Massive postglacial gene flow between European white oaks uncovered genes underlying species barriers

Oaks are dominant forest tree species widely distributed across the Northern Hemisphere, where they constitute natural resources of economic, ecological, social and historical value. Hybridization and adaptive introgressionhave long been thought to be major drivers of their ecological success. Thus, the maintenance of species barriers remains a key question, given the extent of interspecific gene flow. In this study, we scanned the genomes of four European whiteoak species for reproductive barriers. We identified the ecological and phylogenic relationships of these species and inferred a long-term strict isolation followed by a recent and extensive postglacial contact. Then, we made use of the tremendous genetic variation among these species (31 million SNPs) to identify genomic regions for reproductive isolation. A literature-based functional annotation of the underlying genes highlighted important functions driving the reproductive isolationbetween these sister species. These functions were consistent with their ecological preferences and included tolerance to biotic and abiotic constraints. This study holds important implications for the renewal of European forests under global warming.