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Chromosome individuality and somatic pairing in Abies grandis Campbell, John Duncan 1949

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CHROMOSOME INDIVIDUALITY AND SOMATIC PAIRING IN A T h e s i s submitted i n p a r t i a l f u l f i l m e n t o f the requirements f o r the degree o f MASTER OP ARTS In the Department o f BIOLOGY and BOTANY The U n i v e r s i t y o f B r i t i s h Columbia Abies g r a n d i s John Duncan Campbell A p r i l , 1949 Chromosome I n d i v i d u a l i t y and Somatic P a i r i n g i n Abies g r a n d i s by John Duncan Campbell U n i v e r s i t y o f B r i t i s h Columbia, A p r i l , 1949 A b s t r a c t One o f the phenomena uncovered i n c y t o l o g i c a l s t u d i e s o f Abies gra n d i s LIndley, the Lowland White F i r , was the e x i s t e n c e of p a i r i n g o f chromosomes i n somatic c e l l s o f the very young o v u l e s . Huskins (1948) has shown t h a t somatic p a i r i n g i s not r a r e i n the p l a n t world, but i s seldom p r o p e r l y r e corded. I t was thought necessary to undertake a study o f the morphology of the I n d i v i d u a l chromosomes of the t r e e . Counts and measurements were made on the chromosomes r i n s e v e r a l c e l l s , and the measurements reduced to a percentage o f the t o t a l chromosome l e n g t h i n the c e l l . P o s i t i o n s o f the s p i n d l e - f i b r e attachments or centromeres were a l s o t e n t a t i v e l y l o c a t e d and recorded as percentages. The l e n g t h s o f the twelve chromosomes i n the genome are as f o l l o w s : Chromosome 1, 14.4$ u n i t s long; 2, 11.3$ u n i t s ; 3, 10.6$ u n i t s ; 4, 9.6$ u n i t s ; 5, 9.1$ u n i t s ; 6, 8.4$ u n i t s ; 7, 8.1# u n i t s ; 8, 1,2% u n i t s ; 9, 6.4$ u n i t s ; 10, 6,1% u n i t s ; 11, 5,5% u n i t s ; 12, 3.3$ u n i t s . Two chromosomes have c l u b -shaped ends without any c o n s t r i c t i o n , w h i l e f i v e have s i n g l e - 2 -t e r m i n a l knobs. One has three c o n s t r i c t i o n s , one has two, and f o u r have but a s i n g l e c o n s t r i c t i o n . One chromosome i s d i c e n t r i c , f o u r are approximately i s o m e r a l , f i v e a r e d i s t i n c t l y heteromeral, and two have t e r m i n a l centromeres. Somatic c e l l s from young ovules showing apparent p a i r i n g o f chromatin strands a t e a r l y anaphase were examined. In one c e l l s t u d i e d , the p a i r i n g i s so d i s t i n c t and the s i m i l a r i t y between chromatin threads so s t r i k i n g t h a t I t i s thought to be i n d i c a t i o n of some s o r t of reduced m e i o s i s . The d i f f e r e n c e s between somatic p a i r i n g and t r u e m e i o s i s are d i s c u s s e d and a l s o some t h e o r i e s on the reason f o r somatic p a i r i n g . Some problems i n technique o f c o n i f e r c y t o l o g y and the methods used i n t h i s study are set f o r t h . Table o f Contents I I n t r o d u c t i o n 1 I I L i t e r a t u r e 1 I I I M a t e r i a l s and Methods 5 IV Chromosome Morphology 7 V Somatic P a i r i n g 13 VI D i s c u s s i o n 14 VII Appendix - C y t o l o g i c a l Technique 17 V I I I Summary 21 L i t e r a t u r e C i t e d T a b l e o f P l a t e s Acknowledg emen t 3 I would l i k e to acknowledge the i n c a l c u l a b l e h e l p o f f e r e d me by Dr. A.H. Hutchinson of the Department o f B i o l o g y and Botany, U n i v e r s i t y of B r i t i s h Columbia, i n the study o f t h i s problem. He made a v a i l a b l e to me not o n l y m a t e r i a l and equipment, but a l s o suggestions and a v a s t s t o r e o f knowledge on the s u b j e c t . I have a l s o r e c e i v e d v a l u a b l e h e l p from Dr. G.S. A l l e n of the Department of F o r e s t r y , U n i v e r s i t y of B r i t i s h Columbia, and from Dr. V.C. B r i n k o f the Department of Agronomy, U n i v e r s i t y o f B r i t i s h Columbia. To these people and to the U n i v e r s i t y i t s e l f , I am deeply i n d e b t e d . John D. Campbell. I I n t r o d u c t i o n As p a r t o f the program of c y t o l o g i c a l r e s e a r c h on c o n i f e r o u s t r e e s proceeding under Dr. A.H. Hutchinson a t the U n i v e r s i t y o f B r i t i s h Columbia, m i c r o s c o p i c a l examination was made of v a r i o u s stages i n the r e p r o d u c t i v e c y c l e o f the Lowland White F i r , Abies g r a n d i s L i n d l e y . In the somatic t i s s u e o f the maturing ovule were seen a t l e a s t a few c e l l d i v i s i o n s which appeared to resemble m e i o s i s i n many ways; they seemed to c o n t a i n p a i r e d chromosomes a t the metaphase and anaphase. I t was r e a l i z e d t h a t any i n v e s t i g a t i o n o f the natu r e o f these d i v i s i o n s would r e q u i r e p r e c i s e i d e n t i f i c a t i o n o f the I n d i v i d u a l chromosomes i n the genome by t h e i r m o r p h o l o g i c a l appearance. T h i s study attempts to d e s c r i b e i n a p r e l i m i n a r y way, the morphology of the chromosomes o f Abies g r a n d i s as w e l l as t h e i r apparent p a i r i n g . I I L i t e r a t u r e Hutchinson (1915) i n a d e t a