Role of Securin, Separase and Cohesins in female meiosis and polar body formation in Drosophila

Abstract

Chromosome segregation in meiosis is controlled by a conserved pathway that culminates in Separase-mediated cleavage of the α-kleisin Rec8, leading to dissolution of cohesin rings. Drosophila has no gene encoding Rec8, and the absence of a known Separase target raises the question of whether Separase and its regulator Securin (Pim in Drosophila) are important in Drosophila meiosis. Here, we investigate the role of Securin, Separase and the cohesin complex in female meiosis using fluorescence in situ hybridization against centromeric and arm-specific sequences to monitor cohesion. We show that Securin destruction and Separase activity are required for timely release of arm cohesion in anaphase I and centromere-proximal cohesion in anaphase II. They are also required for release of arm cohesion on polar body chromosomes. Cohesion on polar body chromosomes depends on the cohesin components SMC3 and the mitotic α-kleisin Rad21 (also called Vtd in Drosophila). We provide cytological evidence that SMC3 is required for arm cohesion in female meiosis, whereas Rad21, in agreement with recent findings, is not. We conclude that in Drosophila meiosis, cohesion is regulated by a conserved Securin-Separase pathway that targets a diverged Separase target, possibly within the cohesin complex.

Cell Cycle Regulators in Female Meiosis of Drosophila melanogaster

“…The expression of a D-box, KEN-box mutant version of Securin in Drosophila oocytes that were depleted of endogenous Securin by RNAi, produced a delay or failure of homolog segregation in meiosis I and sister segregation in meiosis II. In the same study it was found that RNAi knockdown of the Drosophila Separase gene leads to an identical phenotype [78] (Figure 2). Interestingly, neither Separase knockdown nor stabilized Securin lead to a complete failure of cohesion release in meiosis, though both produced a complete and stable failure of cohesion release in the post-meiotic polar body chromosomes [78].…”

“…In the same study it was found that RNAi knockdown of the Drosophila Separase gene leads to an identical phenotype [78] (Figure 2). Interestingly, neither Separase knockdown nor stabilized Securin lead to a complete failure of cohesion release in meiosis, though both produced a complete and stable failure of cohesion release in the post-meiotic polar body chromosomes [78]. While it is possible that incomplete effects are due to a failure to completely inactivate Separase in these experiments, it could also be that a 2nd pathway for cohesion release operates in parallel with and partially redundant with the Securin/Separase pathway.…”

“…Direct cytological evidence also supports an essential role for the core cohesin complex in meiotic cohesion. Using an arm-specific FISH probe, it was found that SMC3 knockdown, but not knockdown of Rad21, results in absence of chromosome arm cohesion in meiotic prophase [78]. Interestingly, centromere-proximal cohesion appears to persist in these oocytes, either because of incomplete loss of cohesin at these sites or perhaps because a 2nd mechanism contributes to centromeric cohesion [78].…”

“…The roles of Securin and Separase in Drosophila meiosis were investigated, employing centromere-proximal and arm-specific FISH probes to monitor cohesion release in meiosis I and II [78]. The expression of a D-box, KEN-box mutant version of Securin in Drosophila oocytes that were depleted of endogenous Securin by RNAi, produced a delay or failure of homolog segregation in meiosis I and sister segregation in meiosis II.…”

“…Knockdown of cohesin component SMC3 leads to precocious homolog segregation (though not sister segregation) [78], implying a role for the cohesin complex in arm cohesion in Drosophila. Complicating this interpretation is the finding that many cohesin components, including SMC1 and SMC3 are important for synaptonemal complex (SC) assembly or maintenance [79].…”