o t h e r two examples of s c u t e - S i chromosomes c a r r i e d t h e i n v e r s i o n , I n ( l ) d l ^ 9 , and X/0 i n d i -v i d u a l s were n o t g e n e r a t e d from any c r o s s e s . I t was observed t h a t t h e s c u t e - S i + dl49-chromosomes d i d not produce v i a b l e progeny when a s s o c i a t e d w i t h e i t h e r rDNA d e f i c i e n t Y o r X - c h r o -mosomes. S e v e r e l y bobbed f l i e s were r e c o v e r e d when t h e s e s c u t e - S i chromosomes were a s s o c i a t e d w i t h a Ybb-chromosome. T h i s e v i d e n c e s t r o n g l y s u g g e s t s t h a t t h e s e v e r e l e t h a l i t y a s s o c i a t e d with' t h e s e chromosomes i s a r e s u l t o f a bobbed de-f i c i e n c y a n a l o g o u s t o t h e s i t u a t i o n p r e v i o u s l y d e s c r i b e d f o r s c u t e - 8 f v cv. I t i s d i f f i c u l t t o p a r t i t i o n t h e l e t h a l i t y i n t o components o f bobbed l e t h a l i t y and s c u t e l e t h a l i t y as t h e bb~ c o n d i t i o n i s a s s o c i a t e d w i t h complete i n v i a b i l i t y . Thus, e x p e r i m e n t s i n v o l v i n g t h e s e s c u t e - S i + dl49-chromosomes have n o t p r o v i d e d a g r e a t d e a l o f i n f o r m a t i o n r e l a t i n g t o t h e s c u t e l o c u s . 7 1 . P h e n o t y p i c a l l y , s c u t e - S i d i f f e r s somewhat from s c u t e - 8 . I n X/Y males, th e s c u t e - S i genotype c o n f e r s a m o d e r a t e l y s t r o n g s c u t e phenotype w h i l e i n s c u t e - 8 , o n l y r a r e l y a r e mac^ r o c h a e t a e m i s s i n g . S c u t e - S i i n d i v i d u a l s a l s o e x h i b i t a c h a r -a c t e r i s t i c r e d u c t i o n i n abdominal macrochaetes t h a t i s a s s o c i -a t e d w i t h the v a r i e g a t i o n o f the s c u t e gene ( R a f f e l and M u l l e r , 1 9 4 - 0 ) . T h i s f e a t u r e has been p a r t i c u l a r l y u s e f u l i n t h e s e s t u d i e s as h e t e r o z y g o t e s f o r a l l o f the d i f f e r e n t s c u t e i n v e r -s i o n s can be i d e n t i f i e d on the b a s i s o f the abdominal b r i s t l e t r a i t s when o t h e r markers f a i l . I n the X/Y c o n d i t i o n , s c u t e - S i males do n a t e x h i b i t a s e v e r a c haete phenotype o r any o f the o t h e r v a r i e g a t i o n f e a t u r e s t h a t were e v i d e n t i n the s c u t e - 8 f l i e s . I n s c u t e - S i X / 0 males, th e most s t r i k i n g p h e n o t y p i c change was t h e enormous r e d u c t i o n o f maorochaetae from a l l e p i t h e l i a l s u r f a c e s . The major b r i s t l e s , when p r e s e n t , were normal i n appearance and d i d n o t appear bobbed. I n most i n d i -v i d u a l s , t h e m i c r o c h a e t a e were s i g n i f i c a n t l y r e d u c e d i n num-. b e r , b u t n o t as s e v e r l y as seen i n s c u t e - 8 . B r i s t l e p a t t e r n was a l t e r e d but remained d i s t i n g u i s h a b l e i n l i n e s w hich was n o t t h e case i n s c u t e - 8 . I t appears t h a t t h e major p h e n o t y p i c e f f e c t o f the s c u t e - S i i n v e r s i o n i s v a r i e g a t i o n o f the s c u t e gene w h i l e the s c u t e - 8 i n v e r s i o n a f f e c t s t h e e x p r e s s i o n o f t h e achaete gene t o a g r e a t e r e x t e n t . D e v e l o p m e n t a l l y , s c u t e - S i X / 0 males a r e e x t r e m e l y slow, e c l o s i n g on t h e average, 2 - 3 days l a t e r t h a n female s i b s . 7 2 . As w i t h s c u t e - 8 , t h e s e v e r i t y o f phenotype i s d i r e c t l y r e l a t e d t o t h e d u r a t i o n o f development. These s t u d i e s are r e p o r t e d i n C h a p t e r 7 . The s c u t e - S i f l i e s were a l s o among the most s e v e r e l y i n c a p a c i t a t e d i m m e d i a t e l y a f t e r e c l o s i o n as o f t e n X / 0 males c o u l d he r e t r i e v e d from t h e f o o d . The f r e q u e n c y o f t h e s e dead X / 0 i n d i v i d u a l s o f t e n approached 1 $ o f the t o t a l progeny and t h i s r e p r e s e n t e d t h e most common o b s e r v a t i o n o f t r a p p e d males. I t appears t h a t X/ 0 males c a r r y i n g a s c u t e - S i X-chromosome a r e a t an extreme s e l e c t i v e d i s a d v a n t a g e f o l l o w -i n g e c l o s i o n . As w i t h s c u t e - 8 , t h e s e weak f l i e s matured and became f u l l y competent a f t e r 1 - 2 days. The t h i r d s c u t e i n v e r s i o n s t u d i e d was s c u t e - L 8 o f which we had t h r e e examples. None o f the s c u t e - L 8 chromosomes c a r r i e d i n t e r s t i t i a l i n v e r s i o n s , t h u s any observed l e t h a l i t y can be a t t r i b u t e d t o marker e f f e c t s o r d i r e c t e f f e c t s o f t h e s c u t e i n v e r s i o n rearrangement. One o f the s c u t e - L 8 chromo-somes c a r r i e d a v e r y severe a l l e l e o f f o r k e d ( f _ _ _ ) which has been p r e v i o u s l y a s s o c i a t e d w i t h reduced v i a b i l i t y and i n f e r t i l i t y . The d i f f i c u l t y i n m a i n t a i n i n g t h i s s t o c k and t h e complete i n v i a b i l i t y o f X / 0 males suggest t h a t the prob-lems are due t o the marker e f f e c t o f t h e f o r k e d a l l e l e . 36a Recombinant chromosomes c a r r y i n g the f.J a l l e l e demonstrated t h e same l e t h a l i t y p r o f i l e . T h i s marker e f f e c t was d i s t i n -g u i s h e d from t h e bb~ e f f e c t by d e m o n s t r a t i n g t h e v i a b i l i t y o f 36a i n d i v i d u a l s c a r r y i n g b o t h the s c u t e - L 8 fJ chromosome and a bb~ homologue. 73-There was a severe r e d u c t i o n i n v i a b i l i t y when t h e s c u t e - L 8 males l a c k e d a Y-chromosome. The l e t h a l i t y observed f o l l o w e d a s i m i l a r p a t t e r n t o t h a t w i t n e s s e d i n the p r e v i o u s e x p e r i m e n t s w i t h s c u t e - S i , a l t h o u g h fewer s u r v i v o r s were r e -c o v e r e d . Moreover, the c o n t r o l males showed a marked reduc-t i o n i n v a r i a b i l i t y even when a s s o c i a t e d w i t h a Y-chromosome. P h e n o t y p i c a l l y , s c u t e - L 8 mimics the mutant c o n d i t i o n s o b s e r v e d i n s c u t e - S i e xcept t h a t t h e e x p r e s s i o n i s more se-v e r e . X/Y males c a r r y i n g s c u t e - L 8 show a s t r o n g s c u t e pheno-t y p e t h a t i s m a n i f e s t e d by s e v e r e r e d u c t i o n o f macrochaetae e x p r e s s i o n e v i d e n c e d e s p e c i a l l y by the s c u t e l l a r b r i s t l e s (see C h a p t e r 8 ) . I n an X/0 i n d i v i d u a l , t h e macrochaetae are a l m o s t a n n i h i l a t e d . Only r a r e l y are t h e l a r g e b r i s t l e s ob-s e r v e d on t h e head and t h o r a x . The d o r s a l s u r f a c e o f the abdomen i s d e v o i d o f macrochaetae and i s l a r g e l y b a l d except f o r s p a r g e , d i s o r g a n i z e d m i c r o c h a e t a e . The v e n t r a l s u r f a c e e x h i b i t s a s i m i l a r but more s e v e r e phenotype. T h i s phenotype i s t h e s i n g l e , most c h a r a c t e r i s t i c f e a t u r e o f a l l t h e s c u t e - L 8 s t o c k s s t u d i e d . The m i c r o c h a e t a e i n X/X and X/Y s c u t e - L 8 f l i e s a r e e ssen-t i a l l y n o r m a l e x c e p t f o r a s i g n i f i c a n t r e d u c t i o n on b o t h ab-d o m i n a l s u r f a c e s . The X/0 males show v a r i e g a t i o n f o r the achaete l o c u s but t o a l e s s e r e x t e n t t h a n e i t h e r s c u t e - 8 or s c u t e - S i . I n many c a s e s the m i c r o c h a e t a e c o u n t s o v e r l a p p e d t h e w i l d t y pe c o n d i t i o n w h i c h was r a r e l y seen i n s c u t e - S i and n o t seen i n s c u t e - 8 (see C h a p t e r 8 ) . 74. S c u t e - L 8 demonstrated t h e same d e v e l o p m e n t a l p r o f i l e " a s o b s e r v e d i n t h e p r e v i o u s s t u d i e s . The m a j o r i t y o f males are r e c o v e r e d f a r l a t e r t h a n t h e s i b f e m a l e s . A g a i n , t h o s e f l i e s e c l o s i n g a f t e r an extended p e r i o d showed th e most extreme phenotypes. O c c a s i o n a l l y , X / 0 males were found t r a p p e d i n th e f o o d but n o t as f r e q u e n t l y as i n the s c u t e - S i e x p e r i m e n t s . T h i s may be a r e s u l t o f the e a r l i e r l e t h a l i t y o bserved i n s c u t e - L 8 as much fewer s c u t e - L 8 l a r v a e r e a c h p u p a t i o n ( i n p r e p a r a t i o n ) . I t i s a pparent t h a t s c u t e - L 8 i s t h e most extreme o f the s c u t e v a r i e g a t i n g mutants. The s t r i k i n g absence o f macro-chaetae on t h e e p i d e r m a l s u r f a c e s i s a c h a r a c t e r i s t i c f e a t u r e o f t h i s chromosome. A l t h o u g h s l i g h t l y l e s s v i a b l e t h a n i t s o t h e r s c u t e c o u n t e r p a r t s , s c u t e - L 8 e x h i b i t s t h e complement o f d e v e l o p m e n t a l phenomena t h a t c h a r a c t e r i z e the s c u t e i n v e r -s i o n e f f e c t . The f o u r t h chromosome s t u d i e d was s c u t e - V 2 , which i s a p e c u l i a r chromosome as i t has a p r o x i m a l break w i t h i n t h e n u c l e o l u s o r g a n i z e r r e g i o n as suggested by g e n e t i c e v i d e n c e r e p o r t e d i n C h a p t e r 3- The e u c h r o m a t i c break i s v e r y s i m i l a r t o t h a t r e p o r t e d f o r the s c u t e - 8 i n v e r s i o n w i t h the s c u t e gene c o n t a i n e d w i t h i n the rearrangement. The scute-V2 i n v e r -s i o n appears t o be v e r y s i m i l a r t o the p r e v i o u s t h r e e mutants w i t h one major e x c e p t i o n . Recent s t u d i e s i n d i c a t e t h a t b o t h ends o f t h e scute-V2 chromosome c o n t a i n s u f f i c i e n t r i b o s o m a l DNA t o s u r v i v e i n d e p e n d e n t l y . T h i s s u b j e c t w i l l be d e a l t 75. w i t h i n some l e n g t h i n the. d i s c u s s i o n . I n t h e X/0 s i t u a t i o n , t h e v i a b i l i t y o f scute-V2 i s g r e a t l y r e d u c e d . T h i s l e t h a l i t y i s n o t as severe as o b s e r v e d w i t h t h e t h r e e p r e v i o u s s c u t e i n v e r s i o n s and t h e d e v e l o p m e n t a l phenomena a s s o c i a t e d w i t h t h e m o r t a l i t y do not appear t o be as extreme. A d e v e l o p m e n t a l d e l a y i s observed i n the matur-a t i o n o f the scute-V2 X/0 males but t h i s p e r i o d i s much r e -duced as compared t o the same p e r i o d i n s c u t e - S i o r s c u t e - L 8 . There i s no s t a t i s t i c a l d i f f e r e n c e between th e phenotypes o f X/0 f l i e s e c l o s i n g e a r l y i n t h e experiment and t h o s e e c l o s i n g l a t e . Moreover, scute-V2 X/0 males appear t o be v e r y compet-i t i v e w i t h t h e i r female s i b s and are r a r e l y found t r a p p e d i n t h e f o o d . These f a c t o r s a l l suggest t h a t the scute-V2 i n v e r -s i o n i s somewhat d i f f e r e n t t h a n t h e t h r e e p r e v i o u s l y s t u d i e d r e a r r a n g e m e n t s . P h e n o t y p i c a l l y , scute-V2 most c l o s e l y r e sembles s c u t e - 8 as t h e most p r e v a l e n t mutant c o n d i t i o n v i s i b l e i s a c h a e t e . X/Y males c a r r y i n g scute-V2 show o b v i o u s p a t t e r n s o f v a r i e g a -t i o n a s s o c i a t e d w i t h t h e m i c r o c h a e t a e . The s c u t e l l a r b r i s t l e s and o t h e r macrochaetae r e m a i n r e l a t i v e l y u n a f f e c t e d a l t h o u g h a c c e s s o r y b r i s t l e s were n o t an uncommon o c c u r r e n c e on the pos-t e r i o r t h o r a x . The most s t r i k i n g v a r i e g a t i o n p a t t e r n i n the X/Y i n d i v i d u a l s was e x h i b i t e d i n the abdominal b r i s t l e a r r a n g e -ment. There were s t a n d a r d numbers o f macrochaetae p r e s e n t on b o t h d o r s a l and v e n t r a l s u r f a c e s but t h e m i c r o c h a e t a e were g r e a t l y r e d u c e d . T h i s phenotype was v e r y c h a r a c t e r i s t i c i n 76. a l l o f t h e s t u d i e s i n v o l v i n g the scute-V2 chromosome. The l a s t o f t h e s t a n d a r d s c u t e i n v e r s i o n chromosomes t o be s t u d i e d was s c u t e - 4 . I n comparison w i t h the p r e v i o u s f o u r i n v e r s i o n s , s c u t e - 4 i s a t y p i c a l i n t h a t i t does not i n c l u d e t h e n u c l e o l u s o r g a n i z e r r e g i o n . The e u c h r o m a t i c b r e a k p o i n t has been r e p o r t e d t o be v e r y s i m i l a r t o t h a t seen i n b o t h s c u t e - S i and s c u t e - L 8 . The h e t e r o c h r o m a t i c b r e a k p o i n t , how-e v e r , i s . s i t u a t e d i n the d i s t a l - X h e t e r o c h r o m a t i n r a t h e r t h a n t h e p r o x i m a l r e g i o n as i n t h e o t h e r s c u t e i n v e r s i o n s . The s c u t e - 4 male c a r r y i n g a Y-chromosome appears t o be f u l l y c o m p e t i t i v e w i t h h i s female s i b s , w hich i s c o n t r a d i c t o r y t o t h e o b s e r v a t i o n s from t h e o t h e r comparable i n v e r s i o n s . I n t h e X/0 c o n d i t i o n , s c u t e - 4 males appear t o be o f s i m i l a r v i a b i l i t y as t h e i r s i b f e m a l e s or c o n t r o l males w i t h Y-chromo-somes. There i s no d e v e l o p m e n t a l d e l a y o b s e r v e d f o r the s c u t e - 4 / 0 males and t h e y were r a r e l y , i f e v e r found m i r e d i n . t h e f o o d . P h e n o t y p i c a l l y , s c u t e - 4 o b v i o u s l y b e l o n g s w i t h the s c u t e - S i , s c u t e - L 8 group. The major v a r i e g a t i n g system ap-p e a r s t o be t h e s c u t e l o c u s as t h e macrochaetae are most f r e -q u e n t l y mutant. I n the X/Y s i t u a t i o n , s c u t e - 4 males g r e a t l y o v e r l a p w i l d t y p e w i t h r e s p e c t t o t h e i r macrochaetae pheno-t y p e . F o r example, 60% o f a l l normal s c u t e - 4 males produce 4 s c u t e l l a r b r i s t l e s . T h i s i s a much l e s s severe s c u t e pheno-t y p e t h a n t h a t seen i n b o t h s c u t e - S i and s c u t e - L 8 . The m i c r o c h a e t a e are l a r g e l y u n a f f e c t e d by t h e s c u t e - 4 77-i n v e r s i o n . T y p i c a l l y , the t h o r a c i c m i c r o c h a e t a e count approx-i m a t e s t h a t o b s e r v e d i n Oregon R c o n t r o l s t o c k s a l t h o u g h p a t -t e r n d i s t r i b u t i o n i n t h e scute-4 i n d i v i d u a l s i s much more i r r e g u l a r . There are d i s t i n c t d i f f e r e n c e s e v i d e n t on the abdominal s u r f a c e s , however. I n scute-4, t h e r e are s i g n i f i -c a n t r e d u c t i o n s i n b o t h macrochaetae and m i c r o c h a e t a e on b o t h s u r f a c e s and t h i s i s p a r t i c u l a r l y e v i d e n t i n the X/0 c o n d i -t i o n . The scute-4- i n v e r s i o n i s c h a r a c t e r i z e d as p e c u l i a r t o t h i s s t u d y as i t does not e x h i b i t a l e t h a l i t y i n the X/0 s t a t e w h i l e showing an o b v i o u s s c u t e phenotype. I t i s not a s s o c i a t e d -with a d e v e l o p m e n t a l d e l a y and appears t o be d i v o r c e d from an e f f e c t on t h e achaete gene. The scute-4 mutant shows the l e a s t o v e r a l l p h e n o t y p i c change as a r e s u l t o f i t s rearrangement and o v e r l a p s t h e w i l d t ype c o n t r o l s i n b o t h X/Y and X/0 c o n d i t i o n s . The w i l d t y p e c o n t r o l f o r t h e s e s t u d i e s was an Oregon R s t o c k m a i n t a i n e d homozygous w i t h a s t a n d a r d Y-chromosome. C o n t r o l s were r u n on i d e n t i c a l f o o d and s i m u l t a n e o u s l y w i t h t h e e x p e r i m e n t a l c r o s s e s . The Oregon R s t r a i n d i d n o t show a m a l e - s p e c i f i c d e v e l o p m e n t a l d e l a y and b r i s t l e p a t t e r n s were n o t a l t e r e d i n the X/0 c o n d i t i o n . I n a l l e x p e r i m e n t s , X/0 Oregon R males appeared t o be c o m p e t i t i v e w i t h female s i b s and t h e y were e q u a l l y as v i a b l e . 78. P a r e n t a l Source E f f e c t : I I . M a t e r n a l l y D e r i v e d X The second method o f p r o d u c i n g X/0 males i n v o l v e s m a t i n g f e m a l e s c a r r y i n g s c u t e chromosomes t o a t t a c h e d X-Y males w h i c h l a c k f r e e Y-chromosomes (see F i g u r e 3). The c o n t r o l s i t u a t i o n i n v o l v e s X•Y males t h a t c a r r y a f r e e Y-chromosome. The p u r -pose o f t h i s e x e r c i s e was t o d e t e r m i n e i f t h e d e v e l o p m e n t a l a n o m a l i e s a s s o c i a t e d w i t h X/0 s c u t e i n v e r s i o n males were sub-j e c t t o p a r e n t a l m o d i f i c a t i o n . The r e s u l t s o f the m a t e r n a l e f f e c t e x p e r i m e n t s a r e p r o -v i d e d i n T a b l e 3 and v i a b i l i t y comparisons o f p a t e r n a l l y and m a t e r n a l l y d e r i v e d X/0 males are p r e s e n t e d i n F i g u r e 4. I t i s i m m e d i a t e l y apparent t h a t t h e v i a b i l i t y o f t h e s c u t e X/0 i n d i v i d u a l i s c o n d i t i o n a l and appears t o be dependent upon t h e p a r e n t a l source o f t h e X-chromosome. F o r a l l but one o f t h e scute, i n v e r s i o n s s t u d i e d i n t h e s e e x p e r i m e n t s , v i a b i l i t y was enhanced when the i n v e r t e d X-chromosome was i n h e r i t e d from the f e m a l e . I n most s t r a i n s , t h i s d i f f e r e n c e was between f i v e and t e n - f o l d with- t h e s c u t e - 4 i n v e r s i o n b e i n g t h e o n l y e x c e p t i o n . The m a t e r n a l e f f e c t a s s o c i a t e d w i t h t h e s c u t e - 8 chromo-somes was s t r i k i n g . The d i f f e r e n c e between m a t e r n a l and pa-t e r n a l i n h e r i t a n c e was t e n - f o l d and o f t e n h i g h e r . O f t e n , t h e m a t e r n a l l y d e r i v e d X/0 males were comparably v i a b l e t o the f e -male progeny r e c o v e r e d from the same c r o s s . Almost a l l s t r a i n s showed t h i s m a t e r n a l l y d e t e r m i n e d i n c r e a s e . T h i s i s i n marked 7 9 . TABLE 3 The S c u t e M a t e r n a l E f f e c t X / X sc / s c x X-Y/0 CONTROL EXPERIMENT Female Male Sex R a t i o Female Male Sex R a t i ) 8 ^ sc + 1017 1239 1.22 1344 1572 1.17 s c 8 + dl49 1637 2191 1 .34 1577 2019 1.28 8 sc c v V f - - - - - -8 sc s u - f 938 991 1.06 1173 1289 1.10 SI sc ym 710 717 1 .01 1943 894 0 . 4 6 s c S 1 +S yB 939 912 0.97 4 7 1 6 1982 0 . 4 2 s c S 1 +S B 658 688 1 .05 1 4 2 6 528. 0.37 s c S 1 + dl49 - - - - - -SI 61 k sc y - - - - - -L8 3 sc sn 367 339 0.92 780 226 0.29 L8 sc m 4 1 2 360 0.87 607 201 0 . 3 3 g c L 8 f 3 6 a - - - - - -s c V 2 1213 1456 1 .20 1302 1536 1.18 4 sc 771 817 1.06 1021 1009 0 . 