i l e d account o f syngamy i n Abies balsamea d e s c r i b e d the p a i r i n g o f p a t e r n a l and maternal chromosomes, and t w i s t i n g o f the p a i r s , a t syngamy, f o l l o w e d by t r a n s v e r s e s p l i t t i n g and m i g r a t i o n o f the r e s u l t -ant daughter chromosomes to the o p p o s i t e p o l e s . Beal (1934) and Haupt (1941) working on Pinus f a i l e d to c o n f i r m t h i s , but - 2 -Chamberlain (1935) commenting on the cycad S t a n g e r i a and A l l e n (1946) working on Pseudotsuga both suggest t h a t c e r t a i n e m b ryological phenomena o f t h e i r r e s p e c t i v e genera can b e s t be e x p l a i n e d by the occurrence o f post-syngamic p a i r i n g . With r e g a r d to t r u e m e i o s i s i n Abies, Rattenbury (1945) working on the microsporogenesis o f A, g r a n d i s found t h a t , w h i l e c e r t a i n d e t a i l s o f n u c l e o l u s development and heterochromatin t r a n s f e r were p e c u l i a r , a l l the normal prophase p a i r i n g stages were p r e s e n t i n a r e c o g n i z a b l e form. Recent work under Huskins a t Wisconsin (Huskins 1948, Kodani 1948) has shown t h a t p a i r i n g and even s e g r e g a t i o n can be induced i n somatic m i t o s i s by treatment w i t h the sodium s a l t o f r i b o s e n u c l e i c a c i d . Besides t h i s , Huskins c i t e s a l a r g e l i s t o f r e f e r e n c e s g i v i n g evidence o f v a r y i n g c e r t a i n t y to show t h a t n a t u r a l somatic p a i r i n g can e x i s t ; he i n c l u d e s , o f course, the w e l l e s t a b l i s h e d f a c t t h a t the g i a n t chromosomes i n the s a l i v a r y glands o f D r o s o p h i l a and ot h e r D i p t e r a n f l i e s , a re r e a l l y p a i r s o f v e r y c l o s e l y synapsed homologous chromosomes. One o f the be s t d i s c u s s i o n s on chromosome morphology i n C o n i f e r s i s co n t a i n e d i n the paper by Sax and Sax (1933). These workers determined the chromosome number f o r 53 s p e c i e s o f C o n i f e r s , r e p r e s e n t i n g 16 genera, and then determined the r e l a t i v e l e n g t h s of whole chromosomes, and the p o s i t i o n o f the s p i n d l e - f i b r e attachments or "centromeres". They - 3 -c l a s s i f i e d chromosomes as " i s o m e r a l " (with equal arms on e i t h e r s i d e o f the centromere), "heteromeral" (with o b v i o u s l y unequal arms), or " t e r m i n a l " (with o n l y one arm). "The b a s i c chromosome number", they r e p o r t , " i s 12 f o r most Gymnosperms with the ex c e p t i o n o f the Gne t a l e s . . . . In the A b i e t i n e a e 12 p a i r s o f chromosomes are found i n P i c e a , Tsuga, A b i e s , L a r i x , and P i n u s . . . (and) Cedrus... but i n Pseudotsuga there are 13 and i n P s e u d o l a r i x there are 22 p a i r s o f chromosomes." They made counts on three s p e c i e s o f Abies (A. c e p h a l o n i c a , A. c o n c o l o r , A. V e l t c h i i ) f i n d i n g 12 the h a p l o i d number i n each; and they measured the chromosomes o f A. c e p h a l o n i c a and A. c o n c o l o r . T a b l e I, below, i s d e r i v e d from the diagrams i n the paper by Sax and Sax, w i t h chromosomes arranged i n descending order o f t o t a l l e n g t h . F i g u r e s 5 and 6, P l a t e I I , are diagrammatic r e p r e s e n t a t i o n s o f the same t h i n g . I t w i l l be seen from the f i g u r e s and t a b l e s t h a t i n both s p e c i e s , 5 chromosomes appear to be d i s t i n c t l y h e t e r -omeral, w h i l e the oth e r 7 appear to be a l l approximately i s o m e r a l . - 4 -Table I Chromosome Measurements by Sax and Sax (1935) - reduced to per c e n t o f t o t a l chromosome l e n g t h i n c e l l . A bies c e p h a l o n i e a Abies c o n c o l o r Chrom. No. Whole Chrom. Short Arm Long Arm Whole Chrom. Short Arm Long Arm 1 12.00 4.59 7.43 10.16 4.40 5.75 2 10.25 4.59 5.65 9.83 4.40 5.75 3 9.90 4.59 5.30 9.83 3.39 6.44 4 9.55 4.59 4.94 9.49 4.40 5.42 5 9.19 3.88 5.30 9.49 4.06 5.42 6 8.48 3.88 4.59 9.15 4.06 5.08 7 8.48 3.88 4.59 9.15 3.72 5.42 8 6.71 2.12 4.59 9.15 3.72 5.42 9 6.71 1.77 4.94 6.44 1.36 5.08 10 6.71 1.41 5.30 5.76 1.36 4.40 11 6.36 2.12 4.24 5.76 1.68 4.06 12 5.65 1.41 4.24 5.76 1.36 4.40 Sax and Sax a l s o d i s c u s s p o s s i b l e o r i g i n s o f v a r i a t i o n s from the b a s i c h e p l o i d number 12 found i n C o n i f e r s . They suggest: (1) t h a t Pseudotsuga. w i t h 13, may be the r e s u l t o f the d u p l i c a t i o n o f one chromosome; (2) t h a t the Cupressaceae, w i t h 11, may r e p r e s e n t u n i o n o f one chromosome wi t h one or more o t h e r s (complete e l i m i n a t i o n o f one - 5 -chromosome i s u s u a l l y l e t h a l ) ; (3) and that P s e u d o l a r i x , w i t h 22, may p o s s i b l y show d u p l i c a t i o n o f 10 o f i t s o r i g i n a l 12 chromosomes. They concluded t h a t a l l o f these changes have probably been f o l l o w e d by p r o g r e s s i v e r e d u c t i o n o f homology by interchange o f segments and by mutation. I l l M a t e r i a l s and Methods The m a t e r i a l s of Abies g r a n d i s used i n t h i s study came from s e v