From continental Asia into the world: global historical biogeography of the saltbush genus Atriplex (Chenopodieae, Chenopodioideae, Amaranthaceae).
Atriplex is the most species-rich genus of Amaranthaceae and one of the largest C4 clades in eudicots. Distributed predominantly in the arid subtropical and temperate regions worldwide, many Atriplex species dominate the plant communities of harsh and inhospitable inland and coastal habitats. Current threats of aridification and salinisation increase the ecological and economic value of this highly stress tolerant xerophytic genus. We compiled sequence data of approximately 80% (208 spp.) of all currently recognised species and carried out a phylogenetic reconstruction using nuclear-encoded internal and external transcribed spacers. In addition, time divergence estimation analysis and ancestral area reconstruction were carried out to reconstruct the worldwide spread of Atriplex. Our results show that Atriplex originated in continental Asia during the Oligocene and dispersed from there across the world, often via long-distance dispersal from the Aralo-Caspian and the Pontic regions, or the floristic province of Turkestan. The highest alpha diversity was retrieved from arid habitats of Australia and the New World resulting from extensive radiation events of the Late Miocene and Pliocene. Most dispersal events took place into the Mediterranean region. Atriplex invaded most continents several times independently from different regions throughout the continuous cooling trend of the Neogene and the Quaternary. Despite limited resolution power of the used molecular markers, this study allows for a more comprehensive understanding of the evolutionary history of Atriplex and lays the foundation for future evolutionary studies of saltbushes.