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 (RI) 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 RI in first-generation hybrids between differentiated populations of an herbaceous plant (up to 92% reduction in fitness). RI...