e r a l l o c a l i t i e s around the G u l f o f G e o r g i a : young o v u l a t e cones from a s i n g l e t r e e i n the Arboretum o f the U n i v e r s i t y of B r i t i s h Columbia i n March, A p r i l , and May, 1948; most o f the staminate cones from the same Arboretum t r e e d u r i n g March, A p r i l , and May, 1948, and March, 1949; a small amount o f q u i t e v a l u a b l e staminate m a t e r i a l from V i c t o r i a , B.C. ( c o l l e c t e d by R.E. Hammond) i n 1940; and the female gametophyte m a t e r i a l ( c o l l e c t e d by Dr. A.H. Hutchinson) from a s i n g l e t r e e a t F r i d a y Harbour i n the San Juan I s l a n d s , Washington, d u r i n g June and J u l y , 1948. The very scanty m a t e r i a l of Abies l a s i o c a r p a came from Lake Louise, A l b e r t a i n 1921. F i x a t i o n , embedding and s t a i n i n g w i l l be d i s c u s s e d i n the Appendix. Heidenhains Iron Alum Hematoxylin and B e l l i n g ^ Iron-Aceto-Carmine were the most u s e f u l s t a i n s . Drawings were made u s i n g a R e i c h e r t microscope - 6 -(lOx o c c u l a r and lOOx o i l - i m m e r s i o n o b j e c t i v e ) and a camera l u c i d a a d j u s t e d to g i v e a c o n s t a n t t o t a l m a g n i f i c a t i o n on the drawing o f 1500x. Most o f the drawings are here reproduced i n t h e i r o r i g i n a l s i z e , except where noted. The photographs were taken on Ansco panchromatic Supreme F i l m , "120" r o l l - f i l m s i z e , u s i n g the same R e l c h e r t microscope. Rattenbury (1945), working w i t h m a t e r i a l from the U n i v e r s i t y o f B r i t i s h Columbia Arboretum t r e e , has g i v e n an e x c e l l e n t c a l e n d a r o f events i n the process o f microsporo-g e n e s i s . To t h a t we might add the f o l l o w i n g d a t a : At U.B.C. Arboretum: 1948 Male D i p l o t e n e and D i a k i n e s i s - Mar. 18 1948 Male M e i o s i s complete - Mar. 19 1948 Female M e i o s i s o c c u r r e d r a p i d l y - Apr. 23-24 1948 P o l l e n a t i o n - May 6-14 At F r i d a y Harbour, San Juan I s l a n d , Washington: 1948 F e r t i l i z a t i o n - J u l y 4 At U.B.C. Arboretum: 1949 Male D i a k i n e s i s and f i r s t Metaphase - Mar. 9 1949 Male F i r s t Telophase and Second - Mar. 10 Anaphase 1949 Male T e t r a d s - Mar. 11 1949 P o l l e n f u l l y formed - Mar. 20. - 7 -IV Chromosome Morphology Prom the c o n s i d e r a b l e amount of c o l l e c t e d m a t e r i a l , f o u r c e l l s were chosen f o r i n t e n s i v e study of chromosome morphology. These were: C e l l I - P o l l e n Mother C e l l , f i r s t anaphase o f m e i o s i s . - from U.B.C. Arboretum t r e e , Mar. 9, 1949. - Aceto-carmine smear. C e l l I I - P o l l e n Mother C e l l , f i r s t metaphase of m e i o s i s . - from V i c t o r i a , Mar. 18, 1940. - P a r a f f i n s e c t i o n , i r o n alum hematoxylin s t a i n . - C e l l cut i n t o two s e r i a l s e c t i o n s . C e l l I I I - Somatic c e l l from female gametophyte, anaphase. - from F r i d a y Harbour, Washington, J u l y 6, 1948. - P a r a f f i n s e c t i o n , i r o n alum hematoxylin s t a i n . C e l l IV - Somatic c e l l from female gametophyte, metaphase. - from F r i d a y Harbour, Washington, J u l y 6, 1948. - P a r a f f i n s e c t i o n , i r o n alum hematoxylin s t a i n . These c e l l s are shown i n t h e i r e n t i r e t y i n P l a t e s I I I and IV ( F i g s . 8 - 1 3 ) and t h e i r chromosomes are shown i n d i v i d u a l l y , arranged i n descending order of l e n g t h i n P l a t e I ( F i g s . 1 -4) . As can be seen, they show r e c o g n i z a b l e i n d i v i d u a l i t y and d e f i n i t e c o r r e l a t i o n among the f o u r c e l l s . Table ..II, below, shows the l e n g t h s In m i l l i m e t e r s of the chromosomes as drawn, and t h e i r l e n g t h s reduced to percentages of the t o t a l l e n g t h of chromatin i n the g i v e n c e l l , and a l s o the average - 8 -perce n t a g e - l e n g t h f o r each. Table I I Length o f Whole Chromosomes C e l l I C e l l I I C e l l I I I C e l l IV Aver-No. mm. Per- mm. Per- mm. Per- mm. Per- age on cent on cent on cent on cent Percei draw- draw- draw- draw-in g i n g i n g i n g 1 35 14.28 31 14.40 28 15*73 32 : 13.05 14.4 2 25 10.20 25 11.61 22 12.37 27 11.01 11.3 3 25 10.20 21 9.77 21 11.80 26 10*60 10.6 4 24 9,81 20 9.32 17 9.56 24 9.80 9.6 5 23 9.40 20 9.32 15 8.44 23 9.39 9.1 6 22 8.99 19 8.85 13 7*31 21 8.58 8.4 7 21 8*58 18 8.38 12.5 7*03 21 8.58 8 18 7.36 15 6.99 12 6.75 19 7.77 7.2 9 16 6.55 14 6.52 11- 6.18 16 6.54 6.4 10 15 6.14 13 6.06 10.5 5.91 15 6.13 6.1 11 14 5.73 12 5.59 10 5.62 12 4.91 5.5 12 7 2.85 7 3.26 6 3.37 9 3.67 3.3 T o t a l 245 215 178 245 The exact p o s i t i o n of the centromere c o u l d not be determined f o r a l l chromosomes i n a l l the c e l l s . T able I I I , below, based on determinable dimensions, shows l e n g t h s i n m i l l i m e t e r s o f the drawn chromosome arms, l e n g t h s reduced to percentage o f the t o t a l l e n g t h o f chromatin i n the g i v e n c e l l , and percentage average. N o t i c e t h a t chromosome 1 has two centromeres and consequently, three d