Proper chromosome segregation at meiosis I depends on the initial alignment of homologous chromosomes, the establishment of synaptonemal complex (SC) that stabilizes this alignment, and the formation of chiasma between homologs. Previous studies have demonstrated that HIM-3, a non-cohesin component of the meiotic chromosome core, is required for these processes (Zetka et al. 1999). The
him-3(
vv6) mutation results in the substitution of a conserved residue of the HORMA domain (Couteau et al. 2004), believed to mediate protein-protein interactions (Aravind et al. 1998). HIM-3 levels in
vv6 mutant germ lines appear to be normal and the protein is loaded to the chromosome core. Similar to other
him-3 mutants,
vv6 exhibits severe defects in homologue alignment, synapsis and chiasma formation, resulting in a high embryonic lethality (emb) and a high incidence of male (him) phenotype as consequences of chromosome missegregation. Also, the nuclear spatial reorganization of early prophase nuclei in
vv6 is abnormal as indicated by the extension of the transition zone that is populated by crescent-shaped nuclei after DAPI staining. To identify proteins that interact with HIM-3, we performed an EMS-based suppressor screen using the
vv6 allele to isolate candidates that suppress the embryonic lethality characteristic of
vv6. We have isolated 1 semi-dominant (
vv52) and 4 dominant (
vv38,
vv39,
vv41,
vv50) suppressors that increase the number of progeny 3- to 6-fold, suggesting a rescue of the autosomal non-disjunction phenotype of
vv6 mutants. Nevertheless, these strains exhibit various levels of X chromosome non-disjunction, indicating the suppressors differentially affect the segregation of the sex chromosome. In addition to the ability of
vv39 to suppress
him-3(
vv6) segregation defects,
vv39 homozygous mutants exhibit emb (41%) and him (13%) phenotype independent of
vv6, indicating
vv39 defines a gene functioning in chromosome segregation. Consistent with this interpretation, DAPI-stained germ lines of
vv39 mutants are marked by an extended transition zone and variable numbers of univalents in diakinesis nuclei. Antibody stainings against the SC component SYP-1 and HIM-3 has revealed the presence of partially synapsed chromosome cores at pachytene. Together, these results demonstrate that
vv39 is required for essential meiotic processes and functions in the same genetic pathway as
him-3. We are currently further characterizing and mapping this mutation. Supported by NSERC and CIHR.