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

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.