THE CENTROMERE EFFECT IN D. MELANOGASTER

The centromere effect (CE) is a meiotic crossover (CO) patterning phenomenon that ensures CO suppression in pericentromeric regions. Despite being a critical safeguard against nondisjunction and aneuploidy, the mechanisms behind the CE remain unknown. My dissertation research focuses on this gap in knowledge at two levels. First, I ask how spatial/structural aspects of chromosome biology contribute to the CE. I use phenotypic recombination mapping to investigate whether dosage changes in centromere number and repetitive DNA content affect CE strength. I also study the importance of centromere-clustering on pericentromeric CO suppression using flies exhibiting substantial centromere declustering in meiotic nuclei. Despite what previous studies suggest, my results show that the Drosophila CE is robust to changes in centromere number, repetitive DNA content, and centromere-declustering, leading us to conclude that the CE is unlikely to be spatially controlled. These observations inspired me to switch vantage points and look at the CE at a more local level, focusing on the contributions of various genetic mutants. Previous research has shown that various regions of the Drosophila pericentromere, encompassing proximal euchromatin, beta and alpha heterochromatin, undergo varying levels of crossover suppression, raising the question of whether distinct mechanisms establish the CE in these different regions. To address this, I asked whether different pericentromeric regions respond differently to mutations that impair various features implicated in establishing the CE. I look at flies mutant for an essential synaptonemal complex (SC) gene, two meiotic recombination genes, and a heterochromatin formation gene, expecting different regions of the pericentromere to display differing sensitivity to each class of mutant. Surprisingly, I observed a significant redistribution of pericentromeric crossovers from proximal euchromatin towards heterochromatin in the SC and meiotic recombination mutants, and no change in pericentromeric crossover distribution in the heterochromatin mutant. This suggests that meiotic machinery is more important for heterochromatic crossover suppression than chromatin state. Combined, both studies point towards the CE being unaffected by spatial factors and instead being primarily mediated genetically and through the meiotic program, providing novel insight into the mechanisms behind this CO patterning event in Drosophila.