Mitochondrial–Nuclear Epistasis Impacts Fitness and Mitochondrial Physiology of Interpopulation Caenorhabditis briggsae Hybrids

In order to identify the earliest genetic changes that precipitate species formation, it is useful to study genetic incompatibilities that cause only mild dysfunction when incompatible alleles are combined in an interpopulation hybrid. Such hybridization within the nematode species Caenorhabditis briggsae has been suggested to result in selection against certain combinations of nuclear and mitochondrial alleles, raising the possibility that mitochondrial–nuclear (mitonuclear) epistasis reduces hybrid fitness. To test this hypothesis, cytoplasmic–nuclear hybrids (cybrids) were created to purposefully disrupt any epistatic interactions. Experimental analysis of the cybrids suggests that mitonuclear discord can result in decreased fecundity, increased lipid content, and increased mitochondrial reactive oxygen species levels. Many of these effects were asymmetric with respect to cross direction, as expected if cytoplasmic–nuclear Dobzhansky-Muller incompatibilities exist. One such effect is consistent with the interpretation that disrupting coevolved mitochondrial and nuclear loci impacts mitochondrial function and organismal fitness. These findings enhance efforts to study the genesis, identity, and maintenance of genetic incompatibilities that precipitate the speciation process.