9 9 + 587 616 1.05 616 659 1.07 80. c o n t r a s t t o t h e p r e v i o u s e x p e r i m e n t s where X/0 males r e p r e -s e n t e d a low v i a b i l i t y c l a s s . P h e n o t y p i c a l l y , s c u t e - 8 f e m a l e s a r e l e s s mutant t h a n s c u t e - 8 males w i t h r e s p e c t t o macrochaetae e x p r e s s i o n . These f e m a l e s p r e d o m i n a n t l y o v e r l a p w i l d t y p e and o n l y r a r e l y are major b r i s t l e s m i s s i n g . Q u i t e t o t h e c o n t r a r y , s c u t e - 8 f e -males show many more a c c e s s o r y b r i s t l e s on the p o s t e r i o r t h o r a x o r s c u t e l l u m . The m i c r o c h a e t a e e x p r e s s i o n i n s c u t e - 8 f e m a l e s i s somewhat more c o n s i s t e n t t h a n t h a t seen i n the males. P a t t e r n d i s r u p t i o n i s f r e q u e n t but b r i s t l e numbers o v e r l a p w i l d t y p e i n the v a s t m a j o r i t y o f i n d i v i d u a l s . The phenotypes o f the X/0 males whose s c u t e X has been i n h e r i t e d from t h e female a r e l a r g e l y the same as those ob-s e r v e d i n t h e p a t e r n a l e x p e r i m e n t s . There i s one major excep-t i o n however, as t h e v e r y s e v e r e l y mutant i n d i v i d u a l s a r e n e v e r o b s e r v e d i n t h e m a t e r n a l e x p e r i m e n t s . Thus, a p o p u l a -t i o n o f X/0 males from t h e m a t e r n a l c r o s s e s has a comparable p h e n o t y p i c p r o f i l e t o the g e n o t y p i c a l l y i d e n t i c a l i n d i v i d u a l s r e c o v e r e d from t h e p a t e r n a l c r o s s e s p r o v i d e d one n e g l e c t s the f l i e s w i t h a v e r y extended d e v e l o p m e n t a l p e r i o d . U n f o r t u n a t e l y , t o i g n o r e t h i s s i g n i f i c a n t group would s e v e r e l y b i a s the s t u d y . The most o b v i o u s p h e n o t y p i c f e a t u r e o f t h e s e males was t h e achaete p a t t e r n on b o t h t h e t h o r a x and abdomen. The d e v e l o p -m e n t a l d e l a y was g r e a t l y r e d u c e d from t h a t observed i n t h e p a t e r n a l e x p e r i m e n t s . The X/0 males produced i n t h e s e mater-n a l e f f e c t s t u d i e s appear t o be v e r y s t r o n g a t e c l o s i o n and 81. F I G U R E 4 X/0 v i a b i l i t y a s s o c i a t e d w i t h d i f f e r e n t  p a r e n t a l i n h e r i t a n c e o f t h e s c u t e X-chromosomes. P a t e r n a l l y d e r i v e d X - u n l i n e d M a t e r n a l l y d e r i v e d X - l i n e d 8 2 . Sex ratio 8 , sc + 8 sc + dl 49 sc su - f S1 sc y m sc S 1yB+S s c S 1 B+S L8 3 sc sn s c L 8 m sc sc _ V 2 10 20 30 40 50 60 70 80 90 100 110 120 130 — i 1 1 1 1 1 l I I i i i i 117 12 2: 128 / / / / / / / I 13 z 110 /////////////S71T7\ z 46 42 W2 37 /////// o 2.9 33 / / / / / / A 26 118 /////////////////////7-7-A 87 ^ ^ j ^ ^99 97 107 83-were n e v e r found t r a p p e d i n the f o o d . I t i s e v i d e n t t h a t t h e p a r e n t a l s o u r c e o f t h e mutant chromosome has a p r o f o u n d e f f e c t on a l l o f t h e d e v e l o p m e n t a l phenomena a s s o c i a t e d w i t h th e scute-8 chromosome. The second group o f m a t e r n a l e f f e c t s s t u d i e d i n v o l v e d th e s c u t e - S i chromosome. O r i g i n a l l i t e r a t u r e has c i t e d t h e " f a c t " t h a t s c u t e - S i f e m a l e s a r e v i a b l e hut i n f e r t i l e . These e x p e r i m e n t s show t h a t t h i s i n f e r t i l i t y i s not a permanent f e a t u r e o f a l l s c u t e - S i f e m a l e s and may he c i r c u m v e n t e d . The d i s c o v e r y o f t h e c o n d i t i o n a l f e r t i l i t y was f o r t u i t o u s as numerous n o n d i s j u n c t i o n a l s c u t e - S i f e m a l e s were r e c o v e r e d i n one o f t h e p a t e r n a l e f f e c t e x p e r i m e n t s . Some o f t h e s e f e m a l e s were i m m e d i a t e l y mated t o s t a n d a r d males t o determine f e r t i l i t y w h i l e t h e b a l a n c e o f t h e s e f e m a l e s remained i n an i n v e r t e d v i a l . Three days l a t e r , t h e m a j o r i t y o f t h e mated f e m a l e s had d i e d , s t r a n d e d i n t h e f o o d . The r e m a i n i n g f e m a l e s appeared v e r y s t r o n g and were t h e n mated t o s t a n d a r d males. More t h a n 50% o f t h e s e f e m a l e s produced progeny and appeared t o be t y p i c a l o f o t h e r homozygous s c u t e f e m a l e s t h a t had been s t u d i e d . A p p a r e n t l y , the i n f e r t i l i t y o f s c u t e - S i f e m a l e s can be l a r g e l y a v o i d e d i f t h e f e m a l e s are a l l o w e d t o mature i n a d r y environment f o r a few days. T h i s s e e m i n g l y a l l o w s th e f l y t o " c a t c h up" d e v e l o p m e n t a l l y as w i t n e s s e d by t h e change i n motor a b i l i t y between e c l o s i o n and day 3> P h e n o t y p i c a l l y , s c u t e - S i f e m a l e s are v e r y much l i k e s c u t e - S i males. The macrochaetae p a t t e r n s are i n c o n s i s t e n t 8 4 . and many b r i s t l e s a re m i s s i n g a l t o g e t h e r . T h i s phenotype i s o b s e r v e d on a l l e p i d e r m a l s u r f a c e s . The m i c r o c h a e t a e c o u n t s are t y p i c a l l y subnormal b u t n o t d r a s t i c a l l y a l t e r e d i n p a t -t e r n . D e v e l o p m e n t a l l y , homozygous s c u t e - S i f e m a l e s do n o t show a d e l a y as compared t o w i l d t y p e s i b s e c l o s i n g i n the same e x p e r i m e n t . The m a t e r n a l e f f e c t o b s e r v e d w i t h s c u t e - S i i s o f compar-a b l e magnitude t o t h a t seen i n s c u t e - 8 . The X /0 i n d i v i d u a l s r e c e i v i n g the i n v e r t e d X from t h e i r mother were e i g h t t i m e s as v i a b l e as t h e i r p a t e r n a l l y d e r i v e d c o u n t e r p a r t s (see F i g u r e 4 ) . - The s c u t e - S i i n v e r s i o n s c a r r y i n g I n ( l ) d l 4 9 as an i n t e r s t i t i a l i n v e r s i o n cannot be r e c o v e r e d i n homozygous f e m a l e s as t h e y appear t o be bobbed d e f i c i e n c i e s . The mutant appearance o f t h e s c u t e - S l / O males i n t h i s e x p eriment i s v e r y s i m i l a r t o t h a t o b s e r v e d i n the p a t e r n a l e f f e c t e x p e r i m e n t s . The s c u t e l l a r b r i s t l e s and o t h e r macro-chaetae a r e p r o f o u n d l y a f f e c t e d w h i l e t h e b r i s t l e s o f m i c r o -c h a e t a e d e r i v a t i o n show some i n c r e a s e i n v a r i e g a t i o n . A g a i n w i t h s c u t e - S i , t h e r e a r e no s e v e r e l y mutant i n d i v i d u a l s o b s e r v e d t o e c l o s e v e r y l a t e i n t h e m a t e r n a l e x p e r i m e n t s . T h i s produces a p o p u l a t i o n w i t h much l e s s v a r i a t i o n o f b r i s t l e number t h a n i s observed i n t h e p a t e r n a l e x p e r i m e n t s . D e v e l -o p m e n t a l l y , t h e s c u t e - S l / O males a r e s t i l l somewhat d e l a y e d b u t d e v e l o p m e n t a l time i s f a r s h o r t e r t h a n w i t h the p a t e r n a l l y d e r i v e d X /0 males. These e x p e r i m e n t s i n d i c a t e t h a t p r e v i o u s s u g g e s t i o n s o f 85. s c u t e - S i female i n f e r t i l i t y a r o s e as a r e s u l t o f the mat i n g s o f immature f e m a l e s . C r o s s e s i n v o l v i n g f u l l y matured. s c u t e - S i f e m a l e s i n d i c a t e t h a t s c u t e - S i i s n o t a t y p i c a l o f the s c u t e i n v e r s i o n s w i t h r e s p e c t t o the m a t e r n a l e f f e c t and d e v e l o p -m e n t a l phenomena. The s c u t e - L 8 chromosome p r e s e n t e d a problem s i m i l a r t o t h a t e n countered w i t h s c u t e - S i . I t was found t h a t t h e homo-zygous s c u t e - L 8 f e m a l e s were somewhat i n v i a b l e and f r e q u e n t l y 36a s t e r i l e . The i n v e r s i o n c o n t a i n i n g fJ c o u l d n ot be r e c o v -e r e d i n a homozygous s c u t e - L 8 female presumably as a r e s u l t o f t h e s e v e r e e f f e c t o f t h i s f o r k e d a l l e l e . The o t h e r two s c u t e - L 8 chromosomes were homozygous v i a b l e but female v i a b i l -i t y was 39% o f the c o n t r o l ( i n p r e p a r a t i o n ) . Of t h e s e homo-zygous s c u t e - L 8 f l i e s , o n l y 1 i n 5 were f e r t i l e even a f t e r m a t u r i n g f o r extended l e n g t h s o f t i m e . T h i s r e d u c t i o n i n female v i t a l i t y made i t i m p o s s i b l e t o m a i n t a i n a homozygous s t o c k . I t i s apparent t h a t t h e s c u t e - L 8 rearrangement has t h e most profound, e f f e c t on v i a b i l i t y and f e r t i l i t y . The s c u t e - L 8 f e m a l e s were p h e n o t y p i c a l l y s i m i l a r t o the s c u t e - L 8 males w i t h Y-chromosomes. Macrochaetae were d i s -t u r b e d on t h e e n t i r e e p i d e r m i s o f the f l y but most n o t i c e a b l y on the s u r f a c e s o f the abdomen. M i c r o c h a e t a e numbers o v e r -l a p p e d w i l d t y p e but have subnormal a v e r a g e s . These fe m a l e s were a l s o d e v e l o p m e n t a l l y d e l a y e d when compared to w i l d t y p e s i b s . When f e r t i l e , t he s c u t e - L 8 f e m a l e tended t o l a y numerous 86. eggs, most o f w h i c h h a t c h e d . The X/0 males d e r i v e d from t h e s e c r o s s e s c o u l d n o t be d i s t i n g u i s h e d from s i m i l a r males r e c o v e r e d from t h e p a t e r n a l e x p e r i m e n t s . These f l i e s were a l m o s t denuded o f macrochaetae by t h e i n c r e a s e d v a r i e g a t i o n o f t h e s c u t e gene. O f t e n , l e s s t h a n 6 macrochaetae were v i s -i b l e on t h e head and t h o r a x and none were found on t h e abdomen. The m i c r o c h a e t a e were d i s o r g a n i z e d and reduced i n number on t h e t h o r a x and d r a m a t i c a l l y r e d u c e d on the abdomen. A l t h o u g h e x h i b i t i n g a more s e v e r e phenotype i n most r e s p e c t s , s c u t e - L 8 appears t o m a n i f e s t a l l o f t h e phenomena t h u s f a r a s s o c i a t e d w i t h t h e o t h e r s c u t e i n v e r s i o n s . The n e x t chromosome examined was s c u t e - V 2 , w h i c h was n o t a s s o c i a t e d w i t h any problems o f female i n f e r t i l i t y . Homozygous f e m a l e s e x p r e s s e d a phenotype t h a t was a l m o s t i d e n -t i c a l t o t h a t e x p r e s s e d by hemizygous scute-V2 males. The most p r o f o u n d l y a f f e c t e d l o c u s appears t o be achaete as ab-d o m i n a l and t h o r a c i c m i c r o c h a e t a e a r e g r e a t l y r e d u c e d i n b o t h s e x e s . A l l o f t h e macrochaetae a r e t y p i c a l l y p r e s e n t i n a scute-V2 f e m a l e and a c c e s s o r y b r i s t l e s a r e common on the s c u t e l l u m and p o s t e r i o r t h o r a x . T h i s p a r a l l e l s v e r y c l o s e l y t h e phenotype r e p o r t e d above f o r s c u t e - 8 . M a t e r n a l p r o d u c t i o n o f scute-V2/0 males r e s u l t s i n a d r a m a t i c r e d u c t i o n i n l e t h a l i t y and i s e x p r e s s e d as a f o u r -f o l d i n c r e a s e i n the r e c o v e r y o f t h e s e males. P h e n o t y p i c a l l y , t h e y e x h i b i t s c u t e v a r i e g a t i o n and s t r o n g achaete v a r i e g a t i o n t h a t i s p a r t i c u l a r l y e v i d e n t on t h e s u r f a c e s o f the abdomen. 87. D e v e l o p r a e n t a l l y , t h e s e X/0 males a r e n o t s i g n i f i c a n t l y r e t a r d e d and t h r i v e i n normal c u l t u r e s . The scute - V 2 r e s u l t s i n d i c a t e t h a t m a n i f e s t a t i o n s o f t h e rearrangement do n o t a f f e c t v i a b i l -i t y t o t h e same e x t e n t as t h a t seen w i t h t h e o t h e r i n v e r s i o n s . As a r e s u l t o f t h i s moderate b a s e l i n e X/0 v i a b i l i t y , t he mater-n a l e f f e c t does n o t appear t o be as p r o f o u n d as i n p r e v i o u s e x p e r i m e n t s . I t i s e v i d e n t t h a t t h e scute - V 2 chromosome r e p r e -s e n t s a moderate example o f the s c u t e i n v e r s i o n phenomenon. The f i n a l i n v e r s i o n s t u d i e d f o r e v i d e n c e o f m a t e r n a l e f f e c t s was s c u t e - 4 . Homozygous s t o c k s c o n t a i n i n g t h i s c h r o -mosome t h r i v e and t h e r e i s l i t t l e l e t h a l i t y a s s o c i a t e d w i t h t h e p a t e r n a l l y d e r i v e d X/0 c o n d i t i o n . P h e n o t y p i c a l l y , a s c u t e - 4 female i s t h e l e a s t extreme example o f the s c u t e - L 8 , s c u t e - S i , scute-4- group, a l l o f whom e x h i b i t v a r y i n g degrees o f s c u t e v a r i e g a t i o n . The r e s u l t s o f t h i s s t u d y i n d i c a t e t h a t the r e c o v e r y o f m a t e r n a l l y d e r i v e d scute-4/0 males i s a p p r o x i m a t e l y t h e same as t h e f r e q u e n c y o b s e r v e d i n t h e p a t e r -n a l s t u d i e s . P h e n o t y p i c a l l y , t h e X / 0 males d e r i v e d from b o t h s o u r c e s were i d e n t i c a l t h r o u g h o u t the p o p u l a t i o n s examined i n t h i s s t u d y and t h e r e were no males o b s e r v e d t o e c l o s e a f t e r an * extended d e v e l o p m e n t a l p e r i o d . A l t h o u g h s c u t e - 4 e x h i b i t s a s c u t e phenotype and appears t o be o f a s i m i l a r o r i g i n , t h i s chromosome i s a t y p i c a l o f t h e chromosomes p r e v i o u s l y s t u d i e d . T h i s anomaly w i l l be d i s c u s s e d i n g r e a t e r d e t a i l l a t e r i n the t e x t . 88. - C h a p t e r 2 -X/0 V i a b i l i t y o f Recombinant Scute I n v e r s i o n s F o r many y e a r s , D r o s o p h i l a r e s e a r c h e r s have been u s i n g recombinant s c u t e chromosomes, wh i c h i n c l u d e an i n t e r s t i t i a l i n v e r s i o n , as e f f e c t i v e X-chromosome b a l a n c e r s . There e x i s t two groups t h a t can i n t e r c o n v e r t t h e i r l e f t and r i g h t chromo-some ends w i t h o u t p r o d u c i n g a n e u p l o i d y f o r e u c h r o m a t i c genes. As s c u t e - 4 , s c u t e - S i and s c u t e - L 8 have a v e r y s i m i l a r euchro-m a t i c b reak, v i a b l e r e c o m b i n a n t s c a n be produced e x h i b i t i n g any c o m b i n a t i o n o f ends. The i n v e r s i o n s s c u t e - 8 and scute-V2 comprise the o t h e r group and exchange between t h e s e i n v e r s i o n s w i l l produce two d i f f e r e n t , v i a b l e r e c o m b i n a n t s . From the p r e v i o u s s t u d i e s we have e s t a b l i s h e d t h a t t h e p a r t i c u l a r c h r o -mosomes have s p e c i f i c d e v e l o p m e n t a l p r o f i l e s t h a t are depen-dent upon t h e i r chromosomal make-up. T h i s i n v e s t i g a t i o n o f t h e v a r i o u s r e c o m b i n a n t s p r o v i d e s a f u r t h e r d i s s e c t i o n o f t h e mechanisms c o n t r i b u t i n g t o the m a t e r n a l e f f e c t phenomenon. T a b l e 4 and F i g u r e 5 p r e s e n t r e s u l t s c o n c e r n i n g t h e v i a -b i l i t y o f X/0 i n d i v i d u a l s i n h e r i t i n g t h e s e r e c o m b i n a n t , s c u t e i n v e r s i o n chromosomes e i t h e r from t h e female o r from t h e male p a r e n t . O r i g i n a l l y , t h e phenomenon was d i s c o v e r e d as an i n -c i d e n t a l f i n d i n g i n s t u d i e s on compound autosomes (Ha r g e r , 1974) . A t t h a t t i m e , H a r g e r o b s e r v e d t h a t t h e s c u t e i n v e r -s i o n , b a l a n c e r chromosome, d e s i g n a t e d Base, was f a r more v i a b l e i n the X/0 c o n d i t i o n when the s c u t e X-chromosome was TABLE 4 X/0 Male V i a b i l i t y i n Scute X-chromosome Recombinants P a t e r n a l E f f e c t M a t e r n a l E f f e c t CONTROL EXPERIMENT CONTROL EXPERIMENT Sex R a t i o N Sex R a t i o N Sex R a t i o N Sex R a t i o N S1L 8R : SC asc 1.02 1484 0.17 2053 1.06 1279 0.87 3141 Base 0.93 1777 0.11 3266 1.04- 993 0 .95 4107 B i n s c O.96 977 0.10 2316 O.98 11.14- 0.88 2492 I n s c 1.05 828 0.14 1962 1.07 1273 0.93 2119 v i n c y 1.13 783 0.18 3856 1 .15 1535 1.11 3401 S1L L8R sc 1.07 815 0.47 2182 1.04- 1295 O.96 1844 S1L V2R sc 1.17 887 O.76 2589 1 .12 1280 1 .05 1328 S1L 4R sc 0.93 516 0.68 1071 1 .08 713 1.13 952 TABLE 4 C o n t i n u e d L8L SIR sc sc L8L 8R sc sc L8L V2R sc sc L8L 4R sc sc 4L SIR sc sc 4L L8R sc sc 4L 8R sc sc 4L V2R sc . sc 8L V2R sc sc V2L 8R sc sc CONTROL EXPERIMENT Sex Sex R a t i o N R a t i o N 0 . 8 6 792 0.04 1463 0.98 1288 0 . 0 6 2801 1.16 1313 0.81 1314 0 . 9 3 877 0.51 1103 O .92 1106 0 . 0 0 938 1.16 938 0 . 0 0 711 1.10 1679 0 . 0 0 4360 0 . 8 6 538 0 . 4 4 1712 0 . 9 8 1716 0.11 2316 1.17 1286 0 . 2 0 1977 CONTROL EXPERIMENT Sex R a t i o N Sex R a t i o N 0 . 9 3 729 O .39 1004-0 . 9 7 1205 0 . 9 2 2663 1 .26 953 1 . 0 3 1668 1.11 820 1 . 0 5 1381 1.08 6 9 9 0.81 1017 O . 9 3 1514 0 . 8 3 2119 1.14- 1 0 3 0 1 . 