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Distortions of the genetic map of chromosome I in Caenorhabditis elegans Zetka, Monique-Claire


The process of meiosis ensures heritable genetic material is passed faithfully from one generation to the next. To identify the mechanisms involved in this process, the effects of sex, mutation, and rearrangement on meiotic recombination in Caenorhabditis elegans were investigated. The short life cycle and existence of meiotic mutants make this organism an ideal system in which to study meiosis. To determine the effect of sex on meiotic recombination, crossing over was characterized in male gametes and then compared to the frequencies observed in hermaphrodite gametes. Male recombination across chromosome /was approximately one-third less than that observed in the hermaphrodite. This decrease varied with the interval being measured and in one interval, no difference was observed between the sexes. The frequency of recombination in hermaphrodite spermatocytes was two-fold higher than that observed in oocytes and male spermatocytes . Thus, recombination frequencies appear to be a function of gonad physiology rather than sexual phenotype. To test this further, recombination was measured in males sexually transformed by the her-1 mutation. The results indicated that the sexual phenotype, rather than karyotype, determined the recombination frequency characteristic of a certain sex. Like recombination in the hermaphrodite, male recombination was also found to increase with temperature and decrease with age. Therefore, recombination frequency in C. elegans is influenced by physiological factors such as sexual phenotype and age, and environmental factors such as temperature. Mutations in genes that regulate meiosis can affect the frequency of recombination and the distribution of exchange events. A recessive mutation in the gene rec-1 was mapped, and its effects on the distribution of crossing over on LG / were determined. This mutation was mapped to the right end of chromosome /using the duplications sDp1 (which carries a wild-type allele of the gene) and sDp2 (which does not). A high resolution map position was determined using several deficiencies of the right end of the chromosome to map the mutation. The ribosomal deficiency eDf24 failed to complement rec-1, indicating the locus was located within its boundaries. Crossing over in five intervals on chromosome I was measured in rec-1 homozygotes. The frequency of recombination in one interval located in the, centre of the chromosome showed a ten-fold increase, whereas an interval located on the right end showed a three-fold decrease. Despite the changes to the frequencies of recombination in these intervals, the total genetic length of chromosome /remained unchanged, indicating that the rec-1 mutation affected the distribution of a wild-type number of exchange events. This implies that the rec-1(+) gene product is necessary in establishing the distribution of crossovers along the chromosome. Chromosome rearrangements can reduce or eliminate crossing over by physically disrupting the normal organization of the chromosome. In this study, a crossover suppressor for the right end of LG /was isolated and characterized. By inducing markers on the rearrangement and establishing the gene order in the homozygote, hInl(I) was demonstrated to be the first inversion isolated in C. elegans. Crossing over in the heterozygote was characterized, and intrachromosomal (but not interchromosomal) effects were observed. The interaction of hInl(I) with two translocations demonstrated that small homologous regions can pair and recombine efficiently, and that the formation of a chiasma between two homologues is necessary for their proper segregation. Rare recombinants bearing duplications and deficiencies were isolated from inversion heterozygotes, leading to the proposal that hIn1(I) is paracentric with the meiotic centromere to its left. The meiotic behaviour of the inversion was found to be consistent with the proposal that the meiotic chromosomes of C. elegans are monocentric.

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