Suppression of meiotic crossovers in pericentromeric heterochromatin requires synaptonemal complex and meiotic recombination factors in Drosophila melanogaster

The centromere effect (CE) is a meiotic phenomenon that ensures meiotic crossover suppression in pericentromeric regions. Despite being a critical safeguard against nondisjunction, the mechanisms behind the CE remain unknown. Previous studies have 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 question, we asked whether different pericentromeric regions respond differently to mutations that impair various features that may play a role in the CE. In flies with a mutation that affects the synaptonemal complex (SC), a structure is hypothesized to have important roles in recombination and crossover patterning, we observed a significant redistribution of pericentromeric crossovers from proximal euchromatin towards beta heterochromatin but not alpha heterochromatin, indicating a role for the SC in suppressing crossovers in beta heterochromatin. In flies mutant for mei-218 or rec , which encode components of a critical pro-crossover complex, there was a more extreme redistribution of pericentromeric crossovers towards both beta and alpha heterochromatin, suggesting an important role for these meiotic recombination factors in suppressing heterochromatic crossovers. Lastly, we mapped crossovers in flies mutant for Su(var)3-9 . Although we expected a strong alleviation of crossover suppression in heterochromatic regions, no changes in pericentromeric crossover distribution were observed in this mutant, indicating that this vital heterochromatin factor is dispensable to prevent crossovers in heterochromatin. Our results indicate that the meiotic machinery plays a bigger role in suppressing crossovers than the chromatin state.