Cyto-nuclear epistasis: two-locus random genetic drift in hermaphroditic and dioecious species.

We report the findings of our theoretical investigation of the effect of random genetic drift on the covariance of identity-by-descent (ibd) of nuclear and cytoplasmic genes. The covariance in ibd measures of the degree to which cyto-nuclear gene combinations are heritable, that is, transmitted together from parents to offspring. We show how the mating system affects the covariance of ibd, a potentially important aspect of host-pathogen or host-symbiont coevolution. The magnitude of this covariance influences the degree to which the evolution of apparently neutral cytoplasmic genes, often used in molecular phylogenetics, might be influenced by selection acting on unlinked nuclear genes. To the extent that cyto-nuclear gene combinations are inherited together, genomic conflict is mitigated and intergenomic transfer it facilitated, because genes in both organelle and nuclear genomes share the same evolutionary fate. The covariance of ibd also affects the rate at which cyto-nuclear epistatic variance is converted to additive variance necessary for a response to selection. We find that conversion is biased in species with separate sexes, so that the increment of additive variance added to the nuclear genome exceeds that added to the cytoplasmic genome. As a result, the host might have an adaptive advantage in a coevolutionary arms race with vertically (maternally) transmitted pathogens. Similarly, the nuclear genome could be a source of compensatory mutations for its organellar genomes, as occurs in cytoplasmic male sterility in some plant species. We also discuss the possibility that adaptive cytoplasmic elements, such as favorable mitochondrial mutations or endosymbionts (e.g., Wolbachia), have the potential to release heritable nuclear variation as they sweep through a host population, supporting the view that cytoplasmic introgression plays an important role in adaptation and speciation.