0 6 1475 91. FIGURE 5 X / 0 v i a b i l i t y o f s c u t e recombinant  X-chromosomes o f d i f f e r e n t p a r e n t a l o r i g i n P a t e r n a l l y d e r i v e d X - u n l i n e d M a t e r n a l l y d e r i v e d X - l i n e d 92. S E X R A T I O 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0 1 1 0 1 2 0 1 3 0 11 B a s e 9 5 1 8 v i n s c y i n / / / / / / / / / / / / 4 7 S I L L8R —j 7 r / / / / / / / / / / / / / / 9 6 3 9 L 8 L SIR ZL 4 4 8 1 4 L V2R / / . / / / / / / / / ~ 7 11 8 3 8 L V2R / / / / / / 2 0 1 0 6 V 2 L 8 R / / / / / / / / / / A 93-i n h e r i t e d from a homozygous Base f e m a l e . I n t h e s e s t u d i e s , H a r g e r ' s f i n d i n g s have "been reexamined and extended t o i n c l u d e a number o f a d d i t i o n a l b a l a n c e r chromosomes o f t h e c o n s t i t u -t i o n s c u t e - S i l e f t - s c u t e - 8 r i g h t , and h e r e a f t e r t h i s r e a r -rangement w i l l be termed S1L 8R. The r e s u l t s f o r t h i s chromosome i n d i c a t e a c o n s i s t a n t , marked m a t e r n a l e f f e c t t h a t r e p r e s e n t s a s i x t o t e n - f o l d d i f -f e r e n c e between p a t e r n a l d e r i v a t i o n and m a t e r n a l d e r i v a t i o n . I n a l l c a s e s , m a t e r n a l i n h e r i t a n c e o f t h e X-chromosome r e s u l t s i n n e g l i g i b l e l e t h a l i t y o f X/0 males. I n c o n t r a s t , t h e p a t e r -n a l d e r i v a t i o n r e s u l t s i n 80 - 90$ l e t h a l i t y o f t h e s e i n d i v i d -u a l s . P h e n o t y p i c a l l y , S1L 8R/0 males p a r t i a l l y mimic t h e appear-ance o f s c u t e - S i i n d i v i d u a l s . The major phenotype a s s o c i a t e d w i t h t h i s recombinant i n v o l v e s a b e r r a t i o n s o f the macrochaetae such as the s c u t e l l a r b r i s t l e s . I t s h o u l d be mentioned t h a t t h e s c u t e phenotype i s l e s s s e v e r e i n S1L 8R t h a n i n s c u t e - S i . The achaete v a r i e g a t i o n appears t o be s i m i l a r i n b o t h c a s e s . D e v e l o p m e n t a l l y , t h e S1L 8R/0 i n d i v i d u a l s a r e much s l o w e r t o mature t h a n t h e normal progeny p r e s e n t i n the expe r i m e n t . When p a t e r n a l l y d e r i v e d , t h e s e X/0 males o f t e n e c l o s e s e v e r a l days l a t e r and show s e v e r e s c u t e and achaete phenotypes. Ma-t e r n a l l y d e r i v e d , t h e phenotype i s e s s e n t i a l l y t h e same as s c u t e - S l / O males except t h a t t h e macrochaetae are e x p r e s s e d more f r e q u e n t l y . T h i s may be e x p l a i n e d by t h e chromosome con-s t i t u t i o n t h a t o c c u r s i n t h i s r e c o m b i n a n t . Owing t o t h e f a c t 94. t h a t the s c u t e - 8 and. s c u t e - S i e u c h r o m a t i c b r e a k p o i n t s a r e s t a g g e r e d on e i t h e r s i d e o f the s c u t e l o c u s , the S1L 8R recom-b i n a n t c a r r i e s a d u p l i c a t i o n f o r t h e s c u t e gene. The r e c i p r o -c a l recombinant 8L SIR n o r m a l l y cannot be r e c o v e r e d as i t i s d e f i c i e n t f o r t h e s c u t e r e g i o n . The maintenance o f two s c u t e l o c i i n t h e S1L 8R/0 male may r e s u l t i n a r e d u c t i o n o f mutant s c u t e phenotype as t h e two l o c i t o g e t h e r may c o n t r i b u t e enough gene p r o d u c t t o s h i f t t h e phenotype towards w i l d . I n f a c t , a h a i r y wing e f f e c t i s commonly e x p r e s s e d i n S1L 8R homozygotes and h e m i z y g o t e s . The v e r y l a t e e c l o s i n g X/0 males from the p a t e r n a l e x per-i m e n t s showed f e e b l e n e s s and an i n a b i l i t y t o c o n t r o l wing o r l e g f u n c t i o n s , much the same as was seen w i t h p r e v i o u s p a t e r n a l e x p e r i m e n t s . V e r y o f t e n t h e s e f r a i l i n d i v i d u a l s were found dead i n t h e f o o d . I t i s an i n t e r e s t i n g p o i n t t h a t t h e m a j o r i t y o f f l i e s t r a p p e d i n t h e f o o d do not s u r v i v e t o "expand" t h e i r w i ngs; t h e r e f o r e , upon e c l o s i o n , t h e s e f l i e s must be s e v e r e l y d e b i l i t a t e d . These o b s e r v a t i o n s and t h e o t h e r d e v e l o p m e n t a l d a t a s t r o n g l y suggest t h a t the recombinant S1L 8R m a i n t a i n s c h a r a c t e r i s t i c s f r om b o t h o r i g i n a l chromosomes. T h i s i s the f i r s t i n d i c a t i o n t h a t b o t h ends o f t h e recombinant chromosome c o n t r i b u t e s i g n i f i c a n t l y t o t h e p h e n o t y p i c make-up. The r e c o m b i n a n t , S1L L8R, would be e x p e c t e d t o be l a r g e l y l e t h a l i n t h e p a t e r n a l l y d e r i v e d e x p e r i m e n t s and 2 0 - 4 - 0 % v i a b l e i n t h e m a t e r n a l e x p e r i m e n t s . T h i s a p p a r e n t l y i s not th e c a s e . The p a t e r n a l d o n a t i o n o f t h e X-chromosome r e s u l t s 9 5 . i n a k7% v i a b i l i t y o f X/0 males w h i l e m a t e r n a l d o n a t i o n p r o -duces 96%. One would a l s o expect homozygous S1L L8R f e m a l e s t o e x p r e s s r e d u c e d f e r t i l i t y and show poor v i a b i l i t y . However, t h i s was not t h e c a s e . I n f a c t , the homozygous S1L L8R f e m a l e s were e q u a l l y as v i a b l e and as f e r t i l e as t h e c o n t r o l f e m a l e s . T h i s poses an enigma when one c o n s i d e r s t h e p a r e n t a l s t o c k s . P h e n o t y p i c a l l y , t h e s t o c k i s s c u t e - S i i n appearance, a l t h o u g h t h e abdominal b r i s t l e p a t t e r n s a r e somewhat more s e v e r e l y a f f e c t e d as seen i n s c u t e - L 8 . • The X/0 i n d i v i d u a l s ar e m o d e r a t e l y achaete and s t r o n g l y s c u t e . The d e v e l o p m e n t a l d e l a y i s s i g n i f i c a n t l y r e d u c e d when compared t o e i t h e r o f the p a r e n t a l i n v e r s i o n s . The S1L L8R/0 males a r e r a r e l y found i n th e f o o d and a r e not a s s o c i a t e d w i t h extended development. I t seems l i k e l y t h a t t h e d i f f e r e n c e between th e p a r e n t a l chromosomes and t h i s r e combinant r e s i d e s i n s p e c i f i c marker e f f e c t s i n each s t r a i n o r i s a f u n c t i o n o f t h e new h e t e r o c h r o -m a t i c f e a t u r e s g e n e r a t e d i n t h e r e c o m b i n a n t . The r e c o m b i n a n t s S1L V2R and S1L 4R b o t h e x h i b i t moder-a t e l y good v i a b i l i t y when d e r i v e d p a t e r n a l l y and f u l l v i a b i l i t y when d e r i v e d m a t e r n a l l y . The phenotypes a r e p r e d o m i n a n t l y s c u t e - S i i n o r i g i n a l t h o u g h b o t h male and female S1L V2R i n d i v i d u a l s show s t r i k i n g l y mutant abdomens. The d e v e l o p m e n t a l p r o f i l e o f t h e s e i n d i v i d u a l s i s much the same as was observed f o r t h e p r e v i o u s r e c o m b i n a n t , S1L L8R. I t s h o u l d be p o i n t e d out t h a t b o t h S1L V2R and S1L 4R c o n t a i n a c o n s i d e r a b l e dup-l i c a t i o n o f p r o x i m a l X h e t e r o c h r o m a t i n and t h i s may c o n t r i b u t e 96. t o an a l t e r a t i o n o f t h e e x p r e s s i o n o f the v a r i e g a t i n g l o c i . The n e x t group o f recombinant chromosomes i n v o l v e d t h e l e f t p o r t i o n o f s c u t e - L 8 . The chromosome L8L SIR produces a phenotype v e r y s i m i l a r t o s c u t e - L 8 as the macrochaetae are l a r g e l y absent and t h e abdominal r e g i o n s have few b r i s t l e s a t a l l . P a t e r n a l l y , t h i s chromosome i s v e r y l e t h a l and mater-n a l l y t h e v i a b i l i t y r e t u r n s t o o n e - t h i r d o f the n o r m a l . The d e v e l o p m e n t a l d e l a y i s marked i n t h i s s t o c k and i s v e r y sim-i l a r t o t h a t seen i n t h e p a r e n t a l s t o c k s . T h i s v i a b i l i t y p r o f i l e i s somewhat c r y p t i c as the r e c i p r o c a l recombinant S1L L8R has such a d i s t i n c t l y d i f f e r e n t d e v e l o p m e n t a l p r o f i l e . One may suggest t h a t t h e s e r e c o m b i n a n t s have i s o l a t e d the l e t h a l i t y f a c t o r s on t h e L8L SIR chromosome and have g r e a t l y e l i m i n a t e d them from t h e S1L L8R re c o m b i n a n t . F u r t h e r i n v e s -t i g a t i o n i s n e c e s s a r y t o determine t h e n a t u r e o f t h i s anomaly. The L8L 8R chromosome i s e x t r e m e l y l e t h a l when donated p a t e r n a l l y but shows l i t t l e e f f e c t when t r a n s f e r r e d from a f e m a l e . P h e n o t y p i c a l l y , t h e s e X/0 i n d i v i d u a l s e x h i b i t a mod-i f i e d s c u t e - L 8 e x p r e s s i o n o f b r i s t l e s . Abdominal r e g i o n s a r e p r a c t i c a l l y b a l d w h i l e t h e head, t h o r a x and s c u t e l l a r r e g i o n s e x p r e s s many o f t h e normal b r i s t l e s . D e v e l o p m e n t a l l y , the c o n t r a s t between m a t e r n a l e f f e c t and p a t e r n a l e f f e c t i s g r e a t e s t f o r t h i s p a r t i c u l a r chromosome. P a t e r n a l l y d e r i v e d X/0 males are e x t r e m e l y slow d e v e l o p i n g and many e c l o s e l a t e and u s u a l l y d i e i n the f o o d . M a t e r n a l l y d e r i v e d X/0 males a r e s l o w e r i n d e v e l o p i n g t h a n w i l d t y p e , but n o t as d r a s -97-t i c a l l y d e l a y e d as w i t h t h e p a t e r n a l o r i g i n . As w i t h the o t h e r r e c o m b i n a n t s , i t appears t h a t L8L 8R i s f u n d a m e n t a l l y d i f f e r e n t t h a n e i t h e r o f i t s p a r e n t a l chromosomes as suggested by the d e v e l o p m e n t a l d a t a . The r e c o m b i n a n t s , L8L V2R and L8L 4R, e x h i b i t a s c u t e - L 8 phenotype a l t h o u g h much reduced i n t h e L8L V2R i n d i v i d u a l s . These chromosomes are r e a s o n a b l y v i a b l e when donated by a male p a r e n t and a r e f u l l y v i a b l e when c o n t r i b u t e d by the f e -male p a r e n t . As w i t h t h e S1L r e c o m b i n a n t s , b o t h L8L V2R and L8L 4R c a r r y l a r g e h e t e r o c h r o m a t i c d u p l i c a t i o n s t h a t l i k e l y m o d i f y t h e i r p o s i t i o n e f f e c t e x p r e s s i o n i n t h e X/0 males. I t i s e v i d e n t t h a t t h e s c u t e - L 8 phenotype p r e d o m i n a t e s i n t h e s e r e c o m b i n a n t s but i s m o d i f i e d i n the p r e s e n c e o f the d i f f e r e n t r i g h t ends. Recombinants t h a t i n v o l v e the d i s t a l end o f the s c u t e - 4 chromosome o f f e r a s p e c i a l s i t u a t i o n as t h e y r e s u l t i n X - c h r o -mosomes t o t a l l y l a c k i n g a n u c l e o l u s o r g a n i z e r r e g i o n . These i n d i v i d u a l s s u r v i v e i n t h e p r e s e n c e o f a Y-chromosome o r some o t h e r s o u r c e o f rDNA but a r e e n t i r e l y l e t h a l w i t h o u t a c c e s s o r y r i b o s o m a l genes. P h e n o t y p i c a l l y , t h e s e r e c o m b i n a n t s a r e m o d e r a t e l y s c u t e but not a p p r e c i a b l y a c h a e t e . G e n e r a l l y , t h e s e f l i e s show a s c u t e phenotype i n t e r m e d i a t e between s c u t e - 4 and s c u t e - S i . Three o f t h e r e c o m b i n a n t s ( 4 L SIR, 4L_8R, 4L L8R) were e n t i r e l y bobbed l e t h a l as no x/0 f l i e s were e v e r r e c o v e r e d . The 4L 8R e x p e r i m e n t s showed a g r e a t d e a l o f n o n d i s j u n c t i o n a s s o c i a t e d w i t h the male m e i o t i c e v e n t s 98. and t h i s has "been d i s c u s s e d p r e v i o u s l y i n the l i t e r a t u r e (Yamamoto and M i k l o s , 1977). I t i s i n t e r e s t i n g t h a t t h e r e -m a i n i n g two bobbed l e t h a l s g e n e r a t e d e x h i b i t a much reduced l e v e l o f n o n d i s j u n c t i o n as compared t o 4-L 8R. T h i s l i k e l y r e f l e c t s s p e c i f i c d i f f e r e n c e s i n h e t e r o c h r o m a t i c c o n t e n t b e t -ween the t h r e e examples. T h i s o b s e r v a t i o n w i l l be o f p a r t i c -u l a r i n t e r e s t t o t h o s e s t u d y i n g t h e r o l e o f h e t e r o c h r o m a t i n i n chromo some p a i r i n g . The f i r s t i n d i c a t i o n o f t h e p e c u l i a r h e t e r o c h r o m a t i c b r e a k i n scute-V2 was e n c o u n t e r e d i n t h i s e x periment. I t was f i r s t d i s c o v e r e d t h a t t h e recombinant 4L V2R was n o t a bobbed l e t h a l , w h i c h s t r o n g l y s u g g ested t h a t the V2R end o f t h e X-chromosome c o n t a i n e d s u f f i c i e n t r i b o s o m a l genes t o m a i n t a i n a non-mutant phenotype i n an X/0 f l y . The g e n e r a t i o n o f the V2L 8R chromosome and i t s subsequent v i a b i l i t y s uggested t h a t t h e l e f t end o f t h e V2 chromosome a l s o c a r r i e d a c o n s i d e r a b l e number o f r i b o s o m a l genes. These two o b s e r v a t i o n s l e d t o t h e f o r m a t i o n o f two hypotheses: e i t h e r scute-V2 was o r i g i n a l l y b r o k e n i n t h e m i d d l e o f t h e r i b o s o m a l c l u s t e r o r t h e chromo-some was n o t a s i m p l e X i n v e r s i o n and was p r e v i o u s l y i n v o l v e d i n a complex rearrangement t h a t s e c u r e d the second n u c l e o l u s o r g a n i z e r . F o r t h e p u r p o s e s o f t h i s d i s c u s s i o n , the former e x p l a n a t i o n w i l l be adopted owing t o i t s s i m p l i c i t y and t e s t -a b i l i t y . The 4L V2R recombinant showed moderate v i a b i l i t y when p a t e r n a l l y d e r i v e d and a l m o s t normal v i a b i l i t y when m a t e r n a l l y 9 9 . i n h e r i t e d . The X/0 males were v i g o r o u s and showed v e r y l i t t l e d e v e l o p m e n t a l d e l a y as compared t o t h e f e m a l e s o f t h e same exper i m e n t . P h e n o t y p i c a l l y , t h e s e f l i e s were o f the scute-4-mutant c o n d i t i o n , showing macrochaetae d i s t u r b a n c e s but neg-l i g i b l e e f f e c t on m i c r o c h a e t a e . The r e m a i n i n g r e c o m b i n a n t s s t u d i e d were V2L 8R and 8L V2R. Bo t h chromosomes produced a s t a n d a r d m a t e r n a l e f f e c t and showed d e v e l o p m e n t a l p r o f i l e s t h a t were c o n s i s t e n t f o r t h e p a r e n t a l chromosomes. The V2L 8R chromosome i s markedly achaete on a l l e p i d e r m a l s u r f a c e s i n t h e X/0, X/Y and X/X c o n d i t i o n s . T h i s i s n o t obse r v e d i n t h e 8L V2R recombinant where achaete b r i s -t l e s were l a r g e l y normal except f o r the X/0 males. The most f r e q u e n t l y o b s e r v e d mutant phenotypes i n 8L V2R were m i s s i n g s c u t e l l a r b r i s t l e s and t h e p r e s e n c e o f a c c e s s o r y t h o r a c i c b r i s t l e s . 100. -Chapter 3 -M a t e r n a l E f f e c t s o f I n v e r s i o n H e t e r o z y g o t e s The purpose o f the n e x t s e t o f e x p e r i m e n t s was t o t e s t t h e e f f e c t o f t h e m a t e r n a l genotype, when i t was d e f i c i e n t i n h e t e r o c h r o m a t i n and/or the r i b o s o m a l genes, on the s u p p r e s -s i o n o f s c u t e X/0 l e t h a l i t y . P r i o r t o t h e s e e x p e r i m e n t s , the m a t e r n a l e f f e c t had been demonstrated o n l y f o r homozygous s c u t e i n v e r s i o n f e m a l e s . I t seemed w o r t h w h i l e t o prove t h a t t h e m a t e r n a l e f f e c t was a g e n e r a l phenomenon t h a t r e s u l t e d from t h e s c u t e i n v e r s i o n b e i n g i n h e r i t e d from any female r a t h e r t h a n a s p e c i f i c p e c u l i a r i t y o f s c u t e f e m a l e s . Each o f t h e s c u t e chromosomes s t u d i e d were made h e t e r o -zygous w i t h each o f t h e f o l l o w i n g X-chromosomes: s c ^ s c 8 ^ N0~, D f ( l ) b b 1-452, b b - 2 - r l - 3 , X-Y (an a t t a c h e d X j Y ) and a s t a n d a r d 2 X-chromosome marked w i t h v_. The r e s u l t i n g f e m a l e s were mon-i t e r e d f o r t h e i r v i a b i l i t y and t h e n mated t o X-Y/O and X•Y/Y males. The subsequent progeny were s c o r e d e v e r y second day and compared t o t h e i r s t a n d a r d s i b s . The sex r a t i o s e n t e r e d i n T a b l e 5 were c a l c u l a t e d by comparing t h e v i a b i l i t i e s o f t h e X/0 males w i t h t h e v i a b i l i t y o f X/X f e m a l e s r e c o v e r e d from t h e same c r o s s . The v a l u e s p r e s e n t e d i n T a b l e 5 suggest t h a t t h e m a t e r n a l e f f e c t i s i n d e e d a g e n e r a l phenomenon r e s u l t i n g from the gen-d e r o f the s c u t e X d o n a t i n g p a r e n t . I n a l l c a s e s , t h e r e c o v -e r y o f X/0 males from h e t e r o z y g o u s f e m a l e s r e s u l t e d i n enhanced TABLE 5 X/0 V i a b i l i t y from H e t e r o z y g o u s S c u t e Females  s c ^ s c 8 NO" D f ( l ) bb 1452 b b 2 r 1 " 3 X-Y Sex Sex Sex Sex Sex R a t i o N R a t i o N R a t i o N R a t i o N R a t i o N s c 8 +dl49 1.17 1008 1.08 847 1-03 566 1.23 1 6 1 9 1-18 538 Q sc c v v f 0.00 0 0.00 0 0.00 0 0.00 468 0.00 715 C M sc 1 +S 0.36 2516 0.40 1611 O.37 1238 0.4-6 1406 0.31 983 sc +di49 0.00 0 0.00 0 0.00 o 0.00 • 656 0.00 513 s c L 8 s n 3 0.24 538 0.18 369 0.31 716 0.17 648 0.21 898 s c L 8 m 0.16 609 0.24 916 0.26 781 0.19 682 0.22 674 v? sc c 0.93 831 I . 0 3 1239 1.13 772 0.86 1640 1.07 1310 4 sc 0.94 1016 1.13 765 1.05 679 O.96 1172 O.83 844 TABLE 5 C o n t i n u e d 4 sc sc 8 NO-Sex R a t i o N-S1L 8R 0.88 2713 S1L L8R 0.81 615 S1L V2R 0.84 911 S1L 4R 1.14 475 L8L 8R O.96 745 L8L SIR 0-37 1219 L8L V2R 1.18 940 L8L 4R 0.80 592 4L V2R 0.71 628 V2L 8R 1.09 1117 8L V2R 0.82 841 D f ( l ) bb14-52 Sex R a t i o N 1.04 I838 O.87 783 0.97 916 0.90 726 O.92 555 0.22 601 0.90 1056 0.95 539 0.88 786 0.98 492 O.77 1083 ,, 2 r l - 3 bb X-Y Sex R a t i o N 0.91 2063 0.90 912 0.91 583 1.08 830 0.81 538 0.31 399 1.09 949 0.84 670 0.79 983 O.92 1122 0.86 665 Sex R a t i o N 1.24 1616 0.99 836 1.13 654 1.16 610 0.84 ' 664 0.46 716 0.99 788 1.06 1008 1.01 686 1.06 711 O.94 720 Sex R a t i o N O.96 1082 0.91 650 1.06 575 1.04 772 0.91 764-0.26 633 1.07 561 0.97 717 0.85 428 0.97 596 0.89 388 103-v i a b i l i t y when compared t o r e c o v e r y o f X/0 males i n t h e pa-t e r n a l e x p e r i m e n t s . The magnitude o f t h i s m a t e r n a l e f f e c t i s compared w i t h m a t e r n a l e f f e c t s from homozygotes and the p a t e r n a l e f f e c t s ( i . e . p a t e r n a l l y d e r i v e d X-chromosomes) i n F i g u r e 6. The most o b v i o u s o b s e r v a t i o n i s t h a t the m a t e r n a l e f f e c t i s o f s i m i l a r magnitude i n a l l e x p e r i m e n t s . The X/0 v i a b i l i t y i n t h e h e t e r o z y g o t e e x p e r i m e n t s i s o f t e n m a r g i n a l l y l e s s t h a n t h a t observed i n t h e homozygote e x p e r i m e n t s . These d i f f e r e n c e s a r e f r e q u e n t l y i n s i g n i f i c a n t and may r e f l e c t s l i g h t s e g r e g a t i o n d i f f e r e n c e s between t h e d i f f e r e n t p a r e n t a l s t o c k s . The key f e a t u r e r e v e a l e d by t h e s e e x p e r i m e n t s i s t h e u n i v e r s a l i t y o f the m a t e r n a l e f f e c t . The r e s c u e o f the X/0 males i n t h e s e h e t e r o z y g o t e exper-i m e n t s appeared t o be independent o f t h e c o m p o s i t i o n o f the n o n - s c u t e Xhomologue. Two o f t h e chromosomes ( s c ^ s c 8 ^ N0~, D f ( l ) bb"*"-4-52) were c o m p l e t e l y d e f i c i e n t i n r i b o s o m a l DNA and d i d n o t show any p a r t i c u l a r i n f l u e n c e on t h e v i a b i l i t i e s o f t h e i r X/0 progeny. A t h i r d chromosome, b b - 2 - r l - 3 » c a r r i e s a v e r y s m a l l p o r t i o n o f t h e r i b o s o m a l RNA genes which i s i n -s u f f i c i e n t t o m a i n t a i n development i n an X/0 c o n d i t i o n . T h i s chromosome a l s o d i d n o t show any p e c u l i a r e f f e c t on t h e mater-n a l r e s c u e o f X/0 males. A p o i n t o f i n t e r e s t a r i s e s when c o n s i d e r i n g t h e v i a b i l i t y o f s p e c i f i c h e t e r o z y g o t e s . Those i n d i v i d u a l s c a r r y i n g t h e s c u t e - S i i n v e r s i o n p l u s the d l 4 9 i n v e r s i o n were found t o be i n v i a b l e i n t h e p r e s e n c e o f a bobbed l e t h a l chromosome. The s c u t e - 8 chromosome marked w i t h 104. FIGURE 6 • V i a b i l i t y o f s c u t e i n v e r s i o n X/0 males  d e r i v i n g t h e i r X-chromosomes from h e t e r o z y g o u s and  homozygous f e m a l e s o r hemizygous males. P a t e r n a l i n h e r i t a n c e - u n l i n e d M a t e r n a l i n h e r i t a n c e (homozygous) - l i n e d M a t e r n a l i n h e r i t a n c e ( h e t e r o z y g o u s ) - shaded 105. S E X RATIO 10 20 30 4 0 5 0 6 0 