Genetics of Haldane's Rule in Caenorhabditis

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Haldane's rule is the observation that "when in the F1 offspring of two different animals races one sex is absent, rare, or sterile, that sex is the heterozygous [heterogametic] sex" (Haldane, 1922). In the nematode genus Caenorhabditis, Haldane's Rule is observed in C. briggsae::C. remanei hybrids. Implementation in this combination is through sexual transformation of haplo-X hybrids into females. Hybrid sexual transformation likely results from dysgenic interactions among sex determination genes. This hypothesis will be tested by mapping naturally occurring variants that suppress the hybrid sexual transformation phenotype relative to the sex determination genes of C. briggsae and C. remanei. One such variant has been identified in each of these species. Mapping of sexual transformation variants will utilize single-nucleotide polymorphisms(SNPs) as genetic markers. The goal of this project is to map hybrid sexual transformation variants to the nucleotide. This will be accomplished in two stages. A resolution of 0.1 mu (0.1% recombination) will be achieved by analysis of recombinant chromosomes through a single-worm PCR assay. Depending on the genomic region involved, this resolution will correspond to between 5,000 to 150,000 bp. Increased resolution (0.0001 mu) will be achieved through a population-based assay designed to detect recombination between adjacent SNPs. This work will further the understanding of the evolutionary genetics of species formation.--