ORD and cohesin at the intersection of recombination and cohesion during Drosophila meiosis

Proper regulation of chromosome segregation during cell division is essential to prevent aneuploidy. During mitosis as well as meiosis, the cohesin complex physically holds sister chromatids together and is necessary for proper segregation of chromosomes. Meiotic sister-chromatid cohesion is required for normal levels of recombination between homologous chromosomes, maintenance of chiasmata as well as accurate segregation of sisters during meiosis I and meiosis II.;In this dissertation, I describe the localization dynamics of two cohesin subunits, SMC1 and SMC3, during wild-type meiosis in Drosophila females and also characterize cohesin localization defects in mutants where cohesion and/or homologous recombination is disrupted. During prophase I, SMC1 and SMC3 colocalize extensively with ORD, a Drosophila protein necessary for normal levels of recombination and cohesion during both meiotic divisions. In all germline cells, SMC1/3 and ORD are enriched at centromeric regions and also localize along chromatid arms. In pachytene cells, SMC1/3 and ORD immunostaining along chromosome arms overlaps extensively with the synaptonemal complex (SC). This thread-like staining pattern persists following DNase treatment of meiotic chromosome spread preparations, indicating that these proteins are localized along chromosome cores during pachytene. Immunostaining experiments performed in flies lacking ORD demonstrate that cohesin SMCs fail to accumulate at oocyte centromeres. However, thread-like staining along chromosome arms appears normal during early pachytene, but deteriorates during prophase I progression. Interestingly, cohesin SMCs remain associated with chromosome arms in pro-oocytes in ordnull ovaries, and display a localization pattern identical to pro-nurse cells. Based on these results we propose that ORD is necessary to maintain meiotic chromosome cores during pachytene. Additional experiments demonstrate that the Drosophila protein C(2)M is required for chromosome core assembly during early pachytene.;This work provides insight into the mechanisms that govern changes in chromosome morphology that are necessary for homologous recombination during meiosis. In addition, these data indicate that association of cohesin subunits with chromosome arms and centromeres is regulated differently during female meiosis in flies.