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Analysis of chromosome I rearrangements in Caenorhabditis elegans McKim, Kim Stewart


In this thesis, chromosome I rearrangements were used to study the organization of essential genes and regions important for chromosome behaviour in the nematode Caenorhabditis elegans. To facilitate the genetic mapping of mutations in essential genes, rearrangements were isolated using a procedure designed to recover derivative chromosome I duplications shortened by gamma radiation from existing duplications. Sixty-two duplications were isolated in this way. These duplications, along with three deletions isolated in this study and 9 existing deletions of the region, divided the left half of chromosome I into at least 24 regions. Protocols were developed and used to rapidly map mutations into the regions defined by the breakpoints. The techniques and results described demonstrate the feasibility of carrying out a similar analysis on the whole genome. The majority of duplications behaved as if they were free; that is they segregated independently of the euploid chromosome set. While size was an important determinant of mitotic stability, clear exceptions to a size - stability correlation were observed. For example, despite its larger size, hDp72 was lost during cell division more frequently than hDpl8, suggesting features of chromosome structure were important. Shortening of duplications in the unc-11 dpy-5 region caused greater reductions in mitotic stability than similar sized shortenings in the dpy-5 unc-13 region. Therefore, specific sequences appear to influence duplication stability. Some free duplications were also observed to break spontaneously. Breakage occurred at different frequencies for different duplications and correlated with mitotic instability. The meiotic properties of four translocations involving chromosome I were examined. No recombination was observed in any of the translocation heterozygotes along the left (let-362 - unc-13) portion of chromosome I. By isolating a half-translocation chromosome as a free duplication, I mapped the breakpoints of three of the translocations. The boundaries of cross-over suppression coincided with the physical breakpoints. These results agree with the proposal that DNA sequences at the right end of chromosome I are essential for homologue recognition followed by meiotic synapsis and recombination. The published data of other translocations and duplications indicates that each of the other five C. elegans chromosomes has DNA sequences localized to one end that are required for homologue recognition and recombination.

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