As a recurrent pattern of speciation, Haldane's Rule asserts "When in the F1 offspring of two different animal races one sex is absent, rare, or sterile, that sex is the heterozygous sex" (Haldane, 1922). The broad adherence to Haldane's Rule implies that speciation in diverse taxa occurs through similar mechanisms. In Caenorhabditis, Haldane's Rule results from sexual transformation of F1 hybrids. From crosses of C. briggsae AF16 males to C. remanei EM464 females, all adult F1 hybrids were female. This gender bias resulted from sexual transformation, not male-specific lethality. This was ascertained using a single-worm PCR assay to detect the C. briggsae homolog of the X-linked
unc-18 gene, Cb_unc-18. Cb_unc-18 was detected in only half of the adult hybrids consistent with a karyotypic ratio of 1:1. This hybrid sexual transformation phenotype is partially suppressed by the C. briggsae HK104 and C. remanei PB228 variant strains. When either of these strains was used, diplo-X females and haplo-X intersexes were obtained. The effects of HK104 and PB228 were cumulative. From crosses of C. briggsae HK104 males to C. remanei PB228 females, haplo-X hybrids were male rather than intersexual. These results implicate defects in sex determination as a reproductive isolation mechanism in Caenorhabditis. Genetic studies of the HK104 and PB228 variants will be used to identify the dysgenic interactions that cause this reproductive barrier. Those genes involved will be mapped using single-nucleotide polymorphism (SNP) markers. C. remanei SNPs will be obtained from comparisons of PB228 and EM464 sequence data. To facilitate this, a PB228 genomic library has been constructed. Random clones from this library will be sequenced. The corresponding regions of the EM464 genome will be amplified and sequenced. C. briggsae SNPs will soon be available from the Genome Sequencing Center at Washington University. Ultimately, the objective of this research is to characterize the molecular mechanisms that implement this mode of reproductive isolation.