Berent, E. | bioRxiv Science

bioRxiv2024

Analysis of a hypomorphic mei-P26 mutation reveals developmental control of CO patterning mechanisms

Female gametogenesis in Drosophila requires differentiation and mitotic division of germ cells, acquisition of oocyte fate, and entry into meiosis. Each of these processes is well understood individually; however, little is known about the mechanisms that ensure proper temporal integration of germ cell differentiation and meiotic chromosome dynamics. Here, we take advantage of a hypomorphic mutation in mei-P26, a well-characterized gene with multiple diverse functions in germ cell development, to determine the consequences of disrupting the coordination between development and meiosis. While null mutations in mei-P26 lead to tumorous ovaries, the hypomorphic allele mei-P261 allows sufficient germ cell differentiation and fertility to support analysis of meiotic chromosome dynamics. Unlike wildtype germaria, 60% of cysts in mei-P261 germaria co-express the differentiation factor Bag of marbles (Bam) and the oocyte specification factor Orb, suggesting that mitotic division is delayed. In this context, the synaptonemal complex rarely assembles into full length continuous tracks and instead is missing or present only as foci. Despite these phenotypes, meiotic double-strand breaks still form and are repaired as crossovers, but the crossovers are mis-patterned and form in centromere proximal regions rather than chromosome arms. The strength of crossover interference is significantly reduced and the centromere effect is lost, but crossover assurance is intact and the meiosis-specific machinery is used to form crossovers. We suggest a model where the failure to exit mitosis in a timely fashion causes cells to enter meiosis while still receiving mitotic signals, resulting in abnormal meiotic chromosome dynamics and impaired crossover formation. Article summaryFemale gametogenesis in Drosophila requires the precise temporal integration of germ cell differentiation and meiotic entry. Using a hypomorphic mutation in mei-P26, we investigated how disrupting this coordination affects meiotic chromosome dynamics. In mei-P261 mutants, delayed mitotic exit causes cells to enter meiosis while still receiving mitotic signals. This results in fragmented synaptonemal complex and the loss of some but not all CO patterning mechanisms. These results suggest that timely mitotic exit is critical for establishing the proper regulatory landscape required for meiotic recombination.

genetics↗

bioRxiv2022

Heterozygous Inversion Breakpoints Suppress Meiotic Crossovers by Altering Recombination Repair Outcomes

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.

genetics↗