Recombination between chromosomes is required to generate genetic variation, maintain genome integrity through the repair of double strand DNA breaks (DSBs), and ensure proper chromosome segregation during meiosis, the specialized cell division program by which diploid organisms generate haploid gametes such as sperm and eggs. Perturbations in recombination events can compromise these basic cellular functions, ultimately leading to cancer, fertility problems, or birth defects. Meiotic recombination is initiated by DSBs, which are repaired using meiosis-specific mechanisms that favor utilization of the homologous chromosome (instead of the sister chromatid) as the recombination partner and that promote a crossover (rather than noncrossover) outcome of the DSB repair (DSBR) process. Crossover recombination between homologous chromosomes is required to create temporary physical connections that promote proper chromosome segregation during meiosis. In order to promote these recombination pathway and partner preferences required for generating crossovers between homologous chromosomes, germ cells engage a specialized DSBR program at the onset of meiotic prophase. During late meiotic prophase, meiotic cells undergo a second switch in the DSBR requirements, which is proposed to revert to utilization of repair preferences typical of mitotically dividing cells (e.g. favoring the sister chromatid as the repair partner). This proposed switch would enable repair of any remaining DSBs prior to the meiotic divisions, thereby preserving genomic integrity. Utilizing the C. elegans experimental system, the proposed studies will address how different recombination pathways and partners are employed during meiosis to both promote crossover formation between homologs and repair any remaining DSBs prior to the meiotic divisions. To assess the temporal and mechanistic features of these pathway and partner choices, I will expand the scope of our recently published assay system by developing an assay to detect and distinguish the different repair products of DSBs induced at a defined site. With this assay, I will directly test for the hypothesized switch in partner preference during meiotic prophase progression and determine the effect of chromosomal position on specific repair outcomes and partner choices. Also, utilizing a variety of genetic and cytological techniques, I will assess the roles of known meiotic proteins in promoting specific meiotic repair outcomes. Further, I will utilize two new reagents I generated to assess dynamics of the early stages of meiotic DSBR in live worms, establish the roles of meiotic chromosome structures in regulating DSB formation and repair, and determine a relationship between specific repair partner choices and classes of early DSBR stage dynamics. Overall, these studies will reveal how recombination pathway and partner preferences ensure that chromosomes form the connections necessary for chromosome segregation and repair DSBs for maintaining genomic integrity.
Accurate chromosome segregation and repair of DNA breaks during sperm and egg development are essential for preventing fertility problems, miscarraiges, birth defects, and cancer. To promote proper chromosome segregation and repair sites of potentially damaging DNA breaks, the chromosomes of developing sperm and eggs engage in specific exchanges of DNA, called recombination events. The proposed research will develop and utilize new technologies to reveal the temporal and mechanistic features of these essential recombination events to increase our understanding of these processes that are critical to human health.
|Schvarzstein, Mara; Pattabiraman, Divya; Libuda, Diana E et al. (2014) DNA helicase HIM-6/BLM both promotes MutS?-dependent crossovers and antagonizes MutS?-independent interhomolog associations during caenorhabditis elegans meiosis. Genetics 198:193-207|