Phylogeographic evidence for long-term persistence of the Eurasian steppe plant Astragalus onobrychis in the Pannonian region (eastern Central Europe)

Steppe grasslands represent important refuges for rare species in the Pannonian region of eastern Central Europe. Nevertheless, little is known about their regional continuity and potential (post)glacial in-situ survival. Biogeographical studies assume potential source areas for the postglacial (re)colonisation of the Pannonian region in the Western and Central Pontics including the Black Sea coast. However, south-exposed slopes surrounding the Hungarian plain may have also played an important role as small-scale refugia.Using cpDNA sequence markers (atpI-H, rpL16, ycf1) and 190 individuals representing 25 populations, we investigated the genetic diversity and biogeographical history of Astragalus onobrychis within different subregions of its European distribution. We aimed at answering questions about the glacial survival, (post)glacial expansion and/or recent decline of the species throughout this area. A TCS network showed 108 haplotypes, which were divided into two unconnected clusters, representing different ploidy levels. We found octoploid populations widely distributed from the Pannonian region and Transylvania to more eastern areas in the Western Pontics and the Northern Caucasus. On the contrary, di- and tetraploid populations occur in the Dobruja and the Northern Balkan Peninsula, but also in the Central Pontic and the Southern Pannonian subregions. Four main cpDNA haplotype groups basically showed a geographical pattern; the Pannonian region being best-defined. We found high haplotype and moderate to high nucleotide diversity within most populations/subregions. Equilibrating these two genetic diversity measures indicates a long-term – probably glacial – (diploid) persistence of A. onobrychis in the Southern Pannonian subregion, areas along the Black Sea coast, the Northern Balkan Peninsula and the Central Pontic subregion. However, the octoploid lineage likely originated more to the east and expanded to Western Pontic areas and the Pannonian Basin probably already during Pleistocene.