70 8 0 90 100 110 120 130 140 s c 8 dl 49 s c s l +S S C L 8 sn 3 / / / .V2 SC SIL 8R SIL L8R L8LSIR 4L V2R V2L 8R 8L V2R 12 / / / / / / / / / 127 117 / / / / / 42 36 29 24 26 / / / / / / / / / / / _ / / / 118 11 93 95 / / , / /' /' / / / / / / / 47 I 96 / / / / / / / / / / / 81 39 37 44 / / / / / / / / / 81 71 20 ' ' ' / / /-V/ 106 109 11 83 / / / / / / A 82 4 8 -from SC SC NO heterozygotes 106. cv v f p r e s e n t e d t h e same r e s u l t s . The d a t a suggest t h a t t h e s e s p e c i f i c chromosomes r e q u i r e a d d i t i o n a l r i b o s o m a l genes f o r v i a b i l i t y . S t u d i e s i n d i c a t e t h a t a f u l l complement o f r i b o s o m a l DNA i s p r e s e n t i n t h e s e chromosomes (Ba k e r , 1972). The p o s s i b l e muted e x p r e s s i o n o f t h e s e genes m e r i t s f u r t h e r s t u d y and e x p e r i m e n t s a r e now i n p r o g r e s s t o v e r i f y t h e p r e -sence o f " s i l e n t " r i b o s o m a l RNA genes and t o determine t h e mechanism o f t h e i r s u p p r e s s i o n . The second f a c t o r t h a t p r o v e d o f l i t t l e s i g n i f i c a n c e was t h e q u a n t i t y o f h e t e r o c h r o m a t i n p r e s e n t i n the m a t e r n a l 4-L 8 R genome. The chromosome sc sc i s d e f i c i e n t f o r t h e m a j o r i t y o f t h e p r o x i m a l X h e t e r o c h r o m a t i n i n c l u d i n g t h e n u c l e o l u s o r g a n i z e r r e g i o n . D f ( l ) bb1-452 and bb-2-rl-3 are d e f i c i e n t f o r t h e bobbed r e g i o n b u t n o t f o r s i g n i f i c a n t p o r t i o n s o f p h e t e r o c h r o m a t i n . The x_ chromosome c a r r i e s a s t a n d a r d , u n a l -t e r e d h e t e r o c h r o m a t i c r e g i o n w h i l e t h e X • Y chromosome c a r r i e s p r o x i m a l X h e t e r o c h r o m a t i n p l u s a l a r g e p o r t i o n o f t h e Y h e t -e r o c h r o m a t i n . The comparison o f X/0 v i a b i l i t y from f e m a l e s d e f i c i e n t i n h e t e r o c h r o m a t i n w i t h f e m a l e s w i t h e x c e s s h e t e r o -c h r o m a t i n y i e l d s a s m a l l d i f f e r e n c e between t h e two. The phenotypes o f t h e X/0 i n d i v i d u a l s were v e r y s i m i l a r t o t h o s e o b s e r v e d i n t h e homozygous m a t e r n a l e f f e c t e x p e r i -ments. G e n e r a l l y , t h e v a r i e g a t i n g genes were n o t s e v e r e l y e x p r e s s e d and t h e X/0 males were always c o m p e t i t i v e . A use-f u l i n t e r n a l c o n t r o l i n t h e s e s t u d i e s was t h e e c l o s i o n p e r i o d .of homologous males. One can e a s i l y see two o v e r l a p p i n g d i s -.107-t r i b u t i o n s ; one r e p r e s e n t i n g normal males and a n o t h e r i l l u s t r a t i n g t h e d e l a y e d s c u t e males. These e x p e r i m e n t s c l e a r l y demonstrate t h a t m a t r o c l i n o u s s c u t e X/0 i n d i v i d -u a l s s u r v i v e w e l l whether t h e i r s c u t e X-chromosome i s d e r i v e d from a homozygous o r h e t e r o z y g o u s female p a r e n t . Thus, i t appears t h a t t h i s phenomenon i s a s s o c i a t e d w i t h t h e gender o f t h e f l y r a t h e r t h a n an anomaly p e c u l i a r t o th e s c u t e i n v e r s i o n s . The r e s e a r c h a l s o s u g g e s t s t h a t t h e m a t e r n a l " r e s c u e " a s s o c i a t e d w i t h p a r e n t a l i n h e r i t -ance o f the s c u t e X/0 c o n d i t i o n i s somewhat i n s e n s i t i v e t o m o d i f i c a t i o n s i n t h e amount o f X o r Y chromosomal h e t e r o c h r o m a t i n i n the m a t e r n a l genome. The n e x t chap-t e r i s devoted t o a more tho r o u g h a n a l y s i s o f the e f f e c t s o f h e t e r o c h r o m a t i n on s c u t e X/0 v i a b i l i t y . 1 0 8 . - C h a p t e r 4 -M o d i f i c a t i o n o f t h e x/0 V i a b i l i t y  by X-Chromosome H e t e r o c h r o m a t i c D u p l i c a t i o n s  and A l t e r e d Y-Chromosomes The n e x t s e r i e s o f s t u d i e s i n v o l v e d the i n v e s t i g a t i o n o f s c u t e X/0 v i a b i l i t y i n the p r e s e n c e o f s p e c i f i c chromosome fra g m e n t s . These rearra n g e m e n t s c a r r y a number o f f a c t o r s t h a t a re r e p o r t e d t o a f f e c t the e x p r e s s i o n o f s c u t e m u t a t i o n s . A l l o f t h e s e supernumerary f r a g m e n t s a r e p r e d o m i n a t e l y h e t e r o -c h r o m a t i c . . The Ybb~, Dp 1337 and Dp 1187 are d e v o i d o f rRNA c i s t r o n s w h i l e Dp 856 and the s t a n d a r d Y-chromosome have a f u l l complement o f t h e s e genes. There i s a l s o a s m a l l euchro-m a t i c d u p l i c a t i o n c o n t a i n i n g the s c u t e l o c u s on Dp 1337 and Dp 856. The eu c h r o m a t i c d u p l i c a t i o n on Dp 1187 c o v e r s v_ but does n o t i n c l u d e s c u t e . The f i r s t s e t o f c r o s s e s i n v o l v e d m a t i n g compound-X f e m a l e s c a r r y i n g t h e fragment t o t h e s t a n d a r d s c u t e i n v e r s i o n males and d e t e r m i n i n g t h e v i a b i l i t y o f t h e subsequent f r a g -ment c a r r y i n g males. T h i s t e s t s the m o d i f i c a t i o n o f the pa-t e r n a l l e t h a l i t y by the s p e c i f i c f r a g m e n t s . I t was n e c e s s a r y t o d u p l i c a t e t h e s e e x p e r i m e n t s u s i n g two g e n o t y p i c a l l y d i f -f e r e n t donor f e m a l e s as t h e m a t e r n a l c o n s t i t u t i o n i n f l u e n c e s t h e X/0 v i a b i l i t y (see Ch a p t e r 5 ) • T a b l e s 6 and 7 p r e s e n t d a t a concerned w i t h the a l t e r a t i o n o f t h e X/0 v i a b i l i t y i n t h e p r e s e n c e o f s p e c i f i c chromosomal TABLE 6 A l t e r a t i o n o f P a t e r n a l E f f e c t by D u p l i c a t i o n s and A l t e r e d Y's 0 Y Ybb" Dp 1187 s c " NO" Dp 1337 s c + NO" Dp + sc 8^6 N 0 + Sex R a t i o N Sex R a t i o N Sex R a t i o I , Sex R a t i o N Sex R a t i o N Sex R a t i o N SI TJ sc yB 0.04 2004 O.89 2068 0.47 638 O.38 813 0.53 704 O.59 550 L8 sc m 0.01 1859 0.82 1862 0-35 826 0.18 735 0.42 913 0.42 728 8 ^ sc + 0.09 2616 1.23 2311 0 .85 498 0.49 538 1 .20 694 1.13 509 s c 8 + dl49 0.11 2565 0.97 3269 0.77 618 0 .35 855 1.24 580 1.18 892 V2 sc 0.29 2371 1.08 1545 0.89 811 0.61 669 1 .17 804 1.06 738 4 ' sc 0.86 2893 0.97 2629 1.06 936 1 .03 639 l ' .12 727 1 .22 610 S1L 8R 0.09 1935 1.13 .1684 0.82 591 0.41 920 0.18 555 0.26 641 S1L V2R O.79 1610 1 .05 1217 1.00 783 0 .94 746 0.40 725 0.42 866 L8L 8R 0.08 2069 0 .95 1617 O.89 660 O.36 866 0.10 634 0.18 650 L8L V2R 0.48 1262 1.06 988 0.92 816 0.68 807 0.09 580 0.16 718 V2L 8R 0.20 1299 1.17 744 0.96 533 0.44 504 0.60 596 0.66 838 8L V2R 0.11 1043 O.98 912 0.69 800 O.56 777 0.77 755 O.76 809 4L V2R 0.44 1690 0.86 1014 0 .83 691 0.69 562 0.39 690 0.18 713 ( s c x / Y x C(1)RM yw/Dp ) TABLE 7 A l t e r a t i o n o f P a t e r n a l E f f e c t by D u p l i c a t i o n s and A l t e r e d Y's 0 Y Ybb Dp : s c " 1187 N0" Dp : + sc 1337 N0" Dp + sc 856 N0 + Sex R a t i o N Sex R a t i o N Sex R a t i o N , Sex R a t i o N Sex R a t i o N Sex R a t i o N s c S 1 yB 0.06 2317 0.94 2734 0.63 915 0.27 483 0.24 566 0.34^  628 L 8 sc m 0.07 1713 0.86 1239 0.49 729 0.17 609 0.21 841 0.39 992 8 sc + 0.26 1618 1.09 1633 0.61 967 0.35 707 0.39 787 0.53 580 s c 8 + dl49 0 .26 2577 1.03 1691 0.55 707 0.33 816 0.51 692 0.47 836 V2 sc O.39 1370 1.02 1535 0-73 619 0.47 712 0.77 920 0 .84 583 4 sc 0.89 1168 O.96 1787 0.83 682 0.83 498 o ' .9i 549 1.06 491 S I L 8R 0.26 1509 0.99 906 0.49 859 0.30 817 0.16 700 0.11 717 S I L V2R 0 .80 1882 1.09 1573 0.90 538 0.82 541 0.50 945 0.61 662 L 8 L V2R 0.16 1025 0.92 1616 0.68 647 0.25 619 0.09 710 0.05 488 L 8 L 8R 0.55 960 1.05 1382 0.87 629 0.60 833 0.20 944 0.39 579 V2L 8R 0.16 1306 1 .10 1348 0.55 593 0.23 652 0.47 906 0.65 608 8L V2R 0.17 911 O.96 962 0.71 567 0.25 544 0.60 593 O.56 720 4L V2R 0.37 1250 1.02 829 0.68 539 0.41 708 0.13 662 0.22 853 ( C(1)RM yvpn/Dp x s c x / Y ) FIGURE 7 E f f e c t o f Ybb , an a d d i t i o n a l s c u t e  l o c u s , and h e t e r o c h r o m a t i c d u p l i c a t i o n s on  t h e v i a b i l i t y o f s c u t e X / 0 males. I : P a t e r n a l i n h e r i t a n c e o f s c u t e i n v e r s i o n 112. 120 110 100 90 sex ratio 80 70 60 50 40 30 20 10 S1 n SC y B sc8+ dl 49 S1L 8R B a r s r e p r e s e n t : ( l e f t t o r i g h t ) X/0, Dp 1187 ( s c " , b b " ) , Dp 1337 ( s c + , b b " ) , Dp 856 ( s c + , b b + ) , Ybb . 113-f r a g m e n t s . The r e s u l t s a r e g r a p h i c a l l y d e p i c t e d i n F i g u r e 7. I n t h e p r e s e n c e o f Ybb~ o r Dp 1187, t h e X/0 v i a b i l i t y i s d r a m a t i c a l l y i n c r e a s e d . T h i s i s p a r t i c u l a r l y e v i d e n t i n the d a t a p r e s e n t e d i n T a b l e 6 which i n v o l v e s t h e compound-X c h r o -mosome marked w i t h y e l l o w and w h i t e . The b a s e l i n e v i a b i l i t i e s o f X/0 males d e r i v e d from t h i s compound-X a r e e x t r e m e l y low. These c r o s s e s are t h u s t h e most a c c u r a t e gauge o f i n c r e a s e d v i a b i l i t y due t o t h e a c c e s s o r y f r a g m e n t s . I n a l l c r o s s e s where s t a n d a r d X/0 v i a b i l i t y i s v e r y low, t h e p r e s e n c e o f t h e a c c e s s o r y f r a g m e n t s e l e v a t e d v i a b i l i t y . I n t h o s e m a t i n g s t h a t p r e s e n t moderate X/0 v i a b i l i t y , t h e h e t e r o c h r o m a t i c e f f e c t was n o t as pronounced. These i n d i v i d -u a l s may be l a r g e l y i n s e n s i t i v e t o t h e e x t r a h e t e r o c h r o m a t i n as t h e i r o r i g i n a l c o n d i t i o n i s n o t s e v e r e l y mutant and t h u s , may have c i r c u m v e n t e d t h e problem a l l e v i a t e d by the e x c e s s h e t e r o c h r o m a t i n i n t h e o t h e r s t o c k s . The h e t e r o c h r o m a t i c e f f e c t i s c o n s i s t e n t and u n i d i r e c t i o n a l f o r a l l s t o c k s r e -g a r d l e s s o f t h e i r genotype. The r i b o s o m a l genes may e x e r t a p o s i t i v e e f f e c t on t h e X/0 v i a b i l i t y i n t h e s e t e s t s . The r e s c u e o f the X/0 v i a b i l i t y by a s t a n d a r d Y-chromosome has been p r e v i o u s l y d e s c r i b e d i n t h i s t e x t . E x p e r i m e n t s u t i l i z i n g a Y-chromosome d e f i c i e n t i n rRNA c i s t r o n s i n d i c a t e t h a t t h i s fragment d r a m a t i c a l l y i n c r e a s e s t h e v i a b i l i t y o f s c u t e X/0 males. However, the v i a b i l i t y o f s c u t e X/Ybb~ male i s c o n s i s t e n t l y l e s s t h a n t h a t o b s e r v e d f o r the X/Ybb + males i n t h e c o n t r o l e x p e r i m e n t s . 114. T h i s d i f f e r e n c e c o u l d he a s s o c i a t e d w.ith t h e l a c k o f rRNA c i s t r o n s o r some o t h e r f a c t o r i n t h e v i c i n i t y o f the Y bobbed l o c u s . As t h i s Ybb~ chromosome was o b t a i n e d by r a d i a t i o n i n d u c e d breakage, one o n l y knows t h a t the d e l e t i o n o c c u r s on t h e s h o r t arm, d e l e t e s t h e r i b o s o m a l genes and does n o t remove any o f t h e K f e r t i l i t y f a c t o r s . C y t o l o g i c a l p r e -p a r a t i o n s a r e i n c o n c l u s i v e i n q u a n t i t a t i n g the e x t e n t o f the h e t e r o c h r o m a t i c d e l e t i o n . A l t h o u g h i n f o r m a t i o n p e r t a i n i n g t o s p e c i f i c r e g i o n a l d i f f e r e n c e s on t h e Y-chromosome o f D. m e l a n o g a s t e r a r e l a c k i n g , Hess (1968) d e f i n e d r e g i o n s o f t h e D• h y d e i Y-chromosome t h a t have v a r i a b l e e f f e c t s on v a r i -e g a t i n g p o s i t i o n e f f e c t mutants. T h i s p a r t i c u l a r argument i s f a v o r e d i n t h i s t e x t as Dp 856 c a r r i e s a f u l l y f u n c t i o n a l n u c l e o l a r o r g a n i z e r r e g i o n and y e t does n o t e x e r t a d r a m a t i c e f f e c t on i n c r e a s i n g t h e X/0 v i a b i l i t y . The bobbed r e g i o n i n Dp 856 i s d e r i v e d from an X-chromosome and may n o t be a s s o c i a t e d w i t h s i m i l a r f a c t o r s p r e s e n t on the Y-chromosome. These r e s u l t s suggest t h a t t h e r e e x i s t s a d i f f e r e n c e between the bobbed r e g i o n s on the Y and X-chromosomes. I t appears t h a t t h e bobbed r e g i o n a s s o c i a t e d w i t h the Y-chromosome may i n f l u e n c e v i a b i l i t y o f t h e s c u t e X/0 males w h i l e the X bobbed r e g i o n e x e r t s a l e s s e r e f f e c t . The s c u t e d u p l i c a t i o n s p r e s e n t on Dp 1337 and Dp 856 appear t o r e s c u e a s i g n i f i c a n t p o r t i o n o f the X/0 males. The r e s u l t s f o r t h e s e d u p l i c a t i o n s can be compared t o Dp 1187 w h i c h c a r r i e s s i m i l a r amounts o f h e t e r o c h r o m a t i n but l a c k s 115. a f u n c t i o n a l s c u t e l o c u s . I n a l l non-recombinant s c u t e c h r o -mosomes, the p r e s e n c e o f t h e e x t r a s c u t e l o c u s was a s s o c i a t e d w i t h a v i a b i l i t y i n c r e a s e . T h i s i s most c l e a r l y i l l u s t r a t e d i n a comparison o f Dp II87 and Dp 1337 w h i c h d i f f e r o n l y w i t h r e s p e c t t o t h e s c u t e l o c u s . The most p r o f o u n d i n f l u e n c e o f t h e e x t r a s c u t e l o c u s i s o b s e r v e d i n the scute-8 t y p e i n v e r -s i o n s . Scute-V2and scute-8 show a t l e a s t a t w o - f o l d i n c r e a s e i n X/0 v i a b i l i t y when the males c a r r y a s c u t e d u p l i c a t i o n . The s c u t e - S i t y p e i n v e r s i o n s show s i m i l a r r e s u l t s but not as pronounced. The recombinant chromosomes t h a t a r e c r e a t e d from s c u t e - S i t y p e and scute-8 type c a r r y two s c u t e l o c i as a r e s u l t o f the r e c o m b i n a t i o n event. The e f f e c t s o f t h i s anomaly a r e w e l l i l l u s t r a t e d i n t h e s e e x p e r i m e n t s . G a r c i a - B e l l i d o (1979) has r e p o r t e d t h a t t h r e e doses o f t h e s c u t e r e g i o n d e c r e a s e s v i a -b i l i t y i n males. One would e x p e c t many o f t h e r e c o m b i n a n t s t o c a r r y two s c u t e l o c i and t h e d o n a t i o n o f a Dp_ c a r r y i n g a n o t h e r s c u t e l o c u s would r e s u l t i n an X/Dp male p o s s e s s i n g t h r e e s c u t e l o c i . The r e s u l t s p r e s e n t e d i n b o t h T a b l e 6 and 7 a r e c o n s i s t e n t w i t h the h y p o t h e s i s a s s o c i a t e d w i t h t r i p l o -s c u t e male l e t h a l i t y . I n a l l i n d i v i d u a l s t h a t s h o u l d c a r r y two s c u t e l o c i , t h e r e i s a marked r e d u c t i o n o f v i a b i l i t y when th e recombinant X-chromosome i s a s s o c i a t e d w i t h a Dp s c + . These r e s u l t s p r o v i d e f u r t h e r e v i d e n c e t h a t t h e i n v i a b i l i t y a s s o c i a t e d w i t h t h e s c u t e l o c u s d e s c r i b e d by G a r c i a - B e l l i d o i s a fundamental phenomenon o f t h e gene. 116. The work was n e x t extended t o i n c l u d e t h e r e c i p r o c a l e x p eriment i n v o l v i n g t h e m a t e r n a l e f f e c t . The e x p e r i m e n t a l p r o c e d u r e i n v o l v e d m a t i n g homozygous s c u t e f e m a l e s t o a t t a c h e d X•Y males c a r r y i n g t h e chromosomal f r a g m e n t s . The r e s u l t s a r e p r e s e n t e d i n T a b l e 8. C o n s i s t e n t w i t h p r e v i o u s e x p e r i m e n t s , a c l a s s i c m a t e r n a l e f f e c t i s o b s e r v e d when the s c u t e X-chromosome i s i n h e r i t e d f r o m t h e f e m a l e . The b a s e l i n e v i a b i l i t i e s o f a l l s c u t e X/0 males a r e g r e a t l y i n c r e a s e d as compared t o t h e p r e v i o u s p a t e r n a l s t u d i e s . T h i s r e d u c e s s e n s i t i v i t y t o v i a b i l i t y i n c r e a s e s but a m p l i f i e s r e s p o n s e t o v i a b i l i t y d e c r e a s e s . The h e t e r o c h r o m a t i c e f f e c t on v i a b i l i t y o b s e r v e d i n t h e p a t e r n a l e f f e c t e x p e r i m e n t s appears t o be o b s c u r e d i n the m a t e r n a l e x p e r i m e n t s . There a r e l i t t l e v i a b i l i t y d i f f e r e n c e s n o t e d between t h e X/0 males and t h o s e males c a r r y i n g s u r p l u s h e t e r o c h r o m a t i n (Ybb~, Dp 1187). I t i s apparent t h a t t h e • m a t e r n a l e f f e c t s u p p r e s s i o n o f t h e X/0 l e t h a l i t y masks th e h e t e r o c h r o m a t i c e f f e c t p r e v i o u s l y i d e n t i f i e d . The e f f e c t s o f t h e bobbed r e g i o n s a r e s i m i l a r i n t h i s e xperiment t o t h o s e o b s e r v e d i n t h e p a t e r n a l e x p e r i m e n t s . The major f a c t o r i n f l u e n c i n g an i n c r e a s e o f v i a b i l i t y was t h e p r e s e n c e o f a s t a n d a r d Y-chromosome bobbed r e g i o n . However, t h e X-chromosomal bobbed r e g i o n d i d n o t a l t e r t h e X/0 v i a b i l -i t y t o any s i g n i f i c a n t e x t e n t . As was o b s e r v e d i n t h e p r e v i o u s ' e x p e r i m e n t s , i t appears t h a t the Y bobbed r e g i o n i n f l u e n c e s X/0 v i a b i l i t y and i s independent o f t h e mechanism o f t h e mater-TABLE 8 A l t e r a t i o n o f M a t e r n a l E f f e c t "by D u p l i c a t i o n s and A l t e r e d Y's 0 Y Ybb Dp 1187 s c " N0~ Dp + sc 1337 NO" Dp + sc 856 N0 + Sex R a t i o N Sex R a t i o N Sex R a t i o N , Sex R a t i o N Sex R a t i o N Sex R a t i o N SI „ sc yB 0.42 9438 0.97 1628 0.47 783 0.38 621 0.53 694 0.59 709 L8 sc m 0.33 1106 O.87 938 0.35 640 O.38 517 0.42 765 0.42 843 8 ^ sc + 1.17 1924 1 .22 916 1 .05 498 1.09 830 1 .20 746 1.13 521 s c 8 + dl49 1.28 2466 1.34 1138 1.17 622 1-35 789 1.24 490 1.18 562 V2 sc 1.18 2914 1 .20 1262 1.20 816 1.11 753 1.17 685 1.06 811 4 sc 0.99 1238 1 .06 934 1 .06 776 1.03 483 1-.12 627 1.22 736 S1L 8R 0.88 3746 1 .04 1842 0.82 553 1 .01 919 0.58 658 0.46 608 S1L V2R 1.05 1230 1.12 1119 1 .00 590 0.94 660 0.80 539 0.67 666 L8L 8R O.92 992 0.97 1316 0.99 806 0.86 671 0.60 589 0.38 823 L8L V2R 1.03 1008 1.26 1057 0.92 454 1.08 925 0.19 730 0.46 586 V2L 8R 1.06 1314 1.14 922 0.85 618 0.99 841 0.92 460 1.14 591 8L V2R 0.83 1136 0-93 840 0.90 904 0.85 703 O.76 625 O.96 651 4L V2R 0.81 1152 1.08 929 0.93 560 0.84 650 0.58 656 0.50 571 ( X-Y/Dp x 118. FIGURE 8 E f f e c t o f Ybb~, an a d d i t i o n a l s c u t e  l o c u s , and h e t e r o c h r o m a t i c d u p l i c a t i o n s on  t h e v i a b i l i t y o f s c u t e X/0 males. I I : M a t e r n a l i n h e r i t a n c e o f s c u t e i n v e r s i o n 119. S1 D SC y B 8 sc + dl 49 S1L 8R B a r s r e p r e s e n t : ( l e f t t o r i g h t ) X/ 0 , Dp 1187 ( s c _ b b _ ) , Dp 1337 ( s c + b b - ) , Dp 856 ( s c + b b + ) , Y b b _ . 