Evolutionary relationships and diversification of Euphorbia austroanatolica and E. rhytidosperma (Euphorbiaceae), narrow endemics of the Taurus Mountains in the eastern Mediterranean biodiversity hotspot

The Taurus Mountains of southern Anatolia (Turkey) represent a major centre of endemism within the Mediterranean biodiversity hotspot, yet the diversification of plant species occurring there and their evolutionary history remain poorly understood. We use an integrative approach combining double-digest restriction-site associated DNA sequencing, nuclear ITS and plastid ndhF-trnL sequence data, relative genome size (RGS) estimates, and morphometric analyses to infer the evolutionary history of the Taurus endemic Euphorbia austroanatolica and E. rhytidosperma and disentangle their relationships to close relatives distributed in different parts of the Mediterranean Basin. Divergence time estimates suggest rapid radiation across the Mediterranean, likely triggered by late Miocene aridification, and RGS data indicate polyploid origin of the western Mediterranean lineage. In contrast to earlier hypotheses, our results clearly demonstrate that E. austroanatolica, a narrow endemic of westernmost Taurus, is genetically and morphologically distinct from assumingly conspecific eastern Mediterranean E. spinosa. Instead, phylogenomic analyses revealed easterly allopatric and morphologically divergent E. rhytidosperma as closest relative of E. austroanatolica, their closest relatives being species of the E. verrucosa and E. clementei alliances. The mid-Pliocene (∼3.6 Ma) divergence between the two Taurus endemics followed by allopatric speciation was likely triggered by climatically driven range shifts and accompanied by ecological differentiation, which is in line with the species' distinct ecological preferences - E. austroanatolica inhabiting dry serpentine cliffs and E. rhytidosperma occurring in humid rocky gorges. Adaptation to divergent habitats was associated with pronounced morphological differentiation. Our results provide a better understanding of phylogenetic relationships among Mediterranean members of Euphorbia sect. Helioscopia and highlight the Taurus Mountains and their broad spectrum of habitats as a key centre for the diversification of the eastern Mediterranean flora.