The proper assembly of the synaptonemal complex (SC) is essential for the formation of crossover events during meiosis. The SC consists of lateral element (LE) proteins that associate along the chromosomal axes, which in turn are joined by the central region (CR) proteins, resulting in chromosome synapsis. In mice, plants and yeast, SC formation is DSB-dependent. In contrast, SC formation is DSB-independent in both worms and flies. The evolutionarily conserved CRA-1 protein plays a key role in the assembly of the CR and in the stabilization of homologous pairing. Analysis of
cra-1 mutants revealed that DSB formation and repair play an important role in the polymerization of CR components along the unsynapsed chromosomal axes.
cra-1 double mutants with genes in the recombination pathway, exhibit severely impaired loading of CR proteins onto unsynapsed chromosomes. However, the mechanism of CRA-1 function remains unknown. We have undertaken several approaches to investigate this novel mechanism of recombination-dependent polymerization of SC components activated in
cra-1 mutants. To examine the progression of recombination in
cra-1 mutants we assessed the localization of ZHP-3, a marker for crossover sites. Although most chromosomes fail to undergo crossover recombination in
cra-1 mutants, ZHP-3 foci were observed on all chromosomes. These results suggest that crossover recombination is impaired after crossovers have been designated, but before they are implemented to form chiasmata. To test whether the recombination-dependent polymerization of SC components activated in
cra-1 mutants initiates in coordination with the DSB repair machinery, we examined the frequency of co-localization between RAD-51 (a strand-exchange protein) and SYP-1 (a CR protein) upon entrance to meiosis. Frequent co-localization of SYP-1 and RAD-51 was not observed in
cra-1 mutants, indicating that the assembly of SYP-1 may not initiate at all DSB repair sites, but instead, may be restricted to crossover designated sites. If recombination-dependent CR polymerization is coordinated with the DSB repair machinery, it may not be pairing center (PC)-dependent. In wild type, the chromosome localization of CR components requires PC proteins such as ZIM-2, which promotes pairing of chromosome V. However, SYP-1 is still able to localize to chromosome V in
cra-1;
zim-2 mutants. This indicates that in the absence of
cra-1, polymerization of CR components along unsynapsed chromosome axes is mostly PC-independent. These data, in combination with currently ongoing studies, will lead to a better understanding of the interplay between crossover formation and CR assembly in C. elegans meiosis.