A Novel Role for Sister-Chromatid Cohesion Proteins in Promoting Heterochromatin Mediated Association of Achiasmate Homologs in Drosophila Oocytes. Brian C Seitz, Sharon E Bickel. Biological Sciences, Dartmouth College, Hanover, NH.

   During meiosis in Drosophila females, a single crossover between homologous chromosomes is sufficient to ensure their proper orientation and segregation during the first meiotic division. Cohesion along the arms of sisters keeps recombinant homologs physically associated until anaphase I, when arm cohesion is destroyed. However, not all homolog pairs achieve a crossover. X chromosomes are achiasmate in 6-10% of Drosophila oocytes and 4th chromosomes are always achiasmate; still, in the absence of a crossover, these chromosomes segregate with high fidelity. Work by several investigators has demonstrated that homology-dependent interaction between the pericentric heterochromatin of achiasmate homologs during prophase I is required for their accurate segregation at anaphase I. However, identification of chromatin-associated proteins that facilitate this physical interaction has remained elusive. Previous work from our lab (Subramanian and Bickel, 2009) revealed an unexpected role for the meiotic cohesion protein ORD in promoting pericentric heterochromatin-mediated association of achiasmate homologs. This led us to ask whether other cohesion proteins are required for this process. We used the Gal4/UAS inducible system to knock down the cohesion regulators Wapl and Pds5 during meiotic prophase. Using a FISH probe that hybridizes to the 359bp repeat in the pericentric heterochromatin of the X chromosome, we observed significant disruption of FM7a/X association when either Wapl or Pds5 was knocked down in the germ line. In addition, using our standard X meiotic segregation assay, we observed a small but significant increase in FM7a/X nondisjunction in Wapl knockdown oocytes. These genetic data are consistent with those reported by Vernì et al. (2000) for a wapl loss of function allele in wapl/+ heterozygous females. Together, our results support the hypothesis that proteins involved in sister chromatid cohesion not only maintain the association of chiasmate homologs but also play an essential role in promoting the physical association of achiasmate homologs in Drosophila oocytes.