Dominance, Epistasis and the Genetics of Postzygotic Isolation

Corresponding author: Michael Turelli, Center for Population Biology, University of California, 1 Shields Ave., Davis, CA 95616., mturelli{at}ucdavis.edu (E-mail)

Communicating editor: P. D. KEIGHTLEY

The sterility and inviability of species hybrids can be explained by between-locus "Dobzhansky-Muller" incompatibilities: alleles that are fit on their "normal" genetic backgrounds sometimes lower fitness when brought together in hybrids. We present a model of two-locus incompatibilities that distinguishes among three types of hybrid interactions: those between heterozygous loci (H₀), those between a heterozygous and a homozygous (or hemizygous) locus (H₁), and those between homozygous loci (H₂). 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 postzygotic isolation. We show that the severity of H₀ vs. H₁ incompatibilities is key to understanding Haldane's rule, while the severity of H₁ vs. H₂ incompatibilities must also be considered to explain large X effects. Large X effects are not inevitable in backcross analyses but rather—like Haldane's rule—may often reflect the recessivity of alleles causing postzygotic isolation. We also consider incompatibilities involving the Y (or W) chromosome and maternal effects. Such incompatibilities 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.

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