120. e f f e c t . The e l e v a t e d b a s e l i n e v a l u e s f o r s c u t e X/0 v i a b i l i t y i n t h e m a t e r n a l e x p e r i m e n t s p r o v i d e s a s e n s i t i v e system t o t e s t t h e l e t h a l i t y a s s o c i a t e d w i t h -three s c u t e l o c i i n a male. A l l o f t h e s c u t e r e c o m b i n a n t s c a r r y i n g two s c u t e l o c i mani-f e s t a d r a m a t i c r e d u c t i o n i n v i a b i l i t y i n a s s o c i a t i o n w i t h a d u p l i c a t i o n f o r t h e s c u t e l o c u s . T h i s i s c o n s i s t e n t w i t h p r e v i o u s s t u d i e s i n t h i s t h e s i s and w i t h s t u d i e s completed by o t h e r a u t h o r s ( G a r c i a - B e l l i d o and S a n t a m a r i a , 1978). The d a t a suggest t h a t the t r i p l o - s c u t e male l e t h a l i t y i s indepen-dent o f t h e m a t e r n a l e f f e c t . Three doses o f t h e s c u t e gene i n a male r e s u l t s i n a v i a b i l i t y r e d u c t i o n beyond th e b a s e l i n e v i a b i l i t y e s t a b l i s h e d f o r s c u t e X/0 males. T h i s e f f e c t i s o b s e r v e d i n b o t h m a t e r n a l and p a t e r n a l e x p e r i m e n t s . 121 . - C h a p t e r 5 -The Compound-X E f f e c t The o r i g i n a l s t u d i e s i n t h i s r e s e a r c h a r e a i n v o l v e d the m a t i n g o f s c u t e males t o compound-X f e m a l e s t h a t c a r r i e d a f r e e Y-chromosome. I n t h e c o n t r o l s , t h i s female c a r r i e d a marked Y-chromosome t h a t c o u l d be i d e n t i f i e d i n the s c u t e male. The f i r s t compound-X u t i l i z e d was C(1 ).RM y w/0 and r e s u l t s o b t a i n e d from t h e s e c r o s s e s c o r r e s p o n d w e l l w i t h r e -s u l t s p u b l i s h e d by Baker (1972) who e i t h e r used t h i s same compound-X o r one s i m i l a r l y marked. When the y w female was c a r r i e d i n a s t o c k w i t h a f r e e Y-chromosome, i t was e x t r e m e l y d i f f i c u l t t o m a i n t a i n a pure b r e e d i n g s t o c k as a breakdown o f t h e compound-X o c c u r r e d f r e q u e n t l y . I t was d e c i d e d t h a t a n o t h e r compound-X t h a t would n o t break down c o u l d be used f o r t h e e x p e r i m e n t s . C r o s s e s were co n d u c t e d , t h e r e f o r e , w i t h f e m a l e s c a r r y i n g C ( l ) R M Y sEny/0, a compound-X_which d i d not break down a p p r e c i a b l y i n t h e p r e s e n c e o f a Y-chromosome. I t was a s u r p r i s i n g o b s e r v a t i o n t o note t h a t the v i a b i l i t i e s d e t e r m i n e d f o r : t h e s e e x p e r i m e n t s d e v i a t e d s i g n i f i c a n t l y from the p r e v i o u s e x p e r i m e n t s u s i n g C(1)RM y w/0 f e m a l e s . T h i s was t h e f i r s t i n d i c a t i o n t h a t l e t h a l i t y a s s o c i a t e d w i t h the p a t e r n a l e f f e c t was c o n d i t i o n a l t o some e x t e n t on the mater-n a l chromosomal c o n s t i t u t i o n . T a b l e 9 and F i g u r e 9 p r e s e n t d a t a accumulated from the m a t i n g s o f numerous d i f f e r e n t compound-X females t o males TABLE 9 The Compound-X E f f e c t  yw +_ yvbb yvpn Y Eny Sex R a t i o N Sex R a t i o N Sex R a t i o N Sex R a t i o N Sex R a t i o N s c S 1 +S 0.04 2004 0.03 I896 0.04 1512 0.06 1931 0.06 1644 s c S 1 +dl4-9 0.00 1809 0.00 1480 0.00 836 0.00 918 0.00 1009 SI sc ym 0.03 2138 0.07 963 0.04 751 0.15 824 0.12 780 L8 3 sc s n ^ 0.04 1610 0.03 1168 0.05 916 0.10 659 0.12 1138 L8 sc m 0.01 1859 0.04 728 0.03 704 0.07 1326 0.10 956 s c 8 + d l 4 9 0.11 2565 Q.12 1269 0.16 1017 0.26 1157 0.23 1068 8 ^ sc + 0.09 2616 0.08 1352 0.12 696 0.26 922 0.24 788 V2 sc 0.29 2371 . 0 . 2 6 1619 0.23 754 0.39 699 0.46 1041 sc 0.86 2893 0.87 1052 0.95 589 0.89 710 0.99 1164 S I L 8R 0.09 1935 0.18 1736 0.16 653 0.26 2067 O.31 1436 S I L L8R 0.40 1042 0.47 924 0.43 718 0.51 831 0.63 862 S I L V2R 0.79 1610 0.76 836 0.81 830 0.80 750 O.87 904-L8L 8R 0.08 1319 0.06 1124 0.11 919 0.16 690 0.19 886 8L V2R 0.05. 1213 0.11 830 0.14 681 0.17 928 0.22 749 V2L 8R 0.09 1066 0.20 765 0.16 738 0.16 826 0.20 915 4L V2R 0.26 1059 0.44 911 0.35 926 0.37 926 0.44 681 123. FIGURE 9 The Com-pound-X E f f e c t 124. 40 30 s e x r a t i o 20 10 S1 „ SC y B L8 sc m sc 8 S1L 8R B a r s r e p r e s e n t : ( l e f t t o r i g h t ) C ( l ) RM y w, C ( l ) RM y v bb, C ( l ) RM Y s E n y 125-hemizygous f o r the v a r i o u s scute i n v e r s i o n chromosomes. The data i n Table 9 are organized from l e f t to r i g h t on the b a s i s of -the s e v e r i t y of l e t h a l i t y . Often, d i f f e r e n c e s between ad-jacent sets of data are not s i g n i f i c a n t l y d i f f e r e n t but data compared at any gr e a t e r distance always d i f f e r s i g n i f i c a n t l y . I t i s apparent that autosomal modifying f a c t o r s are not r e -sponsible f o r t h i s d i f f e r e n c e as the compound-X stocks were back crossed numerous times to males from the same X•Y/O s t r a i n to randomize such e f f e c t s . I t seems l i k e l y t h a t the X/0 v i a b i l i t y i s s e n s i t i v e to the maternal chromosomal c o n s t i t u t i o n or the maternal c o n t r i -b u t i o n to the zygote. I t i s unfortunate that the heterochro-matic c o n s t i t u t i o n of these d i f f e r e n t compounds has not been c a r e f u l l y analyzed. I t i s p o s s i b l e that the v i a b i l i t y d i f -ferences may r e f l e c t the heterochromatic content i n the mater-n a l genome. The evidence supporting t h i s hypothesis i s l i m -i t e d and i n v o l v e s the increased X/0 v i a b i l i t y observed i n compound-X chromosomes that c a r r y e x t r a Y-chromosomal hetero-chromatin. S i m i l a r experiments were performed w i t h respect to a v a r i e t y of X•Y chromosomes and the maternal donation of the scute X-chromosome. The r e s u l t s i n d i c a t e that the v i a -b i l i t y m o d i f i c a t i o n i s confined to d i f f e r e n c e s w i t h i n the ma-t e r n a l genome. S i g n i f i c a n t d i f f e r e n c e s could not be shown between v i a b i l i t i e s produced by any of the X•Y chromosomes. The c h a r a c t e r i z a t i o n of the heterochromatic content of these chromosomes i s incomplete and l i t t l e i s known concerning the 126. r e l a t i v e p r o p o r t i o n s o f X and Y-chromosomal h e t e r o c h r o m a t i n . T h i s "being t h e c a s e , one cannot i n f e r an a s s o c i a t i o n between t h e h e t e r o c h r o m a t i c c o n t e n t i n t h e gametes and X/0 v i a b i l i t y . There appears t o be a v e r y s i g n i f i c a n t v i a b i l i t y - m o d i f y i n g a b i l i t y a s s o c i a t e d w i t h t h e m a t e r n a l genome but any p r o p o s a l s o f mechanisms would be p u r e l y s p e c u l a t i v e . 127. - C h a p t e r 6 -The Temperature E f f e c t Temperature i s a c l a s s i c m o d i f i e r o f p o s i t i o n - e f f e c t v a r i e g a t i o n and i t i s o f i n t e r e s t t o observe i f t h e x/0 l e -t h a l i t y o b s e r v e d w i t h the s c u t e i n v e r s i o n s i s s u b j e c t t o t h i s v a r i a t i o n . The b r i s t l e phenotype has been shown t o be t e m p e r a t u r e s e n s i t i v e but has a l s o been shown t o be a t y p i c a l o f v a r i e g a t i n g systems. The s c u t e phenotype i s more extreme a t r e d u c e d t e m p e r a t u r e s (14-°, 18°C) and becomes i n c r e a s i n g l y w i l d t y p e as t h e t e m p e r a t u r e s approach 27°C ( C h i l d , 1 9 3 4 , 1940; I v e s , 1938; G e r s h , 1949; Furman e t a l . , 1979)- T h i s i s i n a c c o r d w i t h most v a r i e g a t i n g systems where reduced t e m p e r a t u r e s f a v o r t h e mutant e x p r e s s i o n and e l e v a t e d tem-p e r a t u r e s promote a more w i l d phenotype. I f t h e s c u t e X/0 l e t h a l i t y was a f u n c t i o n o f a s c u t e v a r i e g a t i n g system one would p r e d i c t t h a t X/0 v i a b i l i t y would be r e d uced a t d e c r e a s e d t e m p e r a t u r e s . S h o u l d l e t h a l i t y be a s s o c i a t e d w i t h problems o f rRNA p r o d u c t i o n o r p r o c e s s i n g , one would p r e d i c t e l e v a t e d v i a b i l i t i e s a t l o w e r t e m p e r a t u r e s owing t o i n c r e a s e d d e v e l o p -m e n t a l t i m e and accomodation t o t h e mutant s i t u a t i o n . The d e f i n i t i o n o f a t e m p e r a t u r e e f f e c t c o u l d p o s s i b l y d i s c r i m i n -a t e between t h e s e two p o s s i b i l i t i e s . T a b l e 10 f u r n i s h e s d a t a concerned w i t h the p a t e r n a l e f f e c t d e r i v e d a t 18°, 25° and 29°C. These e x p e r i m e n t s were c a r r i e d out u s i n g t h e f o l l o w i n g p r o t o c o l . Two f e m a l e s and 128. TABLE 10 The P a t e r n a l Temperature E f f e c t 18° C .25 °C 29 °C Sex R a t i o N Sex R a t i o N Sex R a t i o I s c S 1 +S yB 0.02 843 0 .05 1361 0.05 1265 L8 sc m 0.00 638 0.04 1042 0.03 1127 8 ^ sc + 0.12 652 0.23 956 0.29 1044 s c 8 + dl49 0.15 562 0.31 837 0.26 908 V2 sc 0.21 717 0.37 1106 0.45 1072 4 sc 0.77 659 O.83 950 0.88 1046 Base 0.05 604 0.11 1053 0.10 994 v i n c y 0.07 678 0.18 1102 0.15 1078 L8L 8R 0.06 806 0.13 • 1153 0.19 1126 V2L 8R 0.18 591 0.22 916 0.29 1092 8L V2R 0.02 629 0.13 1005 0.15 1135 ( sc A x C(1)RM yvpn/0 ) 129. FIGURE 10 The Temperature E f f e c t I : P a t e r n a l l y i n h e r i t e d X-chromosome 130. 4 0 3 0 sex ra t io 2 0 10 S1 0 S C y B L8 sc m sc 8 S1L 8R B a r s r e p r e s e n t : ( l e f t t o r i g h t ) 18°C, 25°C, 29°C 131 • two males were mated i n a s i n g l e s h e l l v i a l • a t 25 • They were a l l o w e d t o l a y eggs a t t h i s t e m p e r a t u r e f o r 3 days. The p a r e n t s were t h e n t r a n s f e r r e d t o new v i a l s and i m m e d i a t e l y p l a c e d i n an 18° environment t o l a y eggs f o r 2 days. P a r e n t s were a g a i n t r a n s f e r e d t o f r e s h v i a l s and a l l o w e d t o l a y a t 2 9 ° f o r a f u r t h e r 2 days. T h i s p r o t o c o l ensured t h a t the progeny were d e r i v e d from a g e n e t i c a l l y i d e n t i c a l p o p u l a t i o n and t h a t t h e d e v e l o p m e n t a l p e r i o d s were e n t i r e l y a t the d e s i r e d t e m p e r a t u r e . The t e m p e r a t u r e e f f e c t a s s o c i a t e d w i t h X / 0 l e t h a l i t y does n o t seem t o be p r o f o u n d when the s c u t e X-chromosome i s donated by t h e f a t h e r . There i s no c o n s i s t e n t d i f f e r e n c e between v i a b i l i t i e s a t 2 5 ° o r 2 9 ° and t h i s i s a l s o c o m p a t i b l e w i t h t h e o b s e r v e d b r i s t l e phenotypes. There are no d i f f e r -ences d e t e c t a b l e i n terms o f the mutant b r i s t l e phenotype between 2 5 ° and 2 9 ° . There appears t o be a c o n s i s t e n t d i f -f e r e n c e between the f l i e s r a i s e d e i t h e r a t 2 9 ° o r 2 5 ° and t h o s e r a i s e d a t 18° i n terms o f v i a b i l i t y . The i n c r e a s e d l e -t h a l i t y a t 18° i s a l s o a s s o c i a t e d w i t h a more severe e x p r e s -s i o n o f t h e reduced b r i s t l e number phenotype. These d a t a s u p p o r t t h e h y p o t h e s i s t h a t s c u t e X / 0 l e t h a l i t y i s a f u n c t i o n o f t h e i m p r o p e r e x p r e s s i o n o f t h e s c u t e l o c u s . There were no c a s e s where v i a b i l i t y was i n c r e a s e d a t t h e l o w e r tempera-t u r e which i s c o n t r a d i c t o r y t o t h e n o t i o n t h a t the e x p r e s s i o n o f rRNA genes a r e a f f e c t e d by p o s i t i o n e f f e c t . The t e m p e r a t u r e e f f e c t a s s o c i a t e d w i t h the m a t e r n a l 132. e x p e r i m e n t s i n d i c a t e s a more p r e c i s e d e f i n i t i o n o f t h e phen-omenon. These d a t a a r e p r o v i d e d i n T a b l e 11. The most s t r i -k i n g o b s e r v a t i o n i s the r e d u c t i o n o f the m a t e r n a l e f f e c t a t 18°. I n e v e r y experiment except f o r s c u t e - 4 , the X/0 v i a b i l i t y i s s i g n i f i c a n t l y r e d u c e d . P h e n o t y p i c a l l y , one cannot d i s t i n -g u i s h a d i f f e r e n c e between t h e X/0 males g e n e r a t e d a t the t h r e e t e m p e r a t u r e s . T h i s i s l i k e l y a r e s u l t o f t h e m a t e r n a l s u p p r e s s i o n o f t h e s e v e r e s c u t e e x p r e s s i o n s t h a t has been d i s -c u s s e d p r e v i o u s l y . The te m p e r a t u r e e f f e c t r e d u c t i o n o f X/0 v i a b i l i t y a t l o w e r t e m p e r a t u r e s appears t o be a n o t h e r g e n e r a l phenomenon a s s o c i a t e d w i t h t h e s c u t e i n v e r s i o n s . T h i s r e d u c -t i o n i s o b s e r v e d i n b o t h m a t e r n a l and p a t e r n a l e x p e r i m e n t s and seems t o be a s s o c i a t e d w i t h t h e p h e n o t y p i c e x p r e s s i o n o f the s c u t e phenotype. Thus, the e x p e r i m e n t s i n v o l v e d w i t h d e f i n i n g a t e m p e r a t u r e e f f e c t a s s o c i a t e d w i t h the X/0 l e t h a l i t y have produced a n o t h e r l i n e o f e v i d e n c e s u g g e s t i n g t h a t the i n v i a b i l i t y i s p r i m a r i l y a f u n c t i o n o f t h e v a r i e g a t e d e x p r e s -s i o n o f t h e s c u t e gene. 133-TABLE 11 The M a t e r n a l Temperature E f f e c t 18° C 25°C 29 °C Sex R a t i o N Sex R a t i o N Sex R a t i o I s c S 1 +S yB 0.19 765 O.36 947 0.35 1008 L8 sc m 0.09 538 0.29 862 0.23 814 s c 8 + 0.78 641 0.99 904 1.08 836 s c 8 + dl49 0.83 683 1.10 781 1.02 901 V2 sc 0.70 720 0.94 IO36 0.89 998 4 sc O.96 783 0.91 1103 0.87 1216 Base 0.75 661 0.88 811 1.13 940 v i n c y 0.69 594 1.03 739 1.07 812 L8L 8R O.63 554 0.81 836 0.88 843 V2L 8R 0-77 691 0.95 930 0.93 890 8L V2R 0.79 805 0.90 1068 0.84 1022 ( s c X / s c X x X-Y/O ) 134. FIGURE 11 The Temperature E f f e c t I I : M a t e r n a l l y i n h e r i t e d X-chromosome 1 3 5 -n o 100 90 80 70 I sex atio 60 r 50 40 30 y 20 to y sc L8 m sc 8 S 1 L Bars represent: ( l e f t to r i g h t ) 18°C, 25°C, 29°C 1 3 6 . - C h a p t e r 7 -The D e v e l o p m e n t a l D e l a y One o f t h e most prominent f e a t u r e s a s s o c i a t e d w i t h the s c u t e X / 0 c o n d i t i o n has been th e extended p e r i o d o f d e v e l o p -ment. T h i s has been e s t a b l i s h e d as a n o t h e r o f the p e c u l i a r phenomena a s s o c i a t e d w i t h t h e s c u t e i n v e r s i o n s . The d e l a y i s o b s e r v e d i n b o t h m a t e r n a l and p a t e r n a l e x p e r i m e n t s and has been v e r y c a r e f u l l y examined as a r e s u l t o f t h e e x p e r i -m e n t a l d e s i g n . T h i s anomaly has been o b s e r v e d i n p r e v i o u s works and the a u t h o r s suggest t h a t the d e l a y r e p r e s e n t s a d i s f u n c t i o n o f the e x p r e s s i o n o f t h e rRNA genes ( N i x , 1 9 7 3 ; B a k e r , 1 9 7 1 ) - These a u t h o r s a l s o a l l u d e t o a s i m i l a r d e v e l -opmental d e l a y a s s o c i a t e d w i t h f l i e s d e f i c i e n t i n rRNA genes. O t h e r a u t h o r s have s u g g e s t e d t h a t t h i s r e t a r d e d development a l l o w s f o r compensation o f c e l l u l a r rRNA c o n t e n t (Mohan and R i t o s s a , 1 9 7 0 ; M a l v a e t a l , 1 9 7 9 ) . I t i s a s i g n i f i c a n t o b s e r -v a t i o n t h a t t h e d e v e l o p m e n t a l d e l a y was shown t o be d i r e c t l y r e l a t e d t o t h e s e v e r i t y o f t h e bobbed phenotype. The purpose o f t h e s e e x p e r i m e n t s i s t o examine the d e v e l o p m e n t a l p r o f i l e s o f the s c u t e mutants and compare t h e s e r e s u l t s w i t h s i m i l a r d a t a c o l l e c t e d from rDNA d e f i c i e n t i n d i v i d u a l s . F o r the p u r p o s e s o f t h i s d i s c u s s i o n , t h e r e s u l t s o f . s p e c i f i c e x p e r i m e n t s w i l l be i l l u s t r a t e d as t h e y are c o n s i d -e r e d t o be exemplary o f the phenomenon. The d a t a f o r the numerous s c u t e i n v e r s i o n s and bobbed mutants a r e p r e s e n t e d i n 137-T a b l e 12. R e p r e s e n t a t i v e g r a p h i c f i g u r e s o f the d a t a are d e p i c t e d i n F i g u r e s 12 t h r o u g h 14. From t h e s e d a t a , i t i s apparent t h a t t h e r e i s a c o n s i d -e r a b l e d e v e l o p m e n t a l d e l a y a s s o c i a t e d w i t h the s c u t e X/0 c o n d i t i o n . The o n l y i n v e r s i o n d e p a r t i n g from t h i s g e n e r a l i -z a t i o n i s s c u t e - 4 w h i c h has p r e v i o u s l y been c h a r a c t e r i z e d as a t y p i c a l o f t h e numerous s c u t e r e a r r a n g e m e n t s . The c o n t r o l s i t u a t i o n s i n d i c a t e t h a t the r e t a r d e d development can be l a r g e l y s u p p r e s s e d i n the p r e s e n c e o f a Y-chromosome. The r e s u l t s d e p i c t e d i n T a b l e 12 are e x p r e s s e d i n terms o f t h e t i m e r e q u i r e d - t o r e c o v e r 50$ o f males o r f e m a l e s . The d i f f e r e n c e between t h e s e two v a l u e s i s d e f i n e d as the d e v e l o p -m e n t a l d e l a y (see F i g u r e 13)- The bb X-chromosomes are l e t h a l i n t h e X/0 s i t u a t i o n and t h e r e f o r e t h e y were r e c o v e r e d w i t h a Ybb chromosome. The X b b - l e t h a l / Y b b phenotype i s a severe bobbed c o n d i t i o n w i t h f r e q u e n t abdominal e t c h i n g and a n o t i c e -a b l e d e v e l o p m e n t a l d e l a y . The bb m a t e r n a l e f f e c t e x p e r i m e n t s were completed by c r o s s i n g h e t e r o z y g o u s d l 4 9 / b b - l e t h a l f e m a l e s t o X•Y/Ybb males. The d e v e l o p m e n t a l p r o f i l e s i n t h e s e exper-i m e n t s t h u s compare the bb/Ybb males t o the dl49/Ybb males and t h e h e t e r o z y g o u s f e m a l e s . When the s c u t e X i s p a t e r n a l l y i n h e r i t e d , the d e v e l o p -m e n t a l d e l a y appears t o be between two and t h r e e days f o r most o f t h e i n v e r s i o n s s t u d i e d . The d e l a y a s s o c i a t e d w i t h the bobbed chromosomes was one day. The c o n t r o l v a l u e s f o r the s c u t e i n v e r s i o n s i n d i c a t e t h a t t h e d e v e l o p m e n t a l d e l a y i s 138. TABLE 12 The Developmental D e l a y PATERNAL MATERNAL Female Male D e l a y Female Male D e l a y sc m 13 .0 15- 7 2 • 7 12 • 9 14. 9 2. ,0 c o n t r o l 12 .6 13- 7 1 .1 13 • 3 14. 1 0. ,8 s c S 1 +S yB 13 • 3 15- 6 2 • 3 13 .4 14. 7 1 . • 3 c o n t r o l 13 • 5 14. 3 0 .8 12 .8 13- 5 0, • 7 SI sc ym 12 • 7 15- 6 2 • 9 13 .2 15. 0 1, ,8 c o n t r o l 13 • 3 13- 8 0 • 5 12 • 3 12. 7 0. .4 s c 8 +dl4-9 12 .6 15- 2 2 .6 12 .0 12. 8 0, .8 c o n t r o l 13 • 9 14. 8 0 • 9 13 .2 13- 5 0, • 3 8 ^ sc + 12 .0 14. 4 2 .4 12 • 3 13. 6 1, • 3 c o n t r o l 12 .8 13- 2 0 .4- 12 .8 13- 1 0, • 3 V2 sc 12 .8 13- 7 0 • 9 12 • 5 13- 3 0, .8 c o n t r o l 12 • 3 12. 