Proper segregation of homologous chromosomes during meiosis relies on the formation of crossovers between paired homologs, resulting in a structure known as a chiasma. We are investigating the identity and function of a new component of the crossover recombination machinery defined by the
me13 mutation.
me13 mutant hermaphrodites produce a high incidence of males and dead embryos reflecting missegregation of the X chromosome and autosomes at meiosis. DAPI staining of diakinetic chromosomes in the
me13 mutant reveals univalent rather than the wildtype bivalent structures, suggesting a lack of chiasmata, and indicating a defect in meiotic prophase. FISH experiments indicate that homologous chromosomes are appropriately paired in the
me13 mutant. In addition, immunofluorescence experiments visualizing synaptonemal complex components SYP-1 and HIM-3 indicate that homologs are appropriately synapsed. These experiments indicate that the defect may be in recombination per se. Experiments assessing the response to ionizing radiation suggest that the
me13 mutant is not specifically defective either in the formation of double strand breaks (DSBs) at the initiation step of recombination or in DNA repair per se. Furthermore, immunofluorescence staining of RAD-51, a protein that binds DSBs, indicates DSBs are made in the
me13 mutant. RAD-51 foci diminish by late pachytene, but redistribution of HTP-1 and SYP-1, currently hypothesized to correlate with crossover events, does not occur. Together these results suggest that DSBs are repaired but by a pathway that does not result in interhomolog crossover. Whereas many features of the
me13 mutant phenotype resemble defects caused by known mutations in
him-14,
msh-5, and
zhp-3, genes that function in converting processed double strand breaks into interhomolog crossovers, we have mapped the
me13 mutation to a region on the left arm of chromosome III that does not contain any previously known meiotic recombination genes. We will report progress in identifying the molecular defect and analyzing the role of
me13 in the recombination process.