Project Summary In sexually reproducing organisms, meiosis reduces the chromosome complement by half to generate haploid gametes. During meiosis, homologous chromosomes undergo pairing, synapsis, recombination, and faithful segregation. As such, defects in meiosis are a leading cause of both infertility and birth defects in humans. While a large number of meiosis-specific factors are involved, ubiquitously expressed factors also play critical roles. The meiotic process is generally conserved from single-cell eukaryotes to multicellular metazoans; however, species-specific meiosis factors and functions have evolved. Our genomic and proteomic screens have identified a large number of novel mammalian meiosis factors and we plan to determine the molecular function of a number of key factors in the regulation of meiosis in both sexes. Meiotic recombination is essential for genome integrity in gametes and critical for genome evolution by increasing genetic diversity at the population level. Meiotic recombination begins with formation of meiotic DNA double strand breaks (DSBs). There are major knowledge gaps in our understanding of this meiotic process. One main challenge is to understand the mechanism that prevents meiotic DSB formation from getting out of control. Only a subset of the meiotic DSBs are processed into crossovers by a large protein network. What is the molecular mechanism underlining the crosstalk in this protein network? What additional proteins are involved in the processing of DSBs into crossovers? In many species, chromosomal synapsis is tightly coupled with meiotic recombination. However, the process of synapsis initiation and maintenance remains poorly understood. These challenges will be tackled in this application through deciphering novel molecular networks underlying regulation of mammalian meiosis. Our innovative combination of genetic, genomic, proteomic, cell/molecular biological, and biochemical approaches will focus on the following critical but challenging aspects of meiosis: initiation of meiotic DNA double strand breaks, regulation of meiotic recombination, initiation and maintenance of chromosomal synapsis, and origin of sex chromosome aneuploidy. Completion of the proposed studies will provide novel mechanistic insights into key meiotic processes, identify new candidate fertility factors, and elucidate the molecular etiology of sex chromosome aneuploidy.