5 0 .2 12 .1 12. 1 0. .1 4-sc 12 • 7 12. 9 0 .2 13 .1 12. 9 -0, .2 c o n t r o l 12 .2 12. 4 0 .2 13 • 3 13- 3 0 .0 asc 13 .2 15- 7 2 • 5 12 .2 14. 0 1 .8 c o n t r o l 13 .6 13. 6 0 .0 12 .8 12. 8 0 .0 12, .8 14-. ,0 1, .2 13. ,4 14 .8 1, .4 c o n t r o l 13. • 3 13- • 5 0, •2 13. ,0 12 • 9 -0 , .1 bb 14-52 * 13. .0 13. ,8 0, .8 12, -5 13 .6 1, .1 c o n t r o l 12, -9 12, • 7 -0 , .2 13. • 3 13 • 5 0, .2 These numbers r e p r e s e n t days e l a p s e d between removal o f a d u l t s and r e c o v e r y o f 50% o f t h e r e s p e c t i v e c l a s s . * I t was n e c e s s a r y t o r e c o v e r t h e s e genotypes w i t h a Ybb chromosome. The m a t e r n a l e x p e r i m e n t s i n v o l v e d f e m a l e s h e t e r o z y g o u s f o r the bb chromosome mated t o X•Y/Ybb males. 139. FIGURE 12 The D e v e l o p m e n t a l D e l a y I : Raw d a t a 140. sc +dl49/BSY x C(1)RM yw/O Progeny Recovered C u m u l a t i v e % Day Female Male Female Male 9 0 0 0.00 0.00 10 53 0 2.74 0.00 11 286 0 17.52 0.00 12 314 3 33-75 0.82 13 604 18 64 .96 5-71 14 206 39 75.61 16.31 15 270 107 89.56 45.39 16 102 71 94.83 64 .68 17 73 ' 106 98.60 93-48 18 21 7 99.69 95-38 19 6 13 100.00 98.91 20 0 4 100.00 T o t a l 1935 368 F I G U R E 13 The De v e l o p m e n t a l D e l a y C u m u l a t i v e p e r c e n t a g e r e c o v e r y p l o t s a) s c 8 A X C(1 ) R M yw/O b) s c 8 / Y X C ( 1 ) R M y w / Y c) 8/ 8 sc / s c X X - Y / O d) 8 y 8 sc / s c X X - Y / Y f e m a l e p r o g e n y - s o l i d l i n e m a l e p r o g e n y - b r o k e n l i n e 142. Day of eclosion 144. Day of eclosion 145. Cumulat ive Recovery 100 D s c 8 / sc + dl 49 X-Y/ Y 8 9 10 11 12 13 14 15 16 17 18 19 20 Day of eclosion 146. FIGURE 14 The De v e l o p m e n t a l D e l a y I I I : P r o b i t s a n a l y s i s o f d a t a a) s c 8 / Y X C(1)RM yw/O b) s c 8 / Y X C(1)RM y w / Y c) sc / s c X X - Y / O d) 8/ 8 sc / s c X X - Y / Y 147. days 148. C u m u l a t i v e .999 .998 .995 .99 .975 .95 .90 .85 .80 .75 70 .60 .50 .40 .25 .20 / < > • • • /.< • f > • • • • / • • • • • / t • > j f m • • • • • ( • • • >• < > • • • • • • r... — < • • » .15 .05 .025 .01 8 9 10 11 12 13 14 15 16 17 18 19 20 d a y s 149. days 150. days 151 • g r e a t l y r e d u c e d i n t h e s c u t e X/Y males. I n most c a s e s , the X/Y c o n t r o l males s t i l l e x h i b i t e d a d e l a y and t h i s i n d i c a t e s t h a t t h e r e may be an i n h e r e n t d e v e l o p m e n t a l problem a s s o c i a t e d w i t h the s e v e r e s c u t e mutants. T h i s o b s e r v a t i o n has been p r e v i o u s l y r e p o r t e d f o r b o t h t h e scute-L8 and s c u t e - S i i n v e r -s i o n s ( R a f f e l and M u l l e r , 194-0) . The d e v e l o p m e n t a l d e l a y was e s s e n t i a l l y e l i m i n a t e d by t h e p r e s e n c e o f a Y-chromosome i n t h e bobbed e x p e r i m e n t s . T h i s would be e x p e c t e d as t h e Y-chromosome c a r r i e s a f u l l complement o f r i b o s o m a l genes. The m a t e r n a l e f f e c t e x p e r i m e n t s showed v e r y s i m i l a r t r e n d s i n d e l a y a l t h o u g h the X/0 males were s i g n i f i c a n t l y l e s s d e v e l o p m e n t a l l y r e t a r d e d . The c o n t r o l e x p e r i m e n t s i n d i -c a t e t h a t t h e m a t e r n a l l y d e r i v e d X/0 males r e q u i r e a c o n s i d -e r a b l y l o n g e r d e v e l o p m e n t a l p e r i o d t h a n do m a t e r n a l l y d e r i v e d X/Y males. The c o m p a r i s o n o f d e v e l o p m e n t a l r a t e s between p a t e r n a l l y d e r i v e d X/0 males and m a t e r n a l l y d e r i v e d X/0 males r e v e a l s t h a t t h e f o r m e r group i s more s e v e r e l y a f f e c t e d by t h e mutant c o n d i t i o n . I n g e n e r a l , t h e X/0 f l i e s w i t h pa-t e r n a l l y i n h e r i t e d s c u t e X-chromosomes e c l o s e a f u l l day l a t e r t h a n g e n e t i c a l l y i d e n t i c a l males w i t h a m a t e r n a l l y i n -h e r i t e d X-chromosome. The m a t e r n a l e f f e c t c o n t r o l e x p e r i m e n t s p r o v i d e f u r t h e r p r o o f t h a t a d e v e l o p m e n t a l d e l a y i s a s s o c i a t e d w i t h the s e v e r e s c u t e e x p r e s s i o n s . The i n v e r s i o n s scute-L8 and s c u t e - S i b o t h e x h i b i t e d s i g n i f i c a n t d e l a y s even i n the p r e s e n c e o f a Y-chromosome. The bobbed e x p e r i m e n t s do n o t i n d i c a t e a d i f f e r e n c e 152. r e s u l t i n g from th e p a r e n t a l source o f t h e bb chromosome. The d e l a y appears t o he c o n s i s t e n t l y i n t h e range o f one t o one and a h a l f days whether the bobbed chromosome i s i n h e r i t e d p a t e r n a l l y o r m a t e r n a l l y . P h e n o t y p i c a l l y , t h e bb/Ybb males r e c o v e r e d from t h e m a t e r n a l e x p e r i m e n t s were i n d i s t i n g u i s h a b l e from t h e males r e c o v e r e d i n the p a t e r n a l e x p e r i m e n t s . They e x h i b i t e d a s e v e r e l y bobbed b r i s t l e p a t t e r n and abdominal e t c h i n g was a f r e q u e n t o b s e r v a t i o n . I t i s e v i d e n t t h a t t h e s e r i b o s o m a l d e f i c i e n t phenotypes a r e independent o f t h e m a t e r n a l e f f e c t s t h a t have been c h a r a c t e r i z e d f o r t h e s c u t e i n v e r s i o n s . There were two s c u t e i n v e r s i o n chromosomes t h a t d i d not e x h i b i t p r o f o u n d l y d e l a y e d development i n t h e x/0 c o n d i t i o n . Scute-V2 has been p r e v i o u s l y c h a r a c t e r i z e d as m a n i f e s t i n g m i l d e x p r e s s i o n s o f t h e numerous phenomena a s s o c i a t e d w i t h t h e o t h e r s c u t e i n v e r s i o n s . The development o f scute-V2 x/0 males appears t o be r e t a r d e d as compared t o c o n t r o l s but n o t as s e v e r e l y as o b s e r v e d w i t h t h e p r e v i o u s i n v e r s i o n s . There i s no a p p arent d i f f e r e n c e a t t r i b u t a b l e t o the p a r e n t a l s o u r c e o f s c u t e - V 2 . The d e l a y o b s e r v e d i n t h e c o n t r o l s i t u a t i o n s i s s m a l l and may r e s u l t from s i m p l e s a m p l i n g e r r o r . These d a t a a r e e n t i r e l y c o n s i s t e n t w i t h the h y p o t h e s i s t h a t scute-V2 r e p r e s e n t s a m i l d l y a f f e c t e d r e p r e s e n t a t i v e o f t h e s c u t e i n -v e r s i o n d e f e c t s . S c u t e - 4 i s n o t a s s o c i a t e d w i t h s i g n i f i c a n t l e t h a l i t y i n t h e X/0 c o n d i t i o n . P h e n o t y p i c a l l y , t h e s e i n d i v i d u a l s are m o d e r a t e l y s c u t e and r a r e l y a c h a e t e . I n t h e s e d e v e l o p m e n t a l 153-e x p e r i m e n t s , scute-4 d i d not e x h i b i t an a p p r e c i a b l e d e v e l o p -m e n t a l d e l a y e i t h e r i n t h e X/0 o r the X/Y c o n d i t i o n . I t has been p r e v i o u s l y o b s e r v e d t h a t scute-4 shows l i t t l e p h e n o t y p i c change between t h e s e two c o n d i t i o n s which may i n d i c a t e t h a t t h e s c u t e e x p r e s s i o n i n scute-4 may a r i s e from a t r u e muta-t i o n r a t h e r t h a n a v a r i e g a t i n g e x p r e s s i o n . T h i s t h e o r y i s s u p p o r t e d by t h e u n i f o r m phenotype p r e s e n t e d by scute-4 when i t i s s u b j e c t e d t o c l a s s i c a l m o d i f i e r s o f p o s i t i o n e f f e c t v a r i e g a t i o n . A l t e r n a t e l y , t h e v a r i e g a t e d e x p r e s s i o n o f s c u t e may be r a d i c a l l y d i f f e r e n t owing t o t h e s p e c i f i c r e g i o n o f h e t e r o -c h r o m a t i n i n f l u e n c i n g t h e s c u t e gene. T h i s t h e o r y i s espec-i a l l y p l a u s i b l e i n l i g h t o f r e c e n t r e s e a r c h c o n c e r n i n g the non-random d i s t r i b u t i o n o f s a t e l l i t e sequences i n the l a r g e b l o c k s o f h e t e r o c h r o m a t i n . Scute-4 i s b r o k e n i n t h e d i s t a l X h e t e r o c h r o m a t i n w h i l e the b r e a k s o f the c l a s s i c s c u t e i n v e r s i o n s ( s c 8 , s c ^ , s c ^ ) a r e i n t h e p r o x i m a l h e t e r o c h r o -m a t i n . The b r e a k p o i n t o f scute-V2 appears t o be i n t h e mid-d l e o f the rDNA gene c l u s t e r and t h i s s p e c i a l f e a t u r e m e r i t s f u r t h e r s t u d y e l s e w h e r e . I t remains t o be seen whether the s p e c i f i c d e v e l o p m e n t a l d e l a y s a s s o c i a t e d w i t h the s c u t e i n -v e r s i o n s a r e c o r r e l a t e d w i t h d i s t i n c t i v e r e g i o n s o f h e t e r o -c h r o m a t i n . The purpose o f e x a m i n i n g the d e v e l o p m e n t a l d e l a y i n the bobbed l e t h a l i n d i v i d u a l s was t o determine t h e e x t e n t o f t h i s phenomenon and t o c o r r e l a t e the d e l a y w i t h the bobbed pheno-154. t y p e . I n a l l e x p e r i m e n t s , t h e Xbb'VYbb i n d i v i d u a l s were phen-o t y p i c a l l y s e v e r e l y bobbed. The d e l a y o b s e r v e d was a p p r o x i -m a t e l y one day and t h i s e f f e c t was e l i m i n a t e d i n the p r e s e n c e o f a Y-chromosome. The d e v e l o p m e n t a l d e l a y a s s o c i a t e d w i t h a s e v e r e bobbed c o n d i t i o n i s t h u s q u a n t i f i e d . Comparing the d e v e l o p m e n t a l r e s u l t s between t h e bobbed i n d i v i d u a l s and t h e s c u t e i n d i v i d u a l s i t i s apparent t h a t the d e l a y a s s o c i a t e d w i t h t h e s c u t e mutant i s c o n s i d e r a b l y l o n g e r . A l s o , the s c u t e X/0 males a r e not a s s o c i a t e d w i t h a bobbed phenotype o f any k i n d . These -experiments have e s t a b l i s h e d t h e f o l l o w i n g l i n e s o f e v i d e n c e s u g g e s t i n g t h a t t h e d e v e l o p m e n t a l d e l a y i s not a r e s u l t o f rDNA d i s f u n c t i o n . The d e l a y i s e v i d e n t i n t h e X/Y s i t u a t i o n a l t h o u g h a f u l l complement o f rDNA i s p r e s e n t . There i s a d i f f e r e n c e i n d e v e l o p m e n t a l time which i s depen-dent upon t h e p a r e n t a l s o u r c e o f i n v e r s i o n . The d e v e l o p m e n t a l r e t a r d a t i o n a s s o c i a t e d w i t h the s c u t e r e a r r a n g e m e n t s i s more l e n g t h y t h a n t h a t accompanying t h e bobbed mutants. The p e r i o d o f d e l a y o b s e r v e d f o r t h e bobbed mutants i s s u f f i c i e n t t o produce a severe bobbed phenotype w h i l e no such phenotype i s i d e n t i f i e d i n the s c u t e X/0 males. These l i n e s o f ev i d e n c e a r e s u f f i c i e n t t o suggest t h a t t h e d e v e l o p m e n t a l d e l a y and a s s o c i a t e d l e t h a l i t y a r e n o t e n t i r e l y a r e s u l t o f rDNA d i s -f u n c t i o n . 155-- C h a p t e r 8 -The B r i s t l e Phenotypes The most c h a r a c t e r i s t i c p h e n o t y p i c e f f e c t a s s o c i a t e d w i t h t h e s c u t e X/0 c o n d i t i o n i s t h e a l t e r a t i o n o f b r i s t l e o r i e n t a t i o n and e x p r e s s i o n . I n an e f f o r t t o q u a n t i t a t e t h e s e changes, a v a r i e t y o f X/0 males have been g e n e r a t e d and spec-i f i c b r i s t l e f r e q u e n c i e s have been r e c o r d e d . F o r each exper-iment, f o r t y i n d i v i d u a l s were examined and t h e number o f s c u t e b r i s t l e s and t h o r a c i c m i c r o c h a e t a e were r e c o r d e d . I n -d i v i d u a l s o f t h e X/X, X/Y and X/0 genotype were s t u d i e d . A c o m p a r i s o n o f X/0 males w i t h m a t e r n a l l y o r p a t e r n a l l y i n h e r -i t e d s c u t e X-chromosomes was a l s o i n c l u d e d . The v a r i e g a t i o n o f t h e achaete l o c u s was i n v e s t i g a t e d by r e c o r d i n g t h e number o f t h o r a c i c m i c r o c h a e t a e . The t h o r -a c i c e p i d e r m i s t h a t was u t i l i z e d f o r t h i s s t u d y was d e f i n e d by the a r e a e n c l o s e d by t h e p r e s u t u r a l and p o s t e r i o r p o s t a l a r b r i s t l e s . The d a t a f o r t h i s s t u d y a r e p r e s e n t e d i n T a b l e 13 and g r a p h i c a l l y d e p i c t e d i n F i g u r e 15- The m a j o r i t y o f 95$ c o n f i d e n c e i n t e r v a l s l i e between 4 and 10 b r i s t l e s . T h i s s u g g e s t s t h a t t h e s t a n d a r d d e v i a t i o n s range from 12 t o 30 i n t h e s e d a t a . T h i s a l s o i n d i c a t e s t h a t 95$ o f t h e i n d i v i d u a l d a t a p o i n t s w i l l l i e w i t h i n 24 t o 60 b r i s t l e s (two s t a n d a r d d e v i a t i o n s ) o f t h e mean v a l u e e s t i m a t e d f o r t h e s p e c i f i c p o p u l a t i o n . The d a t a c o l l e c t e d on m i c r o c h a e t a e bear out a number o f TABLE 13 M i c r o c h a e t a e Frequency x/x X/Y P a t e r n a l x/o M a t e r n a l X/0 s c S 1 +S yB 227.8 + 4.2 220 •1 ± 4.8 160 . 5 + 10 . 5 182.0 + 5 . 5 L8 sc m 208.2 + 3 . 7 198 . 4 + 4 . 3 179 . 9 + 9 . 3 191 . 3 + 4-2 s c 8 + d l 4 9 1 9 2 . 4 + 5 . 5 203 .2 + 4.2 9 2 . 7 + 11 . 6 131 . 6 + 5-6 V2 sc 124 . 5 + 6 . 6 138 •3 + 8.0 6 8 . 4 + 6 . 4 75-1 + 4 . 7 S1L 8R 219 . 4 + 4 . 5 201 •9 + 5-2 116 . 5 + 9-7 lZj . 1 .9 + 4 . 8 V2L 8R 1 3 6 . 7 + 7-0 127 .8 + 6 . 5 79.8 + 5 . 6 7 6 . 5 + 5 . 5 8L V2R 210.2 + 4 . 3 232 .0 + 3 . 3 192.0 + 4.0 2 0 6 . 9 + 5-1 + 2 5 3 . 9 + 3.1 258 •1 + 3 . 3 240 . 4 + 2 . 4 244 . 9 + 3 - 8 (95$ c o n f i d e n c e i n t e r v a l s ) 157-FIGURE 15 B r i s t l e f r e q u e n c y i n a  s e r i e s o f s c u t e i n v e r s i o n X/0 males. I : The m i c r o c h a e t a e 158. i o f w i l d t y pe L8 sc m S1 n SC y B 8 sc sc V2 B a r s r e p r e s e n t : ( l e f t t o r i g h t ) X/X, X/Y, p a t e r n a l l y d e r i v e d X/0, m a t e r n a l l y d e r i v e d X/0 159. c o n s i s t e n t t e n d e n c i e s . A l l o f t h e s c u t e i n v e r s i o n s r e p o r t e d i n t h e s e e x p e r i m e n t s e x h i b i t e d a reduced e x p r e s s i o n o f t h o r -a c i c m i c r o c h a e t a e . E x c e p t i n g f o r s c u t e - L 8 , the f r e q u e n c y o f m i c r o c h a e t a e i n X/0 males i s g r e a t l y r e d u c e d as compared t o X/X f e m a l e s o r X/Y males. As one v i e w s F i g u r e 15 from l e f t t o r i g h t , one may suggest t h a t t h e d a t a a r e a r r a n g e d i n an i n c r e a s i n g l y mutant p r o g r e s s i o n w i t h r e s p e c t t o the achaete l o c u s . I t i s v e r y e v i d e n t t h a t scute-L8 most c l o s e l y resem-b l e s w i l d t y p e w h i l e scute-V2 e x h i b i t s t h e most severe mu-t a n t c o n d i t i o n . I t i s -also a p p a r e n t t h a t t h e m a t e r n a l l y d e r i v e d X/0 males a r e l e s s mutant t h a n the males w i t h p a t e r n a l l y i n h e r i t e d s c u t e X-chromosomes. T h i s o b s e r v a t i o n i s c o n s i s t e n t t h r o u g h -out b o t h m i c r o c h a e t a e and macrochaetae e x p e r i m e n t s and i s b e l i e v e d t o r e s u l t from s a m p l i n g problems. I n t h e p a t e r n a l e x p e r i m e n t s , t h e r e i s an i s o l a t e d group o f X/0 males r e c o v -e r e d t h a t a r e a s s o c i a t e d w i t h g r e a t l y extended development and a more s e v e r e mutant phenotype. T h i s s p e c i f i c group i s n o t observed i n t h e m a t e r n a l e x p e r i m e n t s and, t h e r e f o r e , s a m p l i n g from t h e s e two p o p u l a t i o n s r e s u l t i n d i f f e r e n t d a t a . T h i s t h e o r y i s s u b s t a n t i a t e d by d a t a r e c o r d e d i n T a b l e 13 • The c o n f i d e n c e i n t e r v a l s and s t a n d a r d d e v i a t i o n s are n o t i c e -a b l y l a r g e r as a r e s u l t o f the s m a l l number o f l i e s t h a t d e v i a t e g r e a t l y from t h e mean i n the p a t e r n a l e x p e r i m e n t s . I t may be p o s s i b l e t o e l i m i n a t e t h i s s o u r c e o f v a r i a n c e by s a m p l i n g the f l i e s p r i o r t o e c l o s i o n o f t h e g r e a t l y d e l a y e d 160. i n d i v i d u a l s . The v a r i e g a t i o n o f t h e s c u t e l o c u s was i n v e s t i g a t e d by r e c o r d i n g t h e number o f s c u t e l l a r b r i s t l e s i n the same popu-l a t i o n as above. These d a t a a r e r e p o r t e d i n T a b l e lk and d e p i c t e d g r a p h i c a l l y i n F i g u r e 16. The w i l d type r e s u l t s v e r y c l o s e l y approach t h e maximum v a l u e o f 160. T h i s v a l u e r e p r e s e n t s a f u l l complement o f f o u r b r i s t l e s p e r s c u t e l l u m . The g r a p h i c a l r e s u l t s o f t h e d a t a i n d i c a t e a number o f c o n s i s t e n t t r e n d s . I n a l l c a s e s , the X/0 c o n d i t i o n i s a s s o -c i a t e d w i t h a d r a m a t i c d e c r e a s e i n t h e number o f s c u t e l l a r b r i s t l e s as. compared t o X/X f e m a l e s o r X/Y males. The most c o n s p i c u o u s t r e n d r e v e a l e d by t h e s e d a t a i s the mutant p r o -g r e s s i o n from l e f t t o r i g h t . I t i s a p parent t h a t s c u t e - L 8 i s the most s e v e r e l y a f f e c t e d w i t h r e s p e c t t o t h e s c u t e l o c u s . Scute-V2 most c l o s e l y r e s e m b l e s w i l d type e x p r e s s i o n o f the s c u t e l l a r b r i s t l e s but shows a c o n s i d e r a b l e r e d u c t i o n i n e x p r e s s i o n i n t h e X/0 c o n d i t i o n . The d a t a a r e i n s u f f i c i e n t t o e s t a b l i s h d i f f e r e n c e s i n s c u t e e x p r e s s i o n between p a t e r n a l l y d e r i v e d X/0 males and X/0 i n d i v i d u a l s i n h e r i t i n g t h e i r X-chromosome m a t e r n a l l y . From a group o f f o r t y i n d i v i d u a l s , t h e r e a r e o n l y 160 b r i s t l e e v e n t s t o m o n i t o r i n a s i n g l e sample. J u d g i n g from the v a r i -ances e s t a b l i s h e d i n p r e v i o u s s t u d i e s f o r macrochaetae ( R e n d e l , I963) i t would be improper t o suggest t h a t a d i f f e r -ence e x i s t s between X/0 males o f d i f f e r e n t p a r e n t a l d e r i v a -t i o n . I n v e s t i g a t i o n o f t h i s phenomenon would r e q u i r e numerous 161. TABLE 14 S c u t e l l a r B r i s t l e Frequency P a t e r n a l M a t e r n a l X/X X/Y X/0 X/0 Q1 sc 1 +S yB 113 95 52 56 s c L 8 m 63 48 14 22 Q sc +dl49 144 129 78 86 s c ^ 147 145 118 114 S1L 8R 132 139 89 99 V2L 8R 151 146 123 129 8L V2R 148 144 131 128 + 155 158 155 157 (raw count o f s c u t e l l a r b r i s t l e s from 40 i n d i v i d u a l s ) t 162. FIGURE 16 B r i s t l e f r e q u e n c y i n a  s e r i e s o f s c u t e i n v e r s i o n X/0 males. I I : The s c u t e l l a r s 163. fo Of w i l d t y p e 100 90 -80 I 70 • ~~ 60 • 1 50 -4 0 - 1 I 30 1 20 10 L8 SI 8 V2 sc m sc y B sc sc B a r s r e p r e s e n t : ( l e f t t o r i g h t ) X/X, X/Y, p a t e r n a l l y d e r i v e d X/0, m a t e r n a l l y d e r i v e d X/0 164. samples t o e s t a b l i s h a t r e n d o f t h i s n a t u r e . The d a t a a r e s u f f i c i e n t l y r o b u s t t o i n d i c a t e d i f f e r e n c e s between s t o c k s , however. A comparison o f F i g u r e 15 and 16 i n d i c a t e t h a t t h e mu-t a n t t r e n d s e s t a b l i s h e d f o r t h e s e f o u r s c u t e i n v e r s i o n s are i n v e r s e l y r e l a t e d . F o r example, s c u t e - L 8 e x p r e s s e s t h e most s e v e r e s c u t e phenotype w h i l e m a n i f e s t i n g a v i r t u a l l y w i l d t y p e achaete phenotype. The scute-V2 d a t a r e p r e s e n t s the c o n v e r s e s i t u a t i o n as t h e achaete l o c u s i s most p r o f o u n d l y a f f e c t e d . T h i s p o l a r i t y o f e x p r e s s i o n may r e p r e s e n t a f u n d -amental p r o p e r t y o f p o s i t i o n e f f e c t v a r i e g a t i o n o p e r a t i n g on s l i g h t l y d i f f e r e n t e u c h r o m a t i c b r e a k p o i n t s i n the a c h a e t e -s c u t e r e g i o n . A l t e r n a t i v e l y , t h i s r e l a t i o n s h i p may r e f l e c t d i s t i n c t i v e c a p a c i t i e s o f s p e c i f i c r e g i o n s o f h e t e r o c h r o m a t i n t o i n d u c e p o s i t i o n e f f e c t v a r i e g a t i o n . The d i s c r i m i n a t i o n o f t h e s e two p o s s i b i l i t i e s r e mains a s u b j e c t f o r f u r t h e r r e s e a r c h . 