i s t i n g u i s h a b l e p a r t s . - 9 -Tabl e I I I Chromosome Arm Lengths. A. Short Arms: Chrom. C e l l I I . C e l l . I I C e l l I I I C e l l IV Aver-No. mm. % mm. % , mm; % mm. % age % 1 ~9~ 3.68 7.5 4.22 3.9 2 10 5.62 5.3 3 8 3.36 10 4.08 3.7 4 12 4.91 8 4.49 4.7 5 10 4.08 9 4.18 6 3.37 11 4.50 4.0 6 9 3.68 8 3.72 8 3.36 3.6 7 8 9 10 5 6 4.08 2.04 2.45 8 6 3.72 2.79 4 4 2.25 2.25 8.5 5 3.47 2.04 3.7 2.4 272 10 0 0 0 0 0 11 4 1.68 2.5 1.19 1.4 12 C e l l T o t a l 0 0 0 0 0 0 0 245 215 • 178 245 B. Long Arms: Chrom. C e l l I C e l l I I C e l l I I I C e l l IV Aver-No. mm. i mm. mm. % mm. % age % 1 14~ 5.72 11 6.19 5.9 2 12 6.75 6.5 3 17 6.95 16 6.54 6.7 4 12 4.91 9 5.06 4.9 5 13 5.32 11 5.12 9 5.06 12 4.91 5.1 6 13 5.32 11 5.12 13 5.31 572" 7 11 4.56 10 4 .65 12.5 5.11 TT? 8 13 5.32 9 4.18 8 4.49 4.7 9 10 4.08 7 3.93 11 4.50 4.2 10 15 6.13 10.5 5.91 6.1 11 10 4.08 9.5 4.42 4.2 12 7 2.85 6 3.37 9 3.68 3.3 C e l l T o t a l 245 215 178 245 C. T h i r d Arm: Chrom. C e l l I No. 1 C e l l T o t a l mm. 12 245 4.91 C e l l I I  mm. % 215 C e l l I I I mm. 9.5 178 5.33 C e l l IV mm. Jo 245 Aver-age j 5.1 - 10 -The average percentage measurements of the chromosomes of Abies g r a n d i s a r e shown d i a g r a m m a t i c a l l y on P l a t e I I , P i g . 7, along w i t h s i m i l a r diagrams ( P i g s . 5 and 6) f o r Abies c e p h a l o n i c a and A. c o n c o l o r based on data g i v e n i n the paper by Sax and Sax (1935). The a v a i l a b l e i n f o r m a t i o n about the i n d i v i d u a l chromosomes may now be gathered t o g e t h e r : Chromosome 1: - 14.4 p e r c e n t - u n i t s i n l e n g t h ; has two centromeres d i v i d i n g i t i n t o three d i s t i n g u i s h a b l e p a r t s : 3.9, 5.9, 5.1 u n i t s . - has a p p a r e n t l y three v i s i b l e c o n s t r i c t i o n s i n c l u d -i n g a very deep one producing a l a r g e t e r m i n a l knob on the 5.1 arm. Chromosome 2: - 11.3 p e r c e n t - u n i t s i n l e n g t h ; arms 5.3 and 6.5 u n i t s ; - the l o n g e r arm terminates i n a smal l but very w e l l d e f i n e d knob. Chromosome 3: - 10.6 p e r c e n t - u n i t s In l e n g t h ; arms 3.7 and 6.7 u n i t s ; - a s l i g h t c o n s t r i c t i o n midway along the s h o r t e r arm. Chromosome 4: - 9.6 p e r c e n t - u n i t s i n l e n g t h ; arms 4.7 and 4.9 u n i t s ; - 11 -- a l a r g e w e l l - s e p a r a t e d knob terminates the 4.9 arm. Chromosome 5: - 9.1 p e r c e n t - u n i t s i n l e n g t h ; arms 4.0 and 5.1 u n i t s ; - The 4.0 end i s club-shaped w i t h no c o n s t r i c t i o n . (The centromere shows up w e l l i n C e l l I V ) . Chromosome 6: - 8.4 p e r c e n t - u n i t s i n l e n g t h ; arms 3.6 and 5.2 u n i t s ; Chromosome 7: - 8.1 p e r c e n t - u n i t s i n l e n g t h ; arms 3.7 and 4.7 u n i t s ; - a . d i s t i n c t c o n s t r i c t i o n i n the middle o f each arm. Chromosome 8: - 7.2 p e r c e n t - u n i t s i n l e n g t h ; arms 2.4 and 4.7 u n i t s . - a p p a r e n t l y moves to the anaphase p o l e s w e l l ahead of the oth e r chromosomes. Chromosome 9: - 6.4 p e r c e n t - u n i t s i n l e n g t h ; arms 2.2 and 4.2 u n i t s ; - t h i c k l y club-shaped a t the 4.2 end. Chromosome 10: - 6.1 p e r c e n t - u n i t s i n l e n g t h ; centromere q u i t e t e r m i n a l i n p o s i t i o n ; a small knob d i s t i n c t l y separated by a c o n s t r i c t i o n a t the end o p p o s i t e - 12 -the centromere. Chromosome 11: * - 5.5 p e r c e n t - u n i t s i n l e n g t h ; arms 1.4 and 4.2 u n i t s ; - the s h o r t e r arm i s l i t t l e b e t t e r than a l a r g e knob. Chromosome 12: - 3.3 p e r c e n t - u n i t s i n l e n g t h ; centromere q u i t e t e r m i n a l ; a s t o u t s h o r t r e g u l a r chromosome. The small amount of m a t e r i a l of Abies l a s i o c a r p a , which was examined, seemed to agree w i t h A. g r a n d i s i n p o s s e s s i n g one ve r y l a r g e chromosome" wit h two centromeres and one chromosome much s m a l l e r than any o f the o t h e r s . Some of t h i s i n f o r m a t i o n on Abies g r a n d i s may be summed up. Whole chromosome l e n g t h s vary i n pe r c e n t a g e - u n i t l e n g t h from 14.4 down to 3.3; they vary i n m i l l i m e t e r - o n -drawing l e n g t h s from 35 to 6 (which i s e q u i v a l e n t to 23 microns to 4 microns s i n c e the m a g n i f i c a t i o n i s a con s t a n t 1500x). The v a r i a t i o n i n s i z e o f chromosome 1 from c e l l t o c e l l i s o n l y from 35 to 28 mm. on drawing o r from 23 microns to 19 microns on the a c t u a l chromosome. Two chromosomes have club-shaped ends without any c o n s t r i c t i o n , w h ile f i v e have s i n g l e t e r m i n a l knobs. One has three c o n s t r i c t i o n s , one has two, and f o u r have but a s i n g l e c o n s t r i c t i o n . One chromosome i s d i c e n t r i c , f o u r are approximately i s o m e r a l , f i v e a re d i s t i n c t l y heteromeral and two have t e r m i n a l centromeres. - 13 -V Somatic P a i r i n g In the m a t e r i a l of young o v u l a t e cones c o l l e c t e d i n the U.B.C. Arboretum i n the s p r i n g of 1948, s e v e r a l somatic c e l l s were seen i n which the chromosomes appeared to be p a i r e d ; a l l such c e l l s were i n the v e r y r a p i d l y growing young ovule or o v u l i f e r o u s b r a c t j u s t about the time o f gynospore m e i o s i s . In many c e l l s , the " p a i r i n g " was merely a l o o s e l y p a r a l l e l arrangement o f prophase and metaphase chromosomes; such an appearance c o n c e i v a b l y c o u l d be caused by a not v e r y e l a b o r a t e c o i n c i d e n c e . But a few c e l l s were l e s s e a s i l y e x p l a i n e d away, n o t a b l y c e l l V, p o r t r a y e d i n P l a t e V, P i g . 