Synaptonemal Complex-Dependent Centromeric Clustering and the Initiation of Synapsis in Drosophila Oocytes

The pairing of homologous chromosomes and the intimate synapsis of the paired homologs by the synaptonemal complex (SC) are essential for subsequent meiotic processes including recombination and chromosome segregation. Here we show that the centromere clustering plays an important role in initiating homolog synapsis during meiosis in Drosophila females. Although centromeres are not clustered prior to the onset of meiosis, all four pairs of centromeres are actively clustered into one or two masses during early meiotic prophase. Within the 16-cell cyst, centromeric clustering appears to define the first step in the initiation of synapsis. Clustering is restricted to the nuclei that form the SC and is dependent on all known SC proteins. Surprisingly, both centromeric clusters and the SC components associated with them persist long after the disassembly of the euchromatic SC at the end of pachytene. The initiation of homologous recombination through the formation of programmed double-strand breaks (DSBs) is not required for either the formation or the maintenance of the centromeric clusters. Our data support a view in which the SC-mediated clustering at the centromeres is the initiating event for meiotic synapsis.