Formation of the synaptonemal complex (SC), or synapsis, between homologs in meiosis is essential for crossing over and chromosome segregation [1-4]. How SC assembly initiates is poorly understood but may have a critical role in ensuring synapsis between homologs and regulating double-strand break (DSB) and crossover formation. We investigated the genetic requirements for synapsis in Drosophila and found that there are three temporally and genetically distinct stages of synapsis initiation. In "early zygotene" oocytes, synapsis is only observed at the centromeres. We also found that nonhomologous centromeres are clustered during this process. In "mid-zygotene" oocytes, SC initiates at several euchromatic sites. The centromeric and first euchromatic SC initiation sites depend on the cohesion protein ORD. In "late zygotene" oocytes, SC initiates at many more sites that depend on the Kleisin-like protein C(2)M. Surprisingly, late zygotene synapsis initiation events are independent of the earlier mid-zygotene events, whereas both mid and late synapsis initiation events depend on the cohesin subunits SMC1 and SMC3. We propose that the enrichment of cohesion proteins at specific sites promotes homolog interactions and the initiation of euchromatic SC assembly independent of DSBs. Furthermore, the early euchromatic SC initiation events at mid-zygotene may be required for DSBs to be repaired as crossovers.
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