Meiosis is a specialized form of cell division in which a single diploid cell undergoes one round of genome duplication followed by two rounds of cell division to produce four haploid gametes. In most organisms, including Drosophila melanogaster, programmed double-strand breaks (DSBs) are created during meiosis that are typically repaired by one of two mechanisms: crossing over, which involves the exchange of flanking markers, or noncrossover gene conversion (NCO), which copies short segments of DNA from a homologous chromosome to repair the break. Crossing over is necessary for the proper segregation of homologous chromosomes at the first meiotic division, a process facilitated by the synaptonemal complex (SC), a large, multi-protein structure that holds homologs together during meiosis. Chromosomes that fail to crossover may not segregate properly, resulting in aneuploid gametes.
In many organisms, including humans, two forces primarily control the distribution of crossovers along the chromosome arm. The strongly polar centromere effect functions to reduce the frequency of centromere-proximal crossovers, while interference ensures that crossovers occurring on the same chromosome arm are widely spaced. It is unknown if these forces control the distribution of NCOs as well. In addition, while it is known that Drosophila mutants that fail to construct SC cannot repair DSBs by crossing over, it is unknown if these breaks can be repaired as NCOs. Finally, the forces that prevent crossing over are of interest as well. In Drosophila, multiply inverted balancer chromosomes are used either to suppress recombination or to prevent the recovery of recombinant chromosomes. While it is known that inversion breakpoints themselves suppress nearby crossover events it is unclear over what distance they act.
In this work, I used whole-genome sequencing to investigate recombination in D. melanogaster. First, I precisely positioned CO and NCO events after a single round of meiosis in 196 individual wild-type males. While I found that CO distribution appears to be controlled, as expected, by the centromere effect and interference, NCOs surprisingly do not seem to respond to these same controls. In addition, I looked for evidence of NCOs in SC-deficient flies and recovered a single NCO event, suggesting that while rare, repair by NCO is possible in these mutants. These data also allowed me to identify novel meiotic events such as transposable element (TE)-mediated copy-number variations, which included evidence of recurrent CNV formation, which is known to contribute to disease in humans. Finally, I identified the precise genomic location of the majority of the inversion breakpoints of several of the most commonly used X and 3rd chromosome balancers in Drosophila. This knowledge allows us to understand over what distance these breakpoints suppress crossing over. This analysis also allowed me to identify several instances of double crossovers, demonstrating that the mechanism by which balancers suppress exchange with their normal-sequence homologs is incomplete.
|Commitee:||Albertini, David F., Bergman, Casey M., Blumenstiel, Justin P., Fenton, Aron W., Fields, Timothy|
|School:||University of Kansas|
|Department:||Molecular & Integrative Physiology|
|School Location:||United States -- Kansas|
|Source:||DAI-B 77/10(E), Dissertation Abstracts International|
|Subjects:||Genetics, Bioinformatics, Physiology|
|Keywords:||Balancer chromosomes, Crossing over, Double strand breaks, Drosophila melanogaster, Gene conversion, Meiosis|
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