Explaining Darwin's corollary to Haldane's rule: the role of mitonuclear interactions in asymmetric postzygotic isolation among toads.

We examine the basis of Darwin's corollary to Haldane's rule, which describes viability and fertility differences between F1 produced from reciprocal crosses. We analyzed asymmetries in hybrid viability from >100 reciprocal crosses involving 36 toad species to test whether relatively high rates of mitochondrial vs. nuclear evolution produce dams with systematically less viable F1 hybrid progeny. We find no such effect, suggesting a predominant role for stochastic accumulation of asymmetric epistatic incompatibilities.