Diploid hybrid origin of Ostryopsis intermedia (Betulaceae) in the Qinghai-Tibet Plateau triggered by Quaternary climate change

Despite the well-known effects that Quaternary climate oscillations had on shaping intraspecific diversity, their role in driving homoploid hybrid speciation is less clear. Here, we examine their importance in the putative homoploid hybrid origin and evolution of Ostryopsis intermedia, a diploid species occurring in the Qinghai-Tibet Plateau (QTP), a biodiversity hotspot. We investigated interspecific relationships between this species and its only other congeners, O.davidiana and O.nobilis, based on four sets of nuclear and chloroplast population genetic data and tested alternative speciation hypotheses. All nuclear data distinguished the three species clearly and supported a close relationship between O.intermedia and the disjunctly distributed O.davidiana. Chloroplast DNA sequence variation identified two tentative lineages, which distinguished O.intermedia from O.davidiana; however, both were present in O.nobilis. Admixture analyses of genetic polymorphisms at 20 SSR loci and sequence variation at 11 nuclear loci and approximate Bayesian computation (ABC) tests supported the hypothesis that O.intermedia originated by homoploid hybrid speciation from O.davidiana and O.nobilis. We further estimated that O.davidiana and O.nobilis diverged 6-11Ma, while O.intermedia originated 0.5-1.2Ma when O.davidiana is believed to have migrated southward, contacted and hybridized with O.nobilis possibly during the largest Quaternary glaciation that occurred in this region. Our findings highlight the importance of Quaternary climate change in the QTP in causing hybrid speciation in this important biodiversity hotspot.