14 - 17, and P l a t e V I I I , P i g . 25 and 26. The c e l l was u n f o r t u n a t e l y s e c t i o n e d by the microtome k n i f e so that complete a n a l y s i s o f the chromosomes was i m p o s s i b l e . But throughout, t h e r e were strand s o f chromatin (chromosomes) i n p a i r s s i m i l a r to those seen about d i p l o t e n e i n a normal m e i o s i s . F i g u r e s 15 and 17 are two apparent p a i r s o f chromosomes which e x h i b i t f a i r l y c l o s e p a i r i n g . F i g u r e 15 (probably chromosome 2) shows a s t r i k i n g degree o f v i s u a l s i m i l a r i t y between the a p p a r e n t l y p a i r e d chromosomes, e s p e c i a l l y between the p a i r o f i d e n t i c a l knobs, and t h e r e i s a p a r t i c u l a r l y c l o s e p a i r i n g a t the base of the knobs. T h i s cannot r e p r e s e n t mere s e p a r a t i o n of the daughter chromatids a t metaphase because these chromosomes a r e i n anaphase. They are a l r e a d y going to the p o l e s . I f the p a i r e d chromosomes - 14 -are daughter chromosomes, the anaphase must be a s e p a r a t i o n o f homologues; i f the p a i r e d chromosomes a r e homologues, then i p s o f a c t o , somatic p a i r i n g i s o c c u r r i n g . We must t h e r e f o r e accept the f a c t t h a t somatic p a i r i n g i n some form does occur i n the o v u l a t e t i s s u e of Abies g r a n d i s . For comparison, a more o r l e s s normal m i t o s i s o c c u r r i n g i n a somatic s c a l e c e l l (designated c e l l VI) i s i n c l u d e d i n P l a t e VI, F i g . 19 and P l a t e V I I I , F i g . 27. Here, appearances of p a i r i n g a r e i n c o n c l u s i v e or wanting. I t should be p o i n t e d out t h a t even i n o v u l a r t i s s u e , t h i s i s the u s u a l type o f m i t o s i s ; c e l l s w i t h d i r e c t evidence o f p a i r i n g are o c c a s i o n a l o n l y . VI D i s c u s s i o n The c h i e f p o i n t a r i s i n g from the o b s e r v a t i o n s on chromosome morphology i s that the chromosomes of Abies g r a n d i s (and p o s s i b l y of A. l a s i o c a r p a ) as seen i n the p r e s e n t study, d i f f e r c o n s i d e r a b l y from the chromosomes o f A. c e p h a l o n i c a and A. c o n c o l o r as d e s c r i b e d by Sax and Sax (1935). The two r e p o r t s can b e s t be c ompared by examining the three f i g u r e s of P l a t e I I i n which three s p e c i e s a r e d i a g r a m m a t i c a l l y p o r t r a y e d . The c h i e f d i f f e r e n c e s are i n the l e n g t h of chromosomes, number 1 i n Abies g r a n d i s being g r e a t l y l o n g e r than number 1 i n the o t h e r species, and number 12 i n A. g r a n d i s being much s h o r t e r than i t s c o u n t e r p a r t ; and i n the p o s i t i o n - 15 -of the centromeres, number 1 being d i c e n t r i c , and numbers 10 and 12 being t e r m i n a l . S i n c e i t i s u n l i k e l y t h a t a d i c e n t r i c chromosome i s p r i m i t i v e , the c o n d i t i o n suggests a t r a n s f e r o f a segment from number 12 to number 1 i n c l u d i n g a p o r t i o n o f a centromere. In t h i s connection, Longley (1941) reviewing the c y t o g e n e t i c s o f Zea s t a t e s that i n fragmentation o f a chromosome, any fragment c o n t a i n i n g a centromere s u r v i v e s , but a fragment l a c k i n g a centromere d i e s u n l e s s i t can a t t a c h i t s e l f to another chromosome. On the other hand, X-ray i r r a d i a t i o n i s known to have caused a s p l i t i n a centromere forming two f u n c t i o n a l centromeres and a l l o w i n g the p e r s i s -tence o f two fragments. The second centromere i n chromosome 1 o f Abies g r a n d i s may be such a centromere fragment a t t a c h e d to another chromosome. On the o t h e r hand there i s another p o s s i b i l i t y . The o u t l i n e o f the chromosomes seen i n the p o l l e n mother c e l l d i v i s i o n i s very i r r e g u l a r w i t h obvious knobs and c o n s t r i c t i o n s . T h i s , coupled w i t h the h i g h number o f n u c l e o l i seen i n the " r e s t i n g " nucleus (See Rattenbury 1945 and a l s o P l a t e VI, P i g . 