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Genetic and developmental studies of proximal segments of chromosome 3 of Drosophila melanogaster Sinclair, Donald A. R.

Abstract

The present work deals with several approaches to the study of regions near the centromere of chromosome 3 of Drosophila melanogaster. The goals of this research were: (i) to examine spontaneous crossing over near the centromere in detail; (ii) to locate the Deformed locus genetically and to determine whether this lesion is a recessive lethal; (iii) to test the efficacy of radiation-induced crossing over as a method of producing proximal aberrations; and (iv) to genetically and developmentally characterize a temperature-sensitive (ts) allele of a Minute locus, located near the centromere. CHAPTER 2 describes a study of recombination, which deals with short genetic regions near the centromere of chromosome 3, using the intervals, st-in-ri-eg²-Ki-p[sup P] and Gl-p[sup P]-Sb-H. The following generalizations have emerged: (i) an excess of multiple crossover chromosomes was recovered, and the intervals which immediately span the centromere showed the highest negative interference; (ii) a positive correlation of simultaneous exchange within closely-linked intervals, was noted for many of the multiple crossovers; and (iii) several classes of reciprocal crossover products were not recovered equally. Three possible explanations for these results are: pre-meiotic exchange, chromatid interference and gene conversion. The results of one experiment also indicated that the interchromosomal effects of C(1)M3 are most pronounced within the st-in and Ki-p[sup P] intervals. CHAPTER 3 describes a genetic study of the Deformed locus. The mapping results confirmed that Dfd is closely linked to Ki. Genetic analysis of the crossover chromosomes suggested that Dfd is homozygous viable and this was confirmed by the synthesis of homozygous Dfd stocks. This indicates that the Dfd locus is not located within section 84F in proximal 3R. CHAPTER 4 deals with experiments involving the use of radiation to produce crossovers near the centromere of chromosome 3, in males. Crossovers originating from exchange nearest the centromere, were associated with clusters more frequently than those originating from exchange within other proximally-adjacent segments. Induced exchange was frequently accompanied by mutation and/or chromosome damage, at or near the site of exchange. This was particularly true for crossovers resulting from exchange in wholly euchromatic segments. It is suggested that many of the radiation-induced crossovers arise through asymmetrical exchange, and that this approach will permit the isolation of proximal aberrations. CHAPTER 5 describes the genetic and developmental analysis of a ts Minute. As a ts allele of a proximally-located Minute locus, Q-III exhibits the classical dominant M traits, recessive lethality, and a highly pleiotropic phenotype, at 29°C. This phenotype was analysed in detail through the use of various temperature shift experiments. Q-III possesses a polyphasic temperature-sensitive period (TSP) for lethality extending from the first larval instar to late pupation. Shorter heat pulses defined discrete larval, larval/pupal, and pupal TSPs for lethality. In addition, homozygous Q-III females exhibit ts sterility and maternal effects, indicating that the Q-III gene product is essential throughout development. Heat-pulse experiments revealed a number of adult developmental abnormalities, involving derivatives of eye-antennal, leg, wing and genital imaginal discs. Many defects, for example, those involving the eye or antenna (eye-antennal disc), male genitalia (genital disc), and scutellum (wing disc), have larval TSPs; whereas others, such as bristle or sex comb traits, have pupal TSPs. It is suggested that the former defects may be related to cell death in the larval anlagen; while the latter are more likely due to blockages in differentiation during pupation. Q-III also interacts in ts fashion with several non-allelic mutations . Thus, at 29°C, Q-III is lethal when combined with DI, Ly and Dfd; suppresses the sex comb phenes of Msc and Pc; and produces wing nicking effects when combined with vg or Sex. TSPs were defined for the vg, Dl and Sex interactions. It is suggested that many of these interactions arec:metabolic rather than specific. The fact that Q-III phenotypically resembles bobbed and ts suppressor of forked[sup ts], strengthens the notion that Minute gene products are active in translation. It is concluded that translational defects can fully account for the pleiotropy of Q-III.

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