Abstract
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 (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 postzygotic isolation.
We show that the severity of H 0 vs. H 1 incompatibilities is key to understanding Haldane’s rule, while the
severity of H 1 vs. H 2 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) chromo-
some 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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