Preventing Age-Associated Oocyte Aneuploidy: Mechanisms Behind the Drosophila melanogaster Centromere Effect

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Project Summary/Abstract During meiosis, crossing-over between homologs facilitates accurate chromosome segregation and prevents aneuploidy, which in turn forestalls miscarriages and chromosomal disorders such as Down syndrome. The regulation and placement of meiotic crossovers acts as a vital safeguard against age-associated meiotic defects and infertility, as the risk of non-disjunction (NDJ) increases with increasing maternal age. Meiotic crossovers (COs) are formed when programmed double-strand breaks (DSBs) are repaired through homologous recombination. However, only a subset of DSBs are repaired to form COs; the rest are repaired as non-crossovers (NCOs). Despite meiotic DSBs being distributed throughout the chromosome, CO placement is intricately regulated by three types of patterning phenomena. One of these, the centromere effect (CE), ensures the exclusion of COs in centromere-proximal regions and is crucial to the meiotic cell as centromere-proximal COs increase the risk of NDJ. Furthermore, increasing maternal age has been shown to weaken the CE, potentially explaining why NDJ incidence increases in older women. Although first observed in Drosophila in 1932, the mechanisms behind the CE remain unknown even today. The experiments proposed here aim to address this gap in knowledge regarding a vital cellular process that prevents mis-segregation events, especially in those with advanced maternal age. Recently, our lab showed that the CE is differentially established in the two classes of heterochromatin found at the Drosophila pericentromere. In the highly repetitive alpha heterochromatin immediately adjacent to the centromere, a complete exclusion of COs is observed, while the less repetitive beta heterochromatin adjacent to proximal euchromatin shows a distance dependent CO suppression. I will build on these results by investigating the mechanisms of how the CE is established in these two classes of pericentric heterochromatin. A prominent question regarding CE mechanisms centers around how pericentromeric heterochromatin and the centromere itself contribute to the CE. The few studies that have addressed pericentric crossing-over in the past century have attempted to establish one as more important than the other in Drosophila but failed to arrive at a consensus. Thus, pericentric heterochromatin has been considered everything from an active participant in CO reduction in adjacent intervals to nothing more than a passive spacer between euchromatin and the centromere. Through the experiments outlined in this proposal, I will ask how pericentric heterochromatin and the centromere contribute to the CE independently of each other, particularly focusing on highly repetitive alpha heterochromatin. Investigating the role of alpha heterochromatin as separate from that of pericentric heterochromatin as a whole in manifesting the CE is a novel area of research within the broader question of how the CE is established. In summary, this proposal will increase our understanding of the mechanisms that safeguard against age- related aneuploidy and infertility through shedding light on meiotic crossover patterning.