18) has l e d to the suggestion t h a t : (a) the h a p l o i d number 12 i s a c t u a l l y a p o l y p l o i d on a b a s i c number o f three o r more l i k e l y f o u r . (b) The c o n s t r i c t i o n s a re p o i n t s o f weakness which a l l o w f a i r l y " f r e q uent" ( i n a geolog-i c a l - t i m e sense) r e - s h u f f l i n g o f the i n t e r v e n i n g segments so t h a t the a c t u a l morphology o f the chromosome i s v a r i a b l e to a - 16 -degree though i m p o s s i b l e by Longley f o r Zea. At any r a t e the chromosomes r e t a i n t h e i r i d e n t i t y and s t r u c t u r e through m i t o s i s as m e i o s i s i n a l l c e l l s examined i n the s p e c i e s Abies grand!s. The d i f f e r e n c e s seem to be between s p e c i e s , not i n s i d e them. We are d e a l i n g w i t h processes and events which cause d i f f e r e n c e s on a s p e c i e s l e v e l - events which must have o c c u r r e d l o n g ago. The matter of p a i r i n g , however, a p p a r e n t l y c o n t i n u e s a l l the time. I t i s not common, but one good example such as C e l l V i s enough to i n d i c a t e i t s e x i s t e n c e . Yet i n a l l p r o b a b i l i t y i t i s not a normal m e i o s i s ; there has not been found any sporophyte somatic c e l l c o n t a i n i n g a h a p l o i d number of chromosomes. Three f a c t s : (a) t h e r e has been no r e d u c t i o n proven; (b) chromosomes a r e not shortened and t h i c k e n e d as much as i n t r u e m e i o s i s ; (c) most of the o t h e r apparent cases of p a i r i n g were even more l a x than C e l l V; these three f a c t s suggest that t h i s i s a p a r t i a l m e i o s i s showing v a r y i n g degrees o f approximation to t r u e m e i o s i s , but probably v e r y seldom, i f ever, a c h i e v i n g t r u e r e d u c t i o n . Processes such as c r o s s i n g - o v e r may take p l a c e , but t h e r e i s no evidence to assume achievment of g e n e t i c s e g r e g a t i o n . Two separate e x p l a n a t i o n s present themselves f o r t h i s phenomenon, one h i s t o r i c a l , the o t h e r p h y s i o l o g i c a l . The f i r s t e x p l a n a t i o n i s that the ovule and o v u l i f e r o u s s c a l e together form a m o d i f i e d sporangium which once contained - 17 -numerous spore mother c e l l s . Now i f a c t u a l f e r t i l e t i s s u e were reduced i n the process of e v o l u t i o n to one c e l l by the p r o g r e s s i v e s t e r i l i z a t i o n o f once r e p r o d u c t i v e t i s s u e , r a t h e r than by a s o r t of squeezing i n of r e p r o d u c t i v e t i s s u e by encroaching s t e r i l e t i s s u e , the p a i r i n g s seen i n the ovule t i s s u e may be v e s t i g i a l meioses, evidences of incomplete s t e r i l i z a t i o n . On the o t h e r hand, experiments by Huskins et a l (Huskins 1948, Kodani 1948) seem to prove t h a t , i n c e r t a i n i n s t a n c e s a t l e a s t , m i t o s i s can become m e i o s i s when the r i g h t chemical stimulus i s g i v e n . I t may be presumed t h a t there i s some agency, chemical or p h y s i c a l , p r e s e n t i n or about the megaspore mother c e l l which s t i m u l a t e s i t to under-go m e i o s i s . Since the observed p a i r i n g s i n Abies g r a n d i s occur i n areas adjacent to the megaspore mother c e l l , i t seems l i k e l y t h a t they r e p r e s e n t a spreading of i n f l u e n c e or an outward d i f f u s i o n of t h i s s t i m u l a t i n g agency i n s u f f i c i e n t q u a n t i t y to cause weak i m i t a t i o n s of m e i o s i s i n n o r m a l l y mi-t o t i c c e l l s . V I I Appendix - C y t o l o g l c a l Technique For t h i s study over 500 s l i d e s were made but fewer than 40 were of any v a l u e . Much of the waste was used up i n experimentation to determine s u i t a b l e technique f o r the c o n i f e r s . There are s e v e r a l d i f f i c u l t i e s p e c u l i a r to t h a t - 18 -group o f p l a n t s : (1) Large q u a n t i t i e s of waterproof r e s i n and hard sclerenchyma i n a l l outer l a y e r s p r e v e n t i n g r a p i d p e n e t r a t i o n e s p e c i a l l y by aqueous f i x a t i v e s , and h i n d e r i n g c a r e f u l d i s s e c t i o n . (2) Large s i z e o f t r e e , i n a c c e s s i b i l i t y of the r e p r o d u c t i v e bodies and r i g i d i t y of the time c a l e n d a r making i t almost i m p o s s i b l e to c o n t r o l or experiment w i t h the l i v i n g p l a n t . (3) Low s u r v i v a l r a t e of egg c e l l s i n n a t u r e . (4) C e l l s small so t h a t chromosomes, w h i l e q u i t e l a r g e , seldom have a chance to spread out f o r examination. (5) Protoplasm damaged by moderate h e a t i n g . D i f f i c u l t i e s 3, 4, and 5 make nece s s a r y c o l l e c t i o n on the spot of l a r g e q u a n t i t i e s of m a t e r i a l so t h a t there i s a b e t t e r chance of f i n d i n g normal c e l l s i n the r i g h t stage w i t h the chromosomes a c c i d e n t a l l y v i s i b l e . For s t u d y i n g chromosome morphology, i t i s b e s t to have whole c e l l s to examine r a t h e r than (as i n c e l l I I ) separated s e c t i o n s . For t h i s reason smears (as i n c e l l I ) a r e much s u p e r i o r to s e c t i o n s wherever smears can be made. Where embedding must be used, i t has been found t h a t the p r o t o -plasm's i n t o l e r a n c e of heat causes l o n g p e r i o d s o f i n f i l t r a t -i o n a t p a r a f f i n m e l t i n g temperatures to plasmolyse the c e l l s v e ry badly. To overcome the two d i f f i c u l t i e s of r e s i n - 19 -wat e r p r o o f i n g and heat p l a s m o l y s i s , a s p e c i a l r a p i d technique was developed which produced f a i r l y s a t i s f a c t o r y r e s u l t s . I t s c h i e f f e a t u r e s were: (a) c u t t i n g i n t o small p i e c e s , (b) Carnoy's f i x a t i v e , (c) Benzene c l e a r i n g , and (d) speed. The schedule i s : Carnoy's f i x . : 3 p a r t s Abs. "Meth. A l e . 