Meiotic Chromosome Synapsis and Recombination in Yeast

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 DESCRIPTION (provided by applicant): We investigate the processes involved in communication along and between homologous chromosomes during meiosis, the specialized cell cycle that underlies gamete formation for sexual reproduction. Importantly, defects in meiosis underlie human infertility and several genetic diseases, notably Downs Syndrome. Proposed research addresses three broad areas. I. We investigate how chromosomal events can occur in an evenly-spaced pattern without direct genetic specification of position. In particular, such patterning is exhibited by meiotic crossovers, manifested originally in the genetic phenomenon of "crossover interference" and subsequently by cytological analyses of crossover-correlated structures along elongated mid-prophase chromosomes and later as points of connections between homologs along partially compacted diplotene chromosomes ("chiasmata"). We are using imaging, proteomics, genetics, mathematical simulations, and laser nanosurgery to address three questions: (A) What molecules are involved in interference? (B) What are the biological implications of this patterning? (C) Does communication occur by redistribution of mechanical stress, as we have proposed? II. We investigate homologous chromosome pairing, a central universal feature of the meiotic program, again by three entry points. (A) We have developed a uniquely powerful system for 3D tracking of homologously-pairing FROS foci in living meiotic yeast cells, with images collected densely over the entire time period of meiosis. This system is poised to address recombination-dependent and -independent pairing and the nature and roles of motion during the pairing period. (B) Meiotic homolog pairing involves both avoidance and active elimination of entanglements (interlocks). By tracking chromosome paths in Sordaria, we can now answer two key questions. (i) What is the mechanism of interlock resolution (by TopoII or by telomere-led movement)? (ii) Does the classical bouquet stage mediate initial homolog contact for pairing or is it involved primarily in ensuring regular topological relationships? (C) Our evidence suggests that repeat-induced point mutation (RIP) in Neurospora crassa involves recombination-independent DNA/DNA pairing. As a second approach to DSB-independent pairing, we are searching for functions required for RIP. III. Meiotic recombination occurs in close physical and functional interaction with chromosome structural axes and, once it forms between axes, the synaptonemal complex (SC). In Sordaria, we will extend recent findings which: (A) begin to elucidate a molecular pathway of events at the critical mid-prophase transition when SC is installed; (B) suggest a new idea for how chromatid axes become "exchanged" at sites of DNA crossovers; and reveal that breast cancer-related BRCA2 has diverse roles for chromosomes beyond those attributable to its canonical role as a Rad51 mediator. Finally, to open an entirely new window onto the meiotic process, we will develop a system for identification and analysis of Sordaria meiotic non-coding RNAs.