Phylogeography of the Euro-Siberian steppe plant Astragalus austriacus: Late Pleistocene climate fluctuations fuelled formation and expansion of two main lineages from a Pontic-Pannonian area of origin

The Euro-Siberian steppes have experienced large-scale range fluctuations due to the climatic changes of the Pleistocene that may have also fuelled reshuffling of past steppe vegetation. These species-rich steppe grasslands were much more widespread during glacials and contracted during interglacials, a dynamic which should also be reflected by the evolutionary history of their biota. Astragalus austriacus is a widespread steppe species with European-western Siberian distribution and an ideal model to study the florogenesis of the Euro-Siberian steppes. Here, we inferred the phylogeography of A. austriacus based on genotyping-by-sequencing (GBS) data from populations sampled from the western edge of the Pannonian region across the Pontic region to the western Siberian region. Additionally, we applied molecular dating using single gene sequence data (ycf1, matK and ITS). We outline an evolutionary scenario in which intraspecific diversification occurred in the eastern part of Europe during the later Pleistocene (0.48–0.24 Ma). From there, the species expanded both eastwards and westwards, likely during a cold period, which is reflected by two main lineages within A. austriacus that today occur in the Pannonian sensu lato and in the Pontic/south-western Siberian regions, respectively. Demographic modelling supported such a scenario and showed that population sizes were larger during the last cold stage and contracted postglacially. Within the Pannonian sensu lato region, strong substructure was detected, likely as a result of repeated disintegration of the continuous cold-stage steppes in Europe. Finally, our results are in line with evolutionary scenarios suggested for other steppe species such as Adonis vernalis.