Ancient introgression shapes the evolutionary history of a California Channel Island relictual species, island oak (Quercus tomentella)

Hybridization, a common phenomenon among plants, can result in the exchange of neutral or beneficial genetic loci, potentially leading to adaptive introgression. It is often difficult to know whether the genetic composition of contemporary species is the result of recent hybridization or reflects ancient introgression, but examination of a species long separated from a congener provides the opportunity to study ancient introgression. Here, we investigate the rare and endangered island oak (Quercus tomentella) that is relictual on the California Channel Islands but was once sympatric with canyon live oak (Q. chrysolepis) on the mainland. Recent studies have shown that contemporary populations of island oak include many individuals with essentially 50 % shared co-ancestry between the two species, but very few individuals of non-admixed canyon live oak on any island. The goal of this study is to assess the extent to which the genetic composition of island oak reflects ancient introgression with canyon live oak when they were sympatric on the mainland at least 2.6–7 million years ago. We used evolutionary demographic models that identify the presence and timing of bottleneck events and the extent and timing of ancient introgression between island oak and canyon live oak. Bidirectional gene flow was found throughout their evolutionary history, suggesting that hybridization is not a recent development and may have introduced adaptive alleles into ancient populations that still persist today.