From the Iranian Plateau into the heart of the Eurasian steppe belt: The phylogeography of Sisymbrium polymorphum (Brassicaceae)

The Eurasian steppe belt is the largest grassland region in the world. The flora of this belt varies greatly between its regions, and its individual elements have different spatiotemporal evolutionary histories. The main aim of this study was to investigate