165-DISCUSSION The p r e c e d i n g s e c t i o n s o f t h i s t h e s i s d e s c r i b e the i n -v e s t i g a t i o n o f many f a c t o r s a s s o c i a t e d w i t h t h e s c u t e i n v e r -s i o n phenotypes. I t has been e s t a b l i s h e d t h a t t h e s e r e a r r a n g e -ments a r e a s s o c i a t e d w i t h a r e d u c t i o n o f macro- and/or micro--c h aetae as a p r i m a r y v a r i e g a t i n g phenotype. The g e n e r a t i o n o f s c u t e X/0 i n d i v i d u a l s i n d i c a t e s t h a t a d e v e l o p m e n t a l d e l a y and i n v i a b i l i t y a r e a l s o a r e s u l t o f t h e rearrangement. These a n o m a l i e s have been s y s t e m a t i c a l l y s t u d i e d and t h e r e s u l t s c a n be c o n s o l i d a t e d t o d e v e l o p a r e a s o n a b l e e x p l a n a t i o n o f t h e phenomena. The p a t e r n a l e f f e c t e x p e r i m e n t s ( C h a p t e r 1) c o r r o b o r a t e t h e r e s u l t s p r e s e n t e d by p a s t a u t h o r s (Hess, 1962; B a k e r , 1971) . The p h e n o t y p i c s t u d i e s o f t h e p a t e r n a l l y d e r i v e d s c u t e X/0 males i n d i c a t e t h a t each i n v e r s i o n i s accompanied by a v e r y c h a r a c t e r i s t i c b r i s t l e e x p r e s s i o n . I n most c a s e s , b o t h t h e genes achaete and s c u t e a r e d i s t u r b e d by t h e r e a r -rangement. D e v e l o p m e n t a l l y , t h e X/0 males are r e t a r d e d and o f t e n e x h i b i t s p a s t i c c o n t r o l o f wings and l e g s . I t i s n o t u n u s u a l t o observe t h e s e f l i e s e n t r a p p e d i n the f o o d as a r e s u l t o f t h e i r d e c r e p i t c o n d i t i o n . The l e t h a l i t y o b s e r v e d i n the p a t e r n a l e x p e r i m e n t s was extreme. I n most c a s e s , t h e X/0 male v i a b i l i t y was 15% o r l e s s t h a t o f the c o n t r o l . I n a l l c a s e s , t h e a d d i t i o n o f a Y-chromosome i n t o t h e genome 166. r e s u l t e d i n r e s t o r a t i o n t o almost normal v i a b i l i t y . The m a t e r n a l e f f e c t e x p e r i m e n t s ( C h a p t e r 1) e x t e n d t h e work i n i t i a t e d by H i d e h H a r g e r (197*0 and c o r r o b o r a t e h e r d a t a f o r t h e S1L 8R chromosome. The most i m p o r t a n t concept d e f i n e d by t h i s e xperiment i s t h e c o n d i t i o n a l l e t h a l i t y a s -s o c i a t e d w i t h t h e s c u t e X/0 males. When t h e s c u t e chromosome i s i n h e r i t e d p a t e r n a l l y , t h e X/0 i n d i v i d u a l s a re l a r g e l y i n -v i a b l e . However, when t h e X/0 male d e r i v e s i t s s c u t e X - c h r o -mosome from t h e f e m a l e , v i a b i l i t y i s g r e a t l y enhanced. T h i s phenomenon i s termed t h e s c u t e m a t e r n a l e f f e c t . A l s o a s s o c i a t e d w i t h t h e m a t e r n a l e f f e c t a r e n o t i c e a b l e r e d u c t i o n s o f s e v e r i t y o f t h e jsc and ac v a r i e g a t i n g phenotypes as compared t o t h e p a t e r n a l e f f e c t phenotypes. The d e v e l o p -m e n t a l d e l a y a s s o c i a t e d w i t h t h e s c u t e X/0 males i s a l s o s i g -n i f i c a n t l y r e d u c e d i n t h e m a t e r n a l e x p e r i m e n t s . The g e n e r a t i o n o f t h e v a r i o u s recombinant s c u t e i n v e r s i o n s p r o v i d e d f u r t h e r m a t e r i a l f o r i n v e s t i g a t i o n . I t i s apparent t h a t t h e r e c o m b i n a n t s produce v e r y s i m i l a r r e s u l t s t o thos e o b s e r v e d i n t h e p r e v i o u s two s t u d i e s ( C h a p t e r 2 ) . The p a t e r -n a l e x p e r i m e n t s e x h i b i t c o n s i d e r a b l e l e t h a l i t y and t h e s u r v i -v i n g X/0 males a r e d e v e l o p m e n t a l l y r e t a r d e d and s e v e r e l y mu-t a n t . The m a t e r n a l t r a n s m i s s i o n o f t h e i n v e r s i o n r e s u l t s i n an i n c r e a s e i n X/0 v i a b i l i t y and a p a r t i a l r e v e r s i o n o f the s c u t e - a c h a e t e phenotype. These e x p e r i m e n t s suggest t h a t the an o m a l i e s a s s o c i a t e d w i t h t h e s c u t e i n v e r s i o n s a r e g e n e r a l phenomena r a t h e r t h a n s p e c i f i c p e c u l i a r i t i e s i n h e r e n t t o t h e 167. p a r e n t a l chromosomes. A number o f a u t h o r s s t u d y i n g m a t e r n a l e f f e c t s have sug-g e s t e d t h a t i n d i v i d u a l s d e r i v e d from a female h e t e r o z y g o u s f o r a v a r i e g a t i n g gene a r e more mutant t h a n t h o s e d e r i v e d from a homozygous female ( N o u j d i n , 1944; L u n i n g , 1954; S p o f f o r d , 1959) . The s t u d y o f t h e h e t e r o z y g o u s m a t e r n a l e f -f e c t s ( C h a p t e r 3) i n d i c a t e t h a t t h e s c u t e X/0 i n d i v i d u a l s have comparable v i a b i l i t y t o t h o s e X/0 f l i e s d e r i v e d from homozygous s c u t e f e m a l e s . The phenotypes t h a t were d e r i v e d from t h e s e e x p e r i m e n t s were n o t a p p r e c i a b l y d i f f e r e n t t h a n t h o s e r e c o v e r e d from homozygous f e m a l e s . The d a t a a l s o sug-g e s t t h a t t h e phenotypes and i n c r e a s e d r e c o v e r y o f t h e X/0 males was independent o f t h e rDNA o r X - h e t e r o c h r o m a t i c con-t e n t o f t h e p a r e n t a l f e m a l e . The i n v e s t i g a t i o n o f f a c t o r s i n f l u e n c i n g the p a t e r n a l e f f e c t i n v o l v e d g e n e r a t i n g X/0 males w i t h s l i g h t l y a l t e r e d genotypes ( C h a p t e r 4 ). A number o f d u p l i c a t i o n s d i f f e r i n g i n rDNA c o n t e n t were t r a n s m i t t e d from t h e female t o t h e X/0 i n -d i v i d u a l . A s m a l l , but c o n s i s t e n t i n c r e a s e i n v i a b i l i t y ap-p e a r s t o be r e l a t e d t o t h e X-chromosome rDNA r e g i o n . Two o f t h e s e s m a l l d u p l i c a t i o n s a l s o c a r r i e d an a c t i v e s c u t e l o c u s and t h e p r e s e n c e o f t h i s f a c t o r i n t h e X/0 genome g r e a t l y i n c r e a s e d t h e i r v i a b i l i t y . F o u r o f the recombinant chromo-somes r e c o v e r e d were d u p l i c a t e d f o r the s c u t e l o c u s . There was a s i g n i f i c a n t d e c r e a s e i n v i a b i l i t y when the t h i r d s c u t e l o c u s was t r a n s f e r r r e d i n t o t h e genome and t h i s male t r i p l o -168. s c u t e l e t h a l i t y has been p r e v i o u s l y d i s c u s s e d by G a r c i a - B e l l i d o and S a n t a m a r i a (1978). As t h e d u p l i c a t i o n s a r e l a r g e l y h e t -e r o c h r o m a t i c , one c o u l d a l s o d etermine t h a t a component o f t h e v i a b i l i t y change was a s s o c i a t e d w i t h t h i s f a c t o r . From t h e s e e x p e r i m e n t s , i t i s apparent t h a t t h e f a c t o r s most p r o -f o u n d l y i n f l u e n c i n g t h e v i a b i l i t y o f the s c u t e X/0 males are t h e p r e s e n c e o f an a c t i v e s c u t e l o c u s and genomic h e t e r o c h r o -m a t i n c o n t e n t . The p r e s e n c e o f an X-chromosomal bobbed r e -g i o n appears t o be much l e s s i n f l u e n t i a l . These e x p e r i m e n t s were a l s o completed, w i t h Y-chromosomes w h i c h d i f f e r e d w i t h r e s p e c t t o t h e i r rDNA c o n t e n t . A d e l a t i o n o f a p o r t i o n o f t h e s h o r t arm o f t h e Y-chromosome r e s u l t e d i n a Ybb" c o n s t i t u t i o n . U n f o r t u n a t e l y , t h e amount o f h e t e r o -c h r o m a t i n l o s t i n t h i s rearrangement i s d i f f i c u l t t o q u a n t i -t a t e owing t o t h e l a c k o f g e n e t i c markers on the Y-chromos;ome. These s t u d i e s i n d i c a t e t h a t the p r e s e n c e o f the Ybb" i n the s c u t e X/0 genome i s a s s o c i a t e d w i t h a reduced l e v e l o f v i a b i l -i t y when compared t o t h e Y b b + c o n t r o l s . T h i s e f f e c t c o u l d be a t t r i b u t a b l e t o t h e l a c k o r rDNA o r a d e f i c i e n c y f o r a n o t h e r f a c t o r removed by t h e d e f i c i e n c y . Other s t u d i e s i n v o l v i n g ; m o d i f i e d Y-chromosomes i n d i c a t e t h a t r e s u l t s o b t a i n e d from such e x p e r i m e n t s a r e h i g h l y v a r i a b l e and t h a t the v a r i e g a t i o n s u p p r e s s i o n a c t i v i t y o f t h e Y-chromosomal m a t e r i a l s i s n o t a f u n c t i o n o f t h e l e n g t h o f t h e fragment (Baker and S p o f f o r d , 1959; Hess, 1970; B a k e r , 1971)- I t seems v e r y p o s s i b l e t h a t v e r y l o c a l i z e d a r e a s o f t h e Y-chromosome may e x e r t p r o f o u n d 169-e f f e c t s on v a r i e g a t i o n . M a t e r n a l ( e f f e c t e x p e r i m e n t s were c a r r i e d , out f o r t h e s e same chromosomes. The major f a c t o r i n f l u e n c i n g X/0 male v i a -b i l i t y a ppears t o be t h e p r e s e n c e o f the s t a n d a r d Y-chromosome bobbed r e g i o n . The o t h e r f a c t o r s m o d i f i e d v i a b i l i t y t o a l e s -s e r e x t e n t . These e x p e r i m e n t s were most s e n s i t i v e t o the l e t h a l i t y a s s o c i a t e d w i t h t h r e e doses o f t h e s c u t e gene. A l l o f t h e recombinant chromosomes c a r r y i n g two s c u t e l o c i showed a d r a m a t i c d e c r e a s e i n v i a b i l i t y when a t h i r d l o c u s was sup-p l i e d t o t h e system. These d a t a i n d i c a t e t h a t the t r i p l o -s c u t e l e t h a l i t y i s a phenomenon independent o f the m a t e r n a l e f f e c t . I n the c o u r s e o f t h e s e s t u d i e s , i t became ap p a r e n t t h a t t h e compound X-chromosome c o n s t i t u t i o n i n f l u e n c e d t h e pheno-t y p e o f s c u t e X/0 males (C h a p t e r 5 ) . F o r t u i t o u s l y , i t wa::: d i s c o v e r e d t h a t t h e v i a b i l i t i e s o f t h e s e i n d i v i d u a l s was v a r i a b l e and dependent upon the p a r t i c u l a r compound-X female from which t h e y were r e c o v e r e d . I t i s s u s p e c t e d t h a t t h i s d i f f e r e n c e i n v i a b i l i t y may be due t o the d i f f e r i n g h e t e r o -c h r o m a t i c c o n t e n t s r e s u l t i n g from the c o n s t r u c t i o n o f com-pound X-chromosomes. The most v i a b l e s c u t e X/0 f l i e s a r e d e r i v e d from a compound-X which c o n t a i n s a c c e s s o r y Y-chromo-somal h e t e r o c h r o m a t i n . U n f o r t u n a t e l y , l i t t l e i s known about the h e t e r o c h r o m a t i c c o n t e n t o f t h e o t h e r a t t a c h e d X-chromo-somes and t h u s , any p r o p o s a l o f a mechanism f o r t h i s pheno-menon would be e n t i r e l y s p e c u l a t i v e . 170. Temperature i s a c l a s s i c m o d i f i e r o f p o s i t i o n - e f f e c t v a r i e g a t i o n ( C h a p t e r 6 ) . F o r t h e s c u t e m u t a t i o n , one o b s e r -v e s a more severe phenotype a t l o w e r t e m p e r a t u r e s and i n c r e a s -i n g l y w i l d phenotypes as one approaches 29°C. Mu t a n t s w i t h d i s t u r b a n c e s i n rRNA f u n c t i o n p r e s e n t an o p p o s i t e developmen-t a l p r o f i l e . A t low t e m p e r a t u r e s , t h e bobbed mutants are a b l e t o compensate f o r t h e m u t a t i o n and t h i s r e s u l t s i n a l e s s mu-t a n t phenotype (Mohan and R i t o s s a , 1970). A t h i g h e r tempera-t u r e s , development i s more r a p i d , compensation i s l i m i t e d , ! and a more s e v e r e phenotype r e s u l t s . The d e f i n i t i o n o f a tempera-t u r e e f f e c t i n v o l v e d w i t h t h e s c u t e X/0 l e t h a l i t y may p r o v i d e e v i d e n c e s u p p o r t i n g one o f t h e s e two d e v e l o p m e n t a l pathways. These e x p e r i m e n t s i n d i c a t e t h a t t h e l e t h a l i t y a s s o c i a t e d w i t h s c u t e X/0 males i s more severe a t reduced t e m p e r a t u r e s . There i s no s i g n i f i c a n t d i f f e r e n c e between v i a b i l i t e s a t 29° o r 25° but t h e r e i s a c o n s i d e r a b l e r e d u c t i o n i n v i a b i l i t y o b s e r v e d a t 18°. T h i s e f f e c t was r e c o g n i z e d i n b o t h m a t e r n a l and p a t e r n a l e x p e r i m e n t s and t h i s p r o v i d e s e v i d e n c e t h a t t h e l e t h a l i t y a s s o c i a t e d w i t h t h e s c u t e i n v e r s i o n s may be con-c o m i t a n t w i t h d i s t u r b a n c e s o f t h e euc h r o m a t i c s c u t e f u n c t i o n s , r a t h e r t h a n d i s r u p t i o n o f t h e rRNA system. B o t h rDNA d e f i c i e n c i e s and s c u t e i n v e r s i o n mutants a r e a s s o c i a t e d w i t h r e t a r d e d development. I n an e f f o r t t o d i s -c r i m i n a t e between t h e s e two phenomena, t h e development o f b o t h bobbed and s c u t e X/0 i n d i v i d u a l s has been s t u d i e d w i t h r e s p e c t t o t h e d e l a y i n m a t u r a t i o n and t h e r e s u l t a n t pheno-171 • t y p e ( C h a p t e r 7 ) • I n the p a t e r n a l e x p e r i m e n t s , the s c u t e mutants a r e d e v e l o p m e n t a l l y d e l a y e d by two days and e x h i b i t c h a r a c t e r i s t i c s c u t e phenotypes f o r each i n v e r s i o n . The bobbed mutants were d e v e l o p m e n t a l l y d e l a y e d by one day i n t h e p a t e r n a l e x p e r i m e n t s and were bobbed i n phenotype. No such bobbed phenotype was e v e r observed i n t h e e x p e r i m e n t s i n v o l v e d w i t h the s c u t e i n v e r s i o n s . The d e v e l o p m e n t a l d e l a y a s s o c i a t e d w i t h the s c u t e mater-n a l e f f e c t s i s s i g n i f i c a n t l y r e d u c e d as compared t o t h e p a t e r n a l d e l a y . No such d i f f e r e n c e was n o t e d f o r t h e bobbed mutants. The m a t e r n a l e f f e c t e x p e r i m e n t s i n v o l v i n g the bobbed mutants a l s o produced s t r o n g l y bobbed i n d i v i d u a l s , The c o r r e l a t i o n o f d e v e l o p m e n t a l d e l a y and bobbed pheno-ty p e has been e s t a b l i s h e d f o r two d i f f e r e n t bobbed mutants. I t i s apparent t h a t a one t o two day d e l a y i s s u f f i c i e n t t o produce a c o n s i d e r a b l y severe bobbed phenotype. The d e l a y o b s e r v e d i n t h e s c u t e e x p e r i m e n t s i s c o n s i s t e n t l y two day:;; and l o n g e r and y e t , t h e s e X/0 males a r e not a s s o c i a t e d w i t h any v i s i b l e . b o b b e d phenotype. These r e s u l t s suggest t h a t t h e s c u t e and bobbed d e v e l o p m e n t a l d e l a y s a r e d i f f e r e n t phenomena. T h i s e v i d e n c e i s s u f f i c i e n t t o suggest t h a t the d e v e l o p m e n t a l d e l a y and a s s o c i a t e d l e t h a l i t y a re n o t e n t i r e l y a r e s u l t o f rDNA d i s f u n c t i o n . The f i n a l c h a p t e r i n v e s t i g a t e d t h e b r i s t l e phenotypes t h a t were a s s o c i a t e d w i t h t h e v a r i e g a t i n g a c h a e t e - s c u t e l o c u s ( C h a p t e r 6 ) . The d a t a c o l l e c t e d on t h e m i c r o c h a e t a e i n d i c a t e 172. t h a t t h e r e a r e c h a r a c t e r i s t i c l e v e l s o f b r i s t l e e x p r e s s i o n f o r each i n v e r s i o n and. t h i s l e v e l i s reduced i n the absence o f a Y-chromosome. There appears t o be a mutant p r o g r e s s i o n t h a t i s a s s o c i a t e d w i t h the s p e c i f i c b r e a k p o i n t s o f the i n -v e r s i o n s (see F i g u r e s 15 and 16). The i n v e r s i o n s c u t e - L 8 , r e p r e s e n t s t h e most, w i l d t y p e mutant w h i l e scute-V2 r e p r e s e n t s t h e most s e v e r e mutant s t o c k w i t h r e s p e c t t o t h e achaete l o c u s . The d a t a concerned w i t h t h e s c u t e gene s u g g e s t s t h a t a mutant p r o g r e s s i o n a l s o e x i s t s f o r t h i s l o c u s . Most s i g n i f i -c a n t l y , t h e p r e g r e s s i o n i s o p p o s i t e t h a t observed f o r t h e achaete l o c u s as s c u t e - L 8 i s t h e most s e v e r e l y a f f e c t e d , w h i l e scute-V2 i s p r e d o m i n a n t l y w i l d t y p e . T h i s d i f f e r e n c e may r e s u l t from t h e s p e c i f i c e u c h r o m a t i c b r e a k p o i n t s w i t h r e s p e c t t o t h e s c u t e l o c u s . I t a l s o may be a d e q u a t e l y e x p l a i n e d by s p e c i f i c a b i l i t i e s o f h e t e r o c h r o m a t i n t o i n d u c e v a r i e g a t i o n . These mutant p r o g r e s s i o n s may r e p r e s e n t d i f f e r e n c e s r e s u l t i n g from t h e v a r i e t y o f h e t e r o c h r o m a t i c b r e a k s . More comprehen-s i v e s t u d i e s must be complete t o e s t a b l i s h t h e b a s i s f o r t h e s e phenomena. The major o b j e c t i v e o f t h i s r e s e a r c h has been t o examine t h e p a r e n t - s o u r c e e f f e c t and t o determine the cause o f t h e s c u t e i n v e r s i o n l e t h a l i t y (Hess, 1962).