Data from: Cytonuclear incompatibility contributes to the early stages of speciation

Genetic incompatibility is a hallmark of speciation. Cytonuclear incompatibilities are proposed to be among the first genetic barriers to arise during speciation. Accordingly, reproductive isolation within species should be heavily influenced by interactions between the organelle and nuclear genomes. However, there are few clear examples of cytonuclear incompatibility within a species. Here we show substantial postzygotic reproductive isolation in first-generation hybrids between differentiated populations of an herbaceous plant (up to 92% reduction in fitness). Reproductive isolation was due primarily to germination and survival, with moderate reproductive isolation for pollen viability. Reproductive isolation for survival was asymmetric and caused by cytonuclear incompatibility, with the strength of incompatibility linearly related to chloroplast genetic distance. This cytonuclear incompatibility may be the result of a rapidly evolving plastid genome. Substantial asymmetric reproductive isolation was also found for germination, but was not associated with cytonuclear incompatibility, indicating endosperm or maternal-zygote incompatibilities. These results demonstrate that cytonuclear incompatibility contributes to reproductive isolation within species, suggesting initial rates of speciation could be influenced by rates of organelle evolution. However, other genetic incompatibilities are equally important, indicating that even at early stages, speciation can be a complex process involving multiple genes and incompatibilities.