The Iberian white-oak syngameon as a legacy of introgression in southern Europe

Abstract

Oaks (Quercus L.) are among the most ecologically important tree genera in the northern hemisphere, with an intricate evolutionary history reflected in a reticulated phylogeny. Oak diversity has been profoundly shaped by introgression and diversification, yet the Iberian Peninsula remains an understudied natural laboratory for understanding these evolutionary processes. We used RAD-seq to characterize 38 taxa (including nothotaxa) and investigate the evolutionary history of the Iberian white oaks, with an emphasis on hybrid swarms. Results led to a readdressing of Iberian white oak species, expanding our current understanding of the phylogeography of the European Section Quercus. Furthermore, molecular evidence led to the circumscription of two new subsections, reflecting the contrast between typical temperate and Atlantic distributed species (Group A), and the submediterranean marcescent oaks (Group B). The former unveiled the recovery of Q. estremadurensis and a Northwestern Iberian lineage represented by Q. broteroana and Q. orocantabrica as southwestern representatives of the broad European pedunculate oaks (Q. robur s.l.). The latter led to the validation of hybrid swarms, emphasizing the Iberian oak syngameon and the importance of gene flow to oak evolution. Ultimately, our approach advances the understanding of European white oak evolution across different evolutionary scales, establishing the Iberian Peninsula as an important reservoir of oak diversity.