Characterizing a lethal mitonuclear incompatibility in naturally hybridizing Xiphophorus swordtails

The evolution of reproductive barriers is the first step in the formation of new species and can help us understand the diversification of life on Earth. These reproductive barriers often take the form of “hybrid incompatibilities,” where alleles derived from two different species no longer interact properly in hybrids. Theory predicts that hybrid incompatibilities may be more likely to arise at rapidly evolving genes and that incompatibilities involving multiple genes should be common, but there has been sparse empirical data to evaluate these predictions. Here, we describe a mitonuclear incompatibility involving three genes in physical contact within respiratory Complex I of naturally hybridizing swordtail fish species. Individuals homozygous for mismatched protein combinations fail to complete embryonic development or die as juveniles, while those heterozygous for the incompatibility have reduced Complex I function and unbalanced representation of parental alleles in the mitochondria...