East Asian origins of European holly oaks (Quercus section Ilex Loudon) via the Tibet-Himalaya

Aim Many subtropical organisms exhibit an East Asian-Tethyan disjunction, a distribution split between East Asia and the Mediterranean. The underlying mechanisms and timing have remained unclear to date. The evolutionary history of Quercus section Ilex Loudon, a representative East Asian-Tethyan disjunct lineage with a rich and widespread fossil record, was investigated to understand the key drivers of this disjunction. Location Eurasia. Methods The phylogeny of Quercus section Ilex was reconstructed using RAD-seq. Divergence times were estimated based on three fossil calibrations. Ancestral range and niche were reconstructed on the calibrated tree to infer the timing of transitions in geographic distributions and niche. Convergence in ecological space was estimated by fitting alternative multiple-regime Ornstein-Uhlenbeck models. Leaf shape affinities among extant and fossil taxa of section Ilex were assessed using geometric morphometric approaches. Results Six clades were well resolved in section Ilex. Ancestral range reconstruction and divergence time dating suggest a wide distribution along the East Tethys seaway, with initial divergence at the mid-Eocene, and all six clades originating before the Miocene. The section dispersed from East Asia to the Mediterranean at the Eocene-Oligocene boundary. A shift toward higher elevations was detected in the Himalayan clade during the middle or late Miocene. European fossil lineages during the early Miocene differ in leaf morphology from later lineages, which we infer to be a consequence of adaptive differentiation or species turnover. Main conclusions Quercus section Ilex was widespread along the East Tethys seaway from the middle Eocene onward. The European holly oaks originated from an East Asian ancestral lineage that dispersed to Europe via the Tibet-Himalaya corridor in the Oligocene. Lowlands along the margins of the Himalayas and through an Oligocene Tibetan valley served as the dispersal route(s) for these species. Changing climates drove Miocene extinction and local adaptation of European lineages.