Dominance, epistasis and the genetics of postzygotic isolation

The sterility and inviability of species hybrids can be explained by betwee n-locus "Dobzhansky-Muller" incompatibilities: alleles that are fit on thei r "normal" genetic backgrounds sometimes lower fitness when brought togethe r in hybrids. We present a model of two-locus incompatibilities that distin guishes among three types of hybrid interactions: those between heterozygou s loci (H-0) those between a heterozygous and a homozygous (or hemizygous) locus (H-1), and those between homozygous loci (H-2)We predict the relative fitnesses of hybrid genotypes by calculating the expected numbers of each type of incompatibility;. We use this model to study Haldane's rule and the large effect of X chromosomes on postygotic isolation. We show that the se verity of He vs. HI incompatibilities is keg to understanding Haldane's rul e, while the severity of H-1, vs. Hg incompatibilities must also be conside red to explain large X effects. Large X effects are not inevitable in backc ross analyses but rather-like Haldane's rule-may often reflect the recessiv ity of alleles causing postzygotic isolation. We also consider incompatibil ities involving the Y or W chromosome and maternal effects. Such incompatib ilities are common in Drosophila species crosses, and their consequences in male- vs. female-heterogametic taxa may explain the pattern of exceptions to Haldane's rule.