-A number o f a u t h o r s have suggested t h a t t h i s phenomenon i s a s s o c i a t e d w i t h a p o s i t i o n e f f e c t s u p p r e s s i o n o f the r i b o s o m a l RNA c i s t r o n s (Baker, 1971; N i x , 1973; P u c k e t t and Snyder, 1973a, b, 1975)- However, d a t a r e p o r t e d by 173-H a r g e r (197*0 i n d i c a t e d ' t h a t t h e l e t h a l i t y o f s c u t e X/0 males was c o n d i t i o n a l and dependent upon the p a r e n t a l s ource o f t h e s c u t e X-chromosome. These i n v e s t i g a t i o n s have p r o v i d e d f u r t h e r e v i d e n c e s u g g e s t i n g t h a t t h e p r i -mary d e f e c t r e s p o n s i b l e f o r l e t h a l i t y i s not t h e v a r i e g a -t i o n o f the rDNA c i s t r o n s . The d a t a from t h e s e s t u d i e s are i n agreement w i t h t h o s e o f p r e v i o u s a u t h o r s (Hess, 1962; Baker, 1971» H a r g e r , 197*0 . i n d e m o n s t r a t i n g a c o n s i s t e n t l e v e l o f SI L8 8 i n v i a b i l i t y o f X/0 males c a r r y i n g the sc , sc , sc ,. V2 . . • . . . and sc i n v e r s i o n s when t h i s rearrangement i s i n h e r i t e d p a t e r n a l l y . - Moreover, t h e p r e v i o u s l y r e p o r t e d m a t e r n a l r e s c u e o f t h i s l e t h a l i t y ( Harger 197*+) has been demon-s t r a t e d f o r a l l f o u r o f the s c u t e i n v e r s i o n chromosomes. I t i s apparent t h a t t h e i n v i a b i l i t y o f s c u t e X/0 males i s a g e n e r a l phenomenon o f t h e s c u t e i n v e r s i o n s and t h e anomaly can be g r e a t l y m o d i f i e d by v a r y i n g p a r e n t a l s o u r c e . S h o u l d th e i n v i a b i l i t y r e s u l t from an i m p a i r e d rDNA f u n c t i o n , one would p r e d i c t c o n t r a s t i n g r e s u l t s . P a t e r n a l l y i n h e r i t e d , the sperm c a r r y i n g t h e s c u t e X-chromosome f e r t i l i z e s an egg t h a t has been amply sup-p l i e d w i t h rRNA from s u r r o u n d i n g n u r s e c e l l s . C o n v e r s e l y , w i t h m a t e r n a l i n h e r i t a n c e , t h e egg would be f u r n i s h e d by rDNA d e f e c t i v e n u r s e c e l l s and one would expect the r e -s u l t i n g z y g o t e t o be l e s s a d e q u a t e l y f u r n i s h e d w i t h rRNA. T h i s h y p o t h e s i s p r e d i c t s t h a t v i a b i l i t y would be enhanced 174. i f sperm c a r r y i n g t h e s c u t e X-chromosome f e r t i l i z e d an egg f u l l y f u r n i s h e d w i t h rRNA. T h i s i s e n t i r e l y oppo-s i t e t o t h e observed r e s u l t s . E x p e r i m e n t s i n v o l v i n g f e m a l e s h e t e r o z y g o u s f o r bb~ chromosomes and the s c u t e i n v e r s i o n s a l s o i n d i c a t e t h a t t h e rDNA complement i s n o t a c r u c i a l f a c t o r . The n u r s e c e l l s i n t h e v i c i n i t y o f the m a t u r i n g egg a r e de-pendent s o l e l y on t h e s c u t e X-chromosome f o r t h e i r tem-p l a t e rDNA. Sc u t e X/0 males d e r i v e d from i n d i v i d u a l s h e t e r o z y g o u s f o r a bb~ a r e comparably v i a b l e t o f e m a l e s h e t e r o z y g o u s f o r a s t a n d a r d X-chromosome. I n the c o u r s e o f t h e s e e x p e r i m e n t s , i t was demon-S i 18 s t r a t e d t h a t b o t h t h e sc and sc i n v e r s i o n s a re n o r m a l l y v i a b l e and f e r t i l e when h e t e r o z y g o u s f o r t h e 4L - 8R sc sc bb chromosome. T h i s was f i r s t r e p o r t e d by N i x (1971) and i s c o n t r a r y t o t h e rDNA d e f e c t h y p o t h e s i s . SI L8 These two i n v e r s i o n s ( sc and sc ) show t h e most severe l e t h a l i t y p r o f i l e s and a r e s e m i - s t e r i l e as homozygous f e m a l e s . S h o u l d t h e s e problems a r i s e from d i f f i c u l t i e s i n p r o d u c i n g rRNA, one would expect f e m a l e s h e t e r o z y g o u s 1 f o r rDNA d e f i c i e n c i e s t o be even more s e v e r e l y a f f e c t e d . S I IJ8 T h i s i s n o t the case as t h e sc and sc i n v e r s i o n s , h e t e r o z y g o u s w i t h an NO~-X chromosome, are n o r m a l l y v i a b l e and f e r t i l e . I t s h o u l d be n o t e d t h a t t h i s anomaly c o u l d be ex-p l a i n e d i n terms o f the rDNA r e g u l a t o r y mechanisms. 175. The phenomenon o f rDNA compensation r e s u l t s i n an i n c r e a s e i n rDNA c o n t e n t s o l e l y i n somatic c e l l s when the X chromo-some i s o p p o s i t e a rDNA d e f i c i e n t chromosome (Shermoen and S I ii8 K i e f e r , 1975) . I t i s p o s s i b l e t h a t t h e sc and sc homozygotes a r e u n a b l e t o compensate f o r t h e i r d e f e c t i n rRNA p r o d u c t i o n as t h e y have a w i l d t y pe complement o f rDNA ( N i x , 1973) - The females- h e t e r o z y g o u s f o r the bb~ chromosome has a s i g n i f i c a n t l y l o w e r t e m p l a t e number of rDNA c i s t r o n s and t h i s may t r i g g e r the compensatory mechanism. I t has been p r e v i o u s l y observed t h a t the r e c o g n i t i o n f a c t o r i n rDNA compensation i s t h e d e f i c -i e n c y o f rDNA c i s t r o n s ( T a r t o f , 1973) and t h i s c o u l d ac-count f o r the v i g o r o f t h e sc / bb and sc / bb f e m a l e s . S I J_J8 A n o t h e r anomaly s p e c i f i c t o t h e sc and sc chromo-somes i s an a b n o r m a l l y f r e q u e n t occurence o f second m e i o t i c d i v i s i o n (M-2) n o n d i s j u n c t i o n i n males. T h i s r e s u l t e d i n homozygous f e m a l e s w i t h and w i t h o u t Y chromo-somes i n many e x p e r i m e n t s and p r o v i d e d the f i r s t oppor-t u n i t y t o t e s t t h e s e f e m a l e s f o r f e r t i l i t y and t h e mat-e r n a l e f f e c t . I t was d e t e r m i n e d t h a t the homozygous f e m a l e s w i t h a Y chromosome were n o r m a l l y f e r t i l e w h i l e the f e m a l e s l a c k i n g a Y chromosome were s e m i - s t e r i l e . I t was d i s c o v e r e d t h a t t h e f e r t i l i t y o f t h e s e f e m a l e s c o u l d be g r e a t l y enhanced i f the f l i e s were i s o l a t e d f o r an i n t e r v a l b e f o r e b e i n g mated.. I t i s apparent t h a t motor f u n c t i o n s g r e a t l y improve d u r i n g the i n t e r v a l and 1 7 6 . t h i s may r e p r e s e n t a m a t u r a t i o n p r o c e s s t h a t i s n e c e s s a r y f o r f e r t i l i t y . The second d i v i s i o n n o n d i s j u n c t i o n was obser v e d SI w i t h t h e sc chromosomes not c a r r y i n g t h e dl - 4 - 9 i n v e r s i o n L8 and t o a l e s s e r e x t e n t , w i t h t h e sc chromosome. The f r e q u e n c y approached 2% i n t h e sc y B +S/ Y s t o c k and was l e s s pronounced i n a l l o t h e r s . One would n o t expect 18 SI sc n o n d i s j u n c t i o n a l f e m a l e s t o o c c u r as o f t e n as sc fe m a l e s as t h e y a r e n o t comparably v i a b l e i n o t h e r ex-p e r i m e n t s ( i n p r e p a r a t i o n ) . T h i s i s a l s o i n d i c a t e d by 1 L8 t h e f a c t t h a t the m a j o r i t y o f sc f e m a l e s were r e c o v e r -ed w i t h Y-chromosomes. An i m p o r t a n t f i n d i n g i l l u s t r a t e d by t h e recombinant e x p e r i m e n t s c o n c e r n s t h e v i a b i l i t y o f S I L 8R/0 males. T h i s rearrangement c a r r i e s t h e n u c l e o l a r o r g a n i z e r o f t h e SI SI sc chromosome but does n o t e x h i b i t a sc v i a b i l i t y g p r o f i l e , b u t r a t h e r t h a t o f t h e sc chromosome. Baker (1971) a t t r i b u t e d t h i s c o n t r a d i c t i o n t o a " n e g a t i v e r e g -u l a t o r " p r o p e r t y o f t h e p r o x i m a l X - h e t e r o c h r o m a t i n . A s i m p l e r and t e s t a b l e h y p o t h e s i s v/ould i n v o l v e t h e v a r i e g a t e d e x p r e s s i o n o f t h e r e a r r a n g e d s c u t e genes. The s e r i e s o f e x p e r i m e n t s i n v o l v i n g h e t e r o c h r o m a t i c X-chromosome d u p l i c a t i o n s suggest t h a t t h e bobbed r e g i o n o f t h e X-chromosome i s n o t the major f a c t o r d e t e r m i n i n g t h e i n c r e a s e d v i a b i l i t y o f scute-X/Dp males. D u p l i c a t i o n b e a r i n g males a r e comparably v i a b l e whether t h e d u p l i -177. c a t i o n c a r r i e s an a c t i v e X n u c l e o l u s o r g a n i z e r o r n o t . I t appears t h a t t h e e x t r a h e t e r o c h r o m a t i n and t h e p r e -sence o f an e x t r a s c u t e gene c o n t r i b u t e s i g n i f i c a n t l y more t o t h e i n c r e a s e d v i a b i l i t y than does added X-chromo-somal rDNA c i s t r o n s . A l t h o u g h t h e X bobbed r e g i o n i s n o t r e s p o n s i b l e f o r a l a r g e i n c r e a s e i n v i a b i l i t y , i t i s apparent t h a t t h e Y-chromosomal bobbed r e g i o n i s . Owing t o problems i n d e f i n i n g t h e e x t e n t o f t h e d e l e t i o n i n t h e Ybb~ chromo-some, i t i s i m p o s s i b l e t o d e t e r m i n e whether t h i s i n c r e a s e i n v i a b i l i t y i s due t o t h e rDNA c i s t r o n s o r o t h e r f a c t o r s i n t h e v i c i n i t y o f t h e Y-bobbed r e g i o n ( B r o s s e a u , 1964) . A d e c r e a s e i n t e m p e r a t u r e r e s u l t s i n a more mutant e x p r e s s i o n o f t h e v a r i e g a t i n g s c u t e l o c u s ( C h i l d , 1935) w h i l e c a u s i n g l e s s mutant e x p r e s s i o n i n rDNA d e f i c i e n t s t r a i n s (Mohan and R i t o s s a , 1970) . The t e m p e r a t u r e e f f e c t d e m onstrated i n t h e s e e x p e r i m e n t s i n d i c a t e t h a t t h e l e t h -a l i t y i s more se v e r e a t r e d u c e d t e m p e r a t u r e s . T h i s e v i d e n c e does n o t s u p p o r t B a k e r ' s h y p o t h e s i s as t h e i n -c r e a s e d d e v e l o p m e n t a l time a s s o c i a t e d w i t h reduced temp-e r a t u r e s h o u l d a l l o w f o r accommodation o f a d e f e c t i n rRNA s y n t h e s i s . The most c o n v i n c i n g p h e n o t y p i c e v i d e n c e t h a t i s c o n t r a r y t o B a k e r ' s h y p o t h e s i s i s t h e b r i s t l e phenotypes o f t h e X/0 males. Other a u t h o r s have shown t h a t t h e r a t e o f rRNA a c c u m u l a t i o n and b r i s t l e l e n g t h i s p r o p o r t i o n a l 1 7 8 . t o t h e number o f rDNA c i s t r o n s (Weinman, 1972; Shermoen and K e i f e r , 1 9 7 5 ) - I i " t h e rDNA c i s t r o n s a r e t r u e l y v a r i e g a t i n g , one would expect .cbo f i n d a range o f bobbed i n d i v i d u a l s among t h e s u r v i v i n g s c u t e X/0 males. These i n d i v i d u a l s were n e v e r o b s e r v e d i n any e x p e r i m e n t s i n -v o l v e d w i t h t h e s c u t e chromosomes. The d e v e l o p m e n t a l d e l a y e s t a b l i s h e d f o r the bobbed i n d i v i d u a l s i s some-what l e s s t h a n t h a t f o u n d f o r t h e s c u t e X/0 males; b u t , n o n e t h e l e s s , i t r e s u l t s i n a p r o f o u n d bobbed phenotype. T h i s i n d i c a t e s t h a t t h e d e v e l o p m e n t a l d e l a y c h a r a c t e r -i s t i c o f t h e s c u t e i n v e r s i o n s i s n o t d i r e c t l y a s s o c i a t e d w i t h a d e f e c t i n v o l v i n g t h e rDNA c i s t r o n s . These r e s u l t s suggest t h a t t h e p r i m a r y b i o c h e m i c a l d e f e c t s , w h i c h cause t h e l e t h a l i t y a s s o c i a t e d w i t h the s c u t e i n v e r s i o n s , do n o t i n v o l v e t h e rRNA genes, but r a t h e r t h e e u c h r o m a t i c , a c h a e t e - s c u t e l o c u s . The r e c e n t g e n e t i c d i s s e c t i o n and d e v e l o p m e n t a l a n a l y s i s o f a c h a e t e - s c u t e mutants i n d i c a t e t h a t t h e f u n c t i o n o f t h i s complex l o c u s i n v o l v e s t h e d i f f e r e n t -i a t i o n o f n e r v e elements ( G a r c i a - B e l l i c o and S a n t a m a r i a , 1978; G a r c i a - B e l l i d o , 1 9 7 9 ) . The e v i d e n c e t h u s f a r r e -p o r t e d s u g g e s t s t h a t a c t i v i t y o f t h e s c u t e gene o c c u r s 24-48 h o u r s p r i o r t o puparium f o r m a t i o n and t h i s i s c o n s i s t e n t w i t h the d i f f e r e n t i a t i o n o f p e r i p h e r a l n erve elements. G a r c i a - B e l l i d o and S a n t a m a r i a ( 1 9 7 8 ) a l s o propose t h a t t h e l e t h a l o f s c u t e ( l ' s c ) r e g i o n may be 179. i n v o l v e d w i t h a c e n t r a l n e r v o u s system f u n c t i o n as mut-a t i o n s a t t h i s l o c u s are a s s o c i a t e d w i t h problems i n c o o r d i n a t i n g motor f u n c t i o n s . There a r e numerous e x p e r i m e n t s i n t h i s t e x t w h i c h suggest t h a t v a r i e g a t i o n a t t h e s c u t e l o c u s r e s u l t s i n the o b s e r v e d l e t h a l i t y . S c u t e i n v e r s i o n X/0 males o f t e n e x h i b i t m o b i l i t y problems d i r e c t l y a f t e r e c l o s i o n . A p p a r e n t l y , t h e f l i e s a re a b l e t o compensate f o r t h i s p roblem as normal m o b i l i t y i s a c h i e v e d a f t e r a 1 or 2 day i n t e r v a l . T h i s i s c o n s i s t e n t w i t h v a r i e g a t i o n o f 1'sc, a l o c u s thought t o be i n v o l v e d w i t h motor f u n c t i o n s G a r c i a - B e l l i c o and S a n t a m a r i a 1978). One o f t h e major f a c t o r s o b s e r v e d t o m o d i f y the l e t h a l i t y was t h e p r e s e n c e o f an a c c e s s o r y s c u t e l o c u s . I n t h e e x p e r i m e n t s i n v o l v i n g h e t e r o c h r o m a t i c d u p l i c a t i o n s and t h e s c u t e gene, a c o n s i s t e n t i n c r e a s e i n v i a b i l i t y c o u l d be a t t r i b u t e d t o t h e p r e s e n c e o f a f u l l y f u n c t i o n a l s c u t e l o c u s . I t was a l s o o b s e r v e d , i n many e x p e r i m e n t s , t h a t t h e s e v e r i t y o f the s c u t e phenotype c o u l d be c o r r e l -a t e d w i t h t h e l e n g t h o f d e v e l o p m e n t a l d e l a y and t h e de-gree o f motor d i s a b i l i t y . The l e t h a l i t y p r o f i l e e s t a b -l i s h e d i n t h e t e m p e r a t u r e - e f f e c t e x p e r i m e n t s a l s o sug-g e s t e d t h e i n v o l v e m e n t o f t h e s c u t e l o c u s . . I t i s t h i s e v i d e n c e t h a t s u p p o r t s the concept t h a t p o s i t i o n e f f e c t v a r i e g a t i o n o f the complex a c h a e t e - s c u t e l o c u s r e s u l t s i n X/0 male l e t h a l i t y r a t h e r t h a n a p o s i t i o n e f f e c t 180. suppression of the rRNA ci s t r o n s . Of p a r t i c u l a r i n t e r e s t to researchers i n many f i e l d s i s the i d e n t i f i c a t i o n of the heterochromatic breakpoint V2 . m sc I t i s apparent that both ends of t h i s inver-sion r e t a i n a complement of rDNA that i s s u f f i c i e n t to produce a normal phenotype. I t seems very l i k e l y that t h i s inversion originated with a heterochromatic break within the rDNA cistrons and a euchromatic break within the achaete-scute locus. The r e s u l t i n g chromosome has rDNA cistrons d i r e c t l y abutting the euchromatic achaete-scute locus (See Figure l ) . The experiments involved V2 with sc suggest variegation f o r ac and Hw and, to a less e r extent, f o r _sc (See Chapter 8 ) . As the achaete-scute region i s supposedly bordered by "euchromatic" rDNA cistrons i n t h i s inversion, the variegation i n t h i s chromosome i s somewhat surprizing and suggests that the: biochemical nature of the nucleolar organizer region may d i f f e r from more c l a s s i c a l euchromatic l o c i . V2 The sc experiments also provide further evidence i n favor of a scute gene l e t h a l i t y rather than ribosomal c i s t r o n disturbance. In t h i s inversion, the ribosomal genes are divided into two blocks and separated at op-posite ends of the chromosome. Of the scute inversions, t h i s i s the only example where the i n t e g r i t y of the nu-c l e o l a r organizer i s d i r e c t l y disturbed and thus one V2 would expect sc /0 males to exhibit severe v i a b i l i t y 181. p r oblems. Q u i t e t o t h e c o n t r a r y , sc /0 males are much 8 S1 T 8 more v i a b l e t h a n s i m i l a r sc , sc , o r sc males. From C h a p t e r 8 one o b s e r v e s t h a t t h e b r i s t l e pheno-t y p e i s p r e d o m i n a n t l y achaete v a r i e g a t i n g and t h i s can be c o r r e l a t e d w i t h t h e o b s e r v e d l e t h a l i t y . The g r e a t e r t h e a c haete c h a r a c t e r o f t h e v a r i e g a t i o n , t h e h i g h e r t h e v i a b i l i t y o f t h e X/0 males. F u r t h e r i n v e s t i g a t i o n and i n t e r p r e t a t i o n o f t h e s e a n o m a l i e s a r e now b e i n g p e r s u e d and t h e d a t a s h o u l d p r o v i d e v a l u a b l e i n f o r m a t i o n r e l a t i n g t o the s t r u c t u r e and r e g u l a t i o n o f t h e rDNA c i s t r o n s . The purpose o f t h i s t h e s i s has been t o e s t a b l i s h a more r e f i n e d knowledge o f t h e f a c t o r s i n f l u e n c i n g t h e l e t h a l i t y a s s o c i a t e d w i t h t h e s c u t e i n v e r s i o n s . The d a t a suggest t h a t t h i s phenomenon i s dependent upon the p a r e n t a l source o f t h e rearrangement and t h u s , t h e mater-n a l e f f e c t has been e s t a b l i s h e d . R e s u l t s from numerous e x p e r i m e n t s suggest t h a t t h e p r i m a r y b i o c h e m i c a l d e f e c t i n v o l v e d w i t h t h e l e t h a l i t y r e s u l t s from p o s i t i o n e f f e c t v a r i e g a t i o n a t the a c h a e t e - s c u t e l o c u s r a t h e r t h a n t h e n u c l e o l a r o r g a n i z e r r e g i o n , as p r e v i o u s l y suggested ( B a k e r , 1971). The d i s c o v e r y o f t h e d i v i s i b i l i t y o f the n u c l e o l u s o r g a n i z e r and i t ' s subsequent a b i l i t y t o i n -duce p o s i t i o n e f f e c t v a r i e g a t i o n i s a t o p i c f o r immed-i a t e a t t e n t i o n . A l t h o u g h i t appears t h a t most o f t h e 182. g e n e t i c d i s s e c t i o n o f t h i s problem has been completed, t h e r e a r e numerous b i o c h e m i c a l and d e v e l o p m e n t a l p r o b -lems which r e m a i n u n e x p l o r e d . The modern t e c h n i q u e s o f f i n e s t r u c t u r a l a n a l y s i s promise t o p r o v i d e t h e n e x t major advance i n t h i s problem. 1 8 3 . BIBLIOGRAPHY Agol, I . J . (1930) Evidence of the d i v i s i b i l i t y of the gene Anat. Rec. 47_ p. 3 8 5 . Agol, J.Y.' (1929) Step allelomorphism i n Drosophila melanogaster. Allelomorph scute - 5 J . Exp. B i o l . 4 # 3 - 4 . Ashton, F. and J . Sc h u l t z (1971) The three dimensional f i n e s t r u c t u r e of chromosomes i n a prophase Drosophila nucleus Chromosoma _3J| p.3 8 3 . Bahn, E. (1971) P o s i t i o n - e f f e c t v a r i e g a t i o n f o r an i s o -amylase i n Droso p h i l a melanogaster Hereditas 6j_ p. 79-Baker, W.K. (1968) P o s i t i o n - e f f e c t v a r i e g a t i o n Adv. Genet. 14 p. 641. Baker, W.K. (1971) Evidence f o r p o s i t i o n e f f e c t suppres-s i o n of the ribosomal RNA c i s t r o n s i n Drosophila melanogaster Proc. N a t l . Acad. Sci.USA 68 p. 2472 . 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