Female gametogenesis in Drosophila requires both differentiation of germline stem cells and the unique cell cycle meiosis. Successful segregation of chromosomes into gametes during meiosis requires the formation of crossovers between homologous chromosomes. The frequency and distribution of crossovers is highly regulated at several levels, including formation of at least one obligate crossover per pair of homologs, crossover interference that keeps crossovers farther apart than expected by chance, and suppression of crossovers in and near the centromere. To achieve crossover patterning, a meiosis specific version of homologous recombination is used to repair programmed double-stranded breaks, and in Drosophila melanogaster , this recombination pathway requires the putative double Holliday junction resolvase MEI-9. Despite our understanding of crossover patterning mechanisms at the phenomenological level, how crossover patterning mechanisms are developmentally controlled in the context of germ cell differentiation is unknown. Here, we take advantage of a hypomorphic mutation in mei-P26, a gene previously associated with regulation of both germ cell mitoses and meiotic induction. We confirm that mei-P26 1 mutants show extended expression of mitotic cell cycle markers but rarely completely block differentiation, indicating that the mitotic cell cycle is mis-regulated. In this developmentally delayed context, we show that mei-P26 1 mutants enter meiosis and complete the earliest stages of prophase, including loading of synaptonemal complex proteins onto the centromere and initial loading onto the arms. Most nuclei do not progress past this to make full length synaptonemal complex, yet meiotic double-strand breaks are induced and crossovers form. However, while these crossovers require MEI-9 and exhibit crossover assurance, the centromere effect and interference are lost. We suggest a model where entry into zygotene of prophase is enough to commit pro-oocytes to using the meiotic homologous recombination machinery and thus ensuring crossover assurance, but that full length synaptonemal complex assembly is required for crossover interference.
Abstract Heterozygous chromosome inversions suppress meiotic crossover (CO) formation within an inversion, potentially because they lead to gross chromosome rearrangements that produce inviable gametes. Interestingly, COs are also severely reduced in regions nearby but outside of inversion breakpoints even though COs in these regions do not result in rearrangements. Our mechanistic understanding of why COs are suppressed outside of inversion breakpoints is limited by a lack of data on the frequency of noncrossover gene conversions (NCOGCs) in these regions. To address this critical gap, we mapped the location and frequency of rare CO and NCOGC events that occurred outside of the dl-49 chrX inversion in D. melanogaster . We created full-sibling wildtype and inversion stocks and recovered COs and NCOGCs in the syntenic regions of both stocks, allowing us to directly compare rates and distributions of recombination events. We show that COs are completely suppressed within 500 kb of inversion breakpoints, are severely reduced within 2 Mb of an inversion breakpoint, and increase above wildtype levels 2-4 Mb from the breakpoint. We find that NCOGCs occur evenly throughout the chromosome and, importantly, occur at wildtype levels near inversion breakpoints. We propose a model in which COs are suppressed by inversion breakpoints in a distance-dependent manner through mechanisms that influence DNA double-strand break repair outcome but not double-strand break location or frequency. We suggest that subtle changes in the synaptonemal complex and chromosome pairing might lead to unstable interhomolog interactions during recombination that permits NCOGC formation but not CO formation.
Heterozygous chromosome inversions suppress meiotic crossover (CO) formation within an inversion, potentially because they lead to gross chromosome rearrangements that produce inviable gametes. Interestingly, COs are also severely reduced in regions nearby but outside of inversion breakpoints even though COs in these regions do not result in rearrangements. Our mechanistic understanding of why COs are suppressed outside of inversion breakpoints is limited by a lack of data on the frequency of noncrossover gene conversions (NCOGCs) in these regions. To address this critical gap, we mapped the location and frequency of rare CO and NCOGC events that occurred outside of the dl-49 chrX inversion in D. melanogaster. We created full-sibling wildtype and inversion stocks and recovered COs and NCOGCs in the syntenic regions of both stocks, allowing us to directly compare rates and distributions of recombination events. We show that COs outside of the proximal inversion breakpoint are distributed in a distance-dependent manner, with strongest suppression near the inversion breakpoint. We find that NCOGCs occur evenly throughout the chromosome and, importantly, are not suppressed near inversion breakpoints. We propose a model in which COs are suppressed by inversion breakpoints in a distance-dependent manner through mechanisms that influence DNA double-strand break repair outcome but not double-strand break formation. We suggest that subtle changes in the synaptonemal complex and chromosome pairing might lead to unstable interhomolog interactions during recombination that permits NCOGC formation but not CO formation.
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