Cohesion without cohesins in drosophila meiosis

Meiosis is essential for sexual reproduction. The proper segregation of chromosomes in meiosis requires multiple functions of a multi-subunit protein complex known as cohesin. Cohesin forms a ring around duplicated sister chromatids and prevents them from separating prematurely. In Drosophila, mitotic cohesin is composed of four subunits: SMC1 and SMC3 and SCC1/RAD21 and SCC3/SA and are required for mitotic cohesion. However, RAD21 is dispensable for meiotic cohesion and although SMC1 and SMC3 are present on meiotic centromeres and on synapsed chromosome arms, as expected, their functions in meiosis remain poorly characterized. Moreover, unlike in other eukaryotes in which screens for meiotic cohesion genes have revealed meiosis-specific cohesin paralogs such as the conserved RAD21 paralog REC8, similar screens in Drosophila identified three novel genes that encode meiosis-specific centromere proteins (SOLO, ORD and SUNN (SOS)) with no apparent similarity to cohesins. Loss-of-function mutations in all three genes disrupt centromere cohesion and SMC1/3 centromere localization, and cause random chromatid segregation. We are investigating the role of cohesins in Drosophila male and female meiosis by using germ-line specific RNAi (RNA interference) to deplete one of the core cohesins - SMC3. When we knockdown SMC3 in the male germline we observe premature loss of cohesion between the sister centromeres of some (but not all) chromosomes during prophase I, and numerical mis-segregation of major autosomes (2nd and 3rd). Surprisingly, however, cohesion and segregation of the sex chromosomes is nearly unaffected by depletion of SMC3. Even more remarkably, SMC3 knockdown in the female germline has no apparent effect on centromere cohesion. This result is probably not due to incomplete depletion of SMC3 as synaptonemal complexes (which require SMC1 and SMC3 as components of the lateral elements) are completely eliminated in the SMC3-depleted oocytes. We show that despite the absence of cohesin proteins from centromeres, SOLO localizes normally to the centromeres in SMC3 knockdown in both male and female germlines. These results suggest the hypothesis that SOLO along with SUNN and ORD can provide cohesion to centromeres in the absence of the cohesins. This is the first clear evidence for a non-cohesin-based cohesion mechanism in any eukaryote.