Chromosome axis proteins regulate synapsis initiation in Drosophila oocytes. Kathryn Landy, Mercedes Gyruicza, Kim McKim. Waksman Institute of Microbiology, Rutgers University, Piscataway, NJ.
Accurate chromosome segregation is crucial for the proper completion of meiosis and involves multiple intricate processes, including synapsis, which tethers homologous chromosomes, and cohesion, which tethers sister chromatids. Synapsis is the process of assembling the synaptonemal complex (SC), a proteinaceous structure that joins homologs along the chromosome axis. The axis runs along the length of each chromosome and is composed of several proteins, including the SC protein C(2)M, ORD and cohesion proteins. We have found that these chromosome axis proteins regulate three distinct stages of synapsis initiation: first, at the centromeres, second at 6-8 ORD-dependent euchromatic sites, and third at 15 to 18 additional C(2)M-dependent euchromatic sites. Double mutant analysis shows that the two types of euchromatic SC initiation can occur independently, however both pathways are dependent on cohesion proteins SMC1 and SMC3. In addition, we have genetic evidence that suggests synapsis initiates at many locations along the chromosomes, which may correspond to crossover sites. This data represents a novel model for SC initiation, which we hope to expand upon in order to gain more insight into the mechanism behind synapsis initiation. To do this, we investigated synapsis in additional cohesion defective backgrounds. When the cohesion proteins Stromalin (SCC3) and Nipped-B (SCC2) are knocked-down by RNAi we observed incomplete SC formation, similar to c(2)M mutants. Surprisingly, they were not needed for centromere synapsis as SMC1 and SMC3 are. We are working to determine how these two proteins and others including RAD21 (SCC1), WAPL, ECO, and PDS5 contribute to the synapsis initiation pathway. This analysis of synapsis initiation begins to unweave the intricate details and relationships that occur during meiotic prophase and gives insight into how proper chromosome segregation is accomplished.