Geographic variation in the structure of oak hybrid zones provides insights into the dynamics of speciation

Studying geographic variation in the rate of hybridization between closely related species could provide a useful window on the evolution of reproductive isolation. Reinforcement theory predicts greater prezygotic isolation in areas of prolonged contact between recently diverged species than in areas of recent contact, which implies that old contact zones would be dominated by parental phenotypes with few hybrids (bimodal hybrid zones), whereas recent contact zones would be characterized by hybrid swarms (unimodal hybrid zones). Here, we investigate how the hybrid zones of two closely related Chinese oaks, Quercus mongolica and Q.similar to liaotungensis, are structured geographically using both nuclear and chloroplast markers. We found that populations of Q.similar to liaotungensis located around the Changbai Mountains in Northeast China, an inferred glacial refugium, were introgressed by genes from Q.similar to mongolica, suggesting historical contact between the two species in this region. However, these introgressed populations form sharp bimodal hybrid zones with Q.similar to mongolica. In contrast, populations of Q.similar to liaotungensis located in North China, which show no sign of ancient introgression with Q.similar to mongolica, form unimodal hybrid zones with Q.similar to mongolica. These results are consistent with the hypothesis that selection against hybrids has had sufficient time to reinforce the reproductive barriers between Q.similar to liaotungensis and Q.similar to mongolica in Northeast China but not in North China.