2 p a r t s Chloroform 1 p a r t G l a c i a l A c e t i c - 1-2 h r s . (no l o n g e r ) Absolute A l c o h o l - 20 min. " " - 20 min. 25$ Benzene - 75% Abs. A l c o h o l - 20 min. 50$ " - 50$ " " - 20 min. Pure Benzene - 20 min. Chips o f c o o l e d P a r a f f i n F r o t h f l o a t i n g on Cold Benzene u n t i l d i s s o l v e d - 30 min. More c h i p s P a r a f f i n F r o t h on Warm Benzene u n t i l Benzene more or l e s s s a t u r a t e d - 20 min. Warm to oven temp. (52°C.) - 5 min. Pure melted P a r a f f i n - 20 min. " " " 20 min. " " " 20 min. Embed. T o t a l about 5 or 6 hours. T h i s was found q u i t e s u c c e s s f u l f o r a l l young sporophyte t i s s u e and f o r gametophyte t i s s u e t h a t was embedded immediately. Storage i n 70$ or 80$ a l c o h o l a f t e r Carnoy's - 20 -f i x a t i v e seemed to harden t i s s u e s so t h a t subsequent embedding plasmolysed the c e l l ; s t o r a g e i n Carnoy's f l u i d caused com-p l e t e breakdown o f a l l c e l l c o n t e n t s . The b e s t method found f o r p r e s e r v a t i o n i s embedding i n p a r a f f i n . The h a r d e s t c e l l s o f a l l to embed ar e p o l l e n mother c e l l s because they always plasmolyse. C e l l I and c e l l I I , approximately the same stage of development, a r e drawn to the same m a g n i f i c a t i o n . C e l l I, embedded i n p a r a f f i n , i s t e r r i f i c a l l y c r enated and i s o n l y o n e - t h i r d the diameter of c e l l I I , an aceto-carmine smear; both c e l l s are t y p i c a l o f t h e i r treatment. I t i s , of course, q u i t e probable t h a t aceto-carmine s w e l l s the c e l l a l i t t l e . However, the chromosome s i z e does not v a r y g r e a t l y . Chromosome 1 i n c e l l I i s 35 mm. l o n g on the drawing (23 microns i n the c e l l ) and Chromosome 1 i n c e l l I I i s 31 mm. long on the drawing (20 microns i n the c e l l ) . Very l i t t l e shrinkage of chromosomes, i f any, seems to have taken p l a c e . But i t s t i l l must be presumed t h a t no matter how treatment s h r i n k s o r s w e l l s the c e l l , i t does not change the p r o p o r t i o n a t e l e n g t h s o f the chromosome. T h i s i s a c t u a l l y a r a t h e r l a r g e presumption by i t s e l f , but i t i s a p p a r e n t l y j u s t i f i e d by the experience o f many c y t o l o g i s t s . Three s t a i n s were used f o r the study o f Abies chromosomes: Heidenhain's I r o n Alum Hematoxylin on p a r a f f i n s e c t i o n s ; Peulgen's Leucobasic F u c h s i n w i t h Light-Green i n - 21 -a l c o h o l (Semmens and Bhaduri 1941) on p a r a f f i n s e c t i o n s , and B e l l i n g ' s Iron Aceto-Carmine smears. The L e u c o b a s i c - F u c h s i n and L i g h t Green, while p r o v i d i n g b e a u t i f u l s l i d e s and c a r e f u l d i f f e r e n t i a t i o n of i n t e r n a l s t r u c t u r e s i n chromosomes, i s not as u s e f u l as Hematoxylin i n s t u d y i n g gross chromosome s t r u c t u r e f o r a p r e l i m i n a r y d e s c r i p t i o n . Hematoxylin s t a i n s both chromonema and sheath a s o l i d b l u e - b l a c k t h a t i s easy to see and to d e s c r i b e ; L e u c o b a s i c - F u c h s i n i s the s t a i n to use f o r c o n t i n u a t i o n of t h i s work. For p o l l e n mother c e l l s , Aceto-Carmine smears are s u p e r i o r to a l l s e c t i o n i n g methods because, b e s i d e s p r o v i d i n g a f a i r l y good s t a i n e d o u t l i n e o f the chromosomes, they show the whole c e l l w i t h a l l i t s chromosomes and they do not plasmolyse the c e l l as do a l l other treatments. V I I I Summary One o f the phenomena uncovered i n c y t o l o g i c a l s t u d i e s of Abies g r a n d i s L i n d l e y , the Lowland White F i r , was the e x i s t e n c e o f p a i r i n g of chromosomes i n somatic c e l l s o f the very young o v u l e s . Huskins (1948) has shown that somatic p a i r i n g i s not r a r e i n the p l a n t world, but i s seldom p r o p e r l y r e c orded. I t was thought n e c e s s a r y to undertake a study of the morphology o f the i n d i v i d u a l chromosomes o f the t r e e . Counts and measurements were made on the chromosomes - 22 -In s e v e r a l c e l l s , and the measurements reduced to a p e r c e n t -age of the t o t a l chromosome l e n g t h In the c e l l . P o s i t i o n s o f the s p i n d l e - f i b r e attachments or centromeres were a l s o t e n t a t i v e l y l o c a t e d and recorded as percentages. The l e n g t h s o f the twelve chromosomes i n the genome are as f o l l o w s : Chromosome 1, 14.4$ u n i t s long; 2, 11.3$ u n i t s ; 3, 10.6$ u n i t s ; 4, 9.6$ u n i t s ; 5, 9.1$ u n i t s ; 6, 8.4$ u n i t s ; 7, 8.1$ u n i t s ; 8, 7.2 $ u n i t s ; 9, 6.4$ u n i t s ; 10, 6.1$ u n i t s ; 11, 5.5$ u n i t s ; 12, 3.3$ u n i t s . Two chromosomes have c l u b -shaped ends without any c o n s t r i c t i o n , w h i l e f i v e have s i n g l e t e r m i n a l knobs. One has three c o n s t r i c t i o n s , one has two, and f o u r have but a s i n g l e c o n s t r i c t i o n . One chromosome i s d i c e n t r i c , f o u r a r e a pproximately i s o m e r a l , f i v e are d i s t i n c t l y heteromeral, and two have t e r m i n a l centromeres. Somatic c e l l s from young ovules showing apparent p a i r i n g of chromatin stra n d s a t e a r l y anaphase were examined. In one c e l l s t u d i e d , the p a i r i n g i s so d i s t i n c t and the s i m i l a r i t y between chromatin threads so s t r i k i n g t h a t i t i s thought to be i n d i c a t i o n o f some s o r t of reduced m e i o s i s . The d i f f e r e n c e s between somatic p a i r i n g and t r u e m e i o s i s are d i s c u s s e d and a l s o some t h e o r i e s on the reason f o r somatic p a i r i n g . Some problems i n technique o f c o n i f e r c y t o l o g y and the methods used i n t h i s study are s e t f o r t h . L i t e r a t u r e C i t e d A l l e n , G.S., 1946, "Embryogeny and Development o f the A p i c a l Meristem o f Pseudotsuga, I" Jour.Bot.33:666 Beal, J.M., 1934, "Chromosome Behavior i n Pinus bankslana f o l l o w i n g F e r t i l i z a t i o n " Bot.Gaz.95:660 Chamberlain, C.J., 1935, "Gymnosperms, S t r u c t u r e and E v o l u t i o n " Chicago, U n i v e r s i t y P r e s s . Haupt, A.W., 1941, "Oogenesis and F e r t i l i z a t i o n i n Plnus  lambertiana and P. monophylla" Bot.Gaz.l02:482 Huskins, C.L., 1948, "Segregation and Reduction i n Somatic T i s s u e , I " Jour.Hered.39:311 Hutchinson, A.H., 1915, " F e r t i l i z a t i o n i n Abies balsamea" Bot.Gaz.60:457 Kodani, M., 1948, "Sodium Ribose-Nucleate and M i t o s i s " Jour.Hered.59:327 Longley, A.E., 1941, "Chromosome Morphology i n Maize and i t s R e l a t i v e s " Bot.Rev.7 Rattenbury, J.A., 1945, " N u c l e o l a r and Chromatin Cycles i n Abie s " M.A. T h e s i s , Univ.B.C. Semmens, C.J. and P.N. Bhaduri, 1941, " S t a i n i n g the N u c l e o l u s " Stain.Tech.16:119 Sax, K a r l , and Sax, H.J., 1933, "Chromosome Number and Morphology i n C o n i f e r s " Jour.Arn.Arb.14:356 T a b l e of P l a t e s P l a t e I. Chromosomes o f Abies grand!s arranged i n descend-i n g o r d e r of l e n g t h . P i g . 1 - Prom C e l l I - P o l l e n mother c e l l a t f i r s t anaphase - Aceto-Carmine smear. P i g . 2 - From C e l l I I - P o l l e n mother c e l l a t f i r s t metaphase - Ir o n Alum Hematoxylin s e c t i o n . F i g . 3 - From C e l l I I I - Female gametophyte somatic c e l l a t anaphase - I r o n Alum Hematoxylin s e c t i o n . F i g . 4,- From C e l l IV - Female gametophyte somatic c e l l a t metaphase - Iron Alum Hematoxylin s e c t i o n . P l a t e I I . Diagrams o f chromosome l e n g t h s i n A b i e s . F i g . 5 - Abies c e p h a l o n l c a - m o d i f i e d from Sax and Sax. F i g . 6 - A. c o n c o l o r - m o d i f i e d from Sax and Sax. F i g . 7 - A. g r a n d i s - averages from C e l l s I, I I , I I I , and IV. P l a t e I I I . P i g . 8 - C e l l I i n e n t i r e t y . P l a t e IV. P i g . 9 - C e l l I I - upper h a l f . P i g . 10 - C e l l I I - lower h a l f . F i g . 11 - C e l l I I I . P i g . 12 - Diagram o f chromosomes i n C e l l I I I . F i g . 13 - C e l l IV. P l a t e V. C e l l V showing Somatic P a i r i n g a t e a r l y anaphase. A somatic c e l l on the t i p of a r a p i d l y expanding o v u l i f e r o u s s c a l e a t the time of female m e i o s i s . I r o n Alum Hematoxylin s e c t i o n - C e l l cut i n two. P i g . 14 - upper h a l f . P i g . 15 - two c l o s e l y p a i r e d chromosomes i n upper h a l f . P i g . 16 - lower h a l f . P i g . 17 - two c l o s e l y p a i r e d chromosomes i n lower h a l f . P l a t e VI. Normal somatic c e l l s . P i g . 18 - Two somatic c e l l s a t " r e s t i n g " stage i n young o v u l i f e r o u s s c a l e showing numerous n u c l e o l i , I r o n Alum Hematoxylin s e c t i o n . P i g . 19 - C e l l VI - Somatic c e l l a t anaphase i n young o v u l i f e r o u s s c a l e showing more or l e s s normal m i t o s i s - I r o n Alum Hematoxylin s e c t i o n . P l a t e V I I . Microphotographs. P i g . 20 - C e l l I. P i g . 21 - C e l l I I - upper h a l f . P i g . 22 - C e l l I I - lower h a l f . P i g . 23 - C e l l I I I . P i g . 24 - C e l l IV. P l a t e V I I I . Microphotographs; P i g . 25 - C e l l V - upper h a l f . P i g . 26 - C e l l V - lower h a l f . P i g . 27 - C e l l VI. P L A T E I F I G . I 1 5 0 0 X F I G . 2 I 5 0 0 X F l G . 3 1 5 0 0 X F I G . 4 I 5 0 0 X P L A T E n F I G . 5 F I G . 6 F I G . 7 P L A T E EE P L A T E W F I G . 13 1 5 0 0 X P L A T E F I G . 1 6 1 5 0 0 X F I G . 1 7 7 5 0 0 X P L A T E in 4* PLATE Vi l • FIG.20 500 X ¥ 1 FIG,23 500 X FIG.21 500X % i w FIG. 22 500 X FIG.24 500X P L A T E F I G . 2 5 5 0 0 X F I G . 2 6 5 0 0 X F I G . 2 7 5 0 0 X 

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