Cohesin and microtubule dependent mechanisms regulate sister centromere fusion during meiosis I

Sister centromere fusion is a process unique to meiosis that promotes co-orientation of the sister kinetochores, ensuring they attach to microtubules from the same pole. We have found that the kinetochore protein SPC105R/KNL1 and Protein Phosphatase 1 (PP1-87B) are required for this process. The analysis of these two proteins, however, has shown that two independent mechanisms maintain sister centromere fusion during meiosis I in Drosophila oocytes. Double depletion experiments demonstrated that the precocious separation of centromeres in Spc105R RNAi oocytes is Separase-dependent, suggesting cohesin proteins must be maintained at the core centromeres. In contrast, precocious sister centromere separation in Pp1-87B RNAi oocytes does not depend on Separase or Wapl. Further analysis with microtubule destabilizing drugs showed that PP1-87B maintains sister centromeres fusion by regulating microtubule dynamics. Additional double depletion experiments demonstrated that PP1-87B has this function by antagonizing Polo kinase and BubR1, two proteins known to promote kinetochore-microtubule (KT-MT) attachments. These results suggest that PP1-87B maintains sister centromere fusion by inhibiting stable KT-MT attachments. Surprisingly, we found that loss of C(3)G, the transverse element of the synaptonemal complex (SC), suppresses centromere separation in Pp1-87B RNAi oocytes. This is evidence for a functional role of centromeric SC in the meiotic divisions. We propose two mechanisms maintain co-orientation in Drosophila oocytes: one involves SPC105R to protect cohesins at sister centromeres and another involves PP1-87B to regulate spindle forces at end-on attachments.