Regulation of meiotic cohesion and chromosome core morphogenesis during pachytene inDrosophilaoocytes

Abstract

During meiosis, cohesion between sister chromatids is required for normal levels of homologous recombination, maintenance of chiasmata and accurate chromosome segregation during both divisions. In Drosophila, null mutations in the ord gene abolish meiotic cohesion, although how ORD protein promotes cohesion has remained elusive. We show that SMC subunits of the cohesin complex colocalize with ORD at centromeres of ovarian germ-line cells. In addition, cohesin SMCs and ORD are visible along the length of meiotic chromosomes during pachytene and remain associated with chromosome cores following DNase I digestion. In flies lacking ORD activity, cohesin SMCs fail to accumulate at oocyte centromeres. Although SMC1 and SMC3 localization along chromosome cores appears normal during early pachytene in ord mutant oocytes, the cores disassemble as meiosis progresses. These data suggest that cohesin loading and/or accumulation at centromeres versus arms is under differential control during Drosophila meiosis. Our experiments also reveal that the α-kleisin C(2)M is required for the assembly of chromosome cores during pachytene but is not involved in recruitment of cohesin SMCs to the centromeres. We present a model for how chromosome cores are assembled during Drosophila meiosis and the role of ORD in meiotic cohesion, chromosome core maintenance and homologous recombination.

Functional links between Drosophila Nipped-B and cohesin in somatic and meiotic cells

“…In contrast to Nipped-B, bright staining for the cohesion proteins Ord and Smc1/Smc3 is visible in centromeric heterochromatin, which is identified by bright diamidinophenylindole staining and immunostaining for the CID centromere-specific histone ( Fig. 6; Khetani and Bickel 2007;Webber et al 2004). These data suggest that Nipped-B may not associate strongly with meiotic centromeres and are consistent with the lack of Nipped-B signal at the centromeres of polytene chromosomes.…”

“…In particular, the retention of Smc1 and Smc3 protein at the meiotic chromosome centromeres in heterozygous Nipped-B mutants and the normal meiotic transmission of the J21A minichromosome with a weak centromere indicate that centromeric cohesion is not altered, although the chromosome cores disintegrate early when the dosage of Nipped-B protein is reduced. This contrasts sharply with the chromosome nondisjunction and lack of cohesin binding to the meiotic centromeres in ord mutants (Khetani and Bickel 2007). Thus, the evidence suggests that either Nipped-B does not play a critical role in the function of cohesin at meiotic centromeres or that its role at centromeres is not as dosage sensitive as its function along the chromosome cores.…”

“…6). It also colocalizes with the Ord protein, which is required for meiotic cohesion in both sexes and SC maintenance in oocytes (Bickel et al 1997;Khetani and Bickel 2007;Webber et al 2004), and with C(3)G, a transverse filament protein that resides within the central element of the SC (Anderson et al 2005). These data argue that Nipped-B may play a direct role in regulating meiotic cohesion as well as SC structure and function.…”

“…Z-stacks were collected and deconvolved using the Improvision Volocity software. Meiotic chromosome spreads were prepared and immunostained as described previously (Khetani and Bickel 2007). For all experiments with meiotic cells, a mixture of guinea pig affinity-purified antibodies raised against Smc1 and Smc3 peptides (Khetani and Bickel 2007) was utilized to visualize cohesin.…”

“…In homozygous ord mutants, Smc1 and Smc3 fail to accumulate at centromeres, and the Smc1 and Smc3 threadlike staining along chromosome arms is already disrupted in germarium region 2B, which is earlier than in the heterozygous Nipped-B mutants (Khetani and Bickel 2007). The ord mutants also show elevated levels of meiotic nondisjunction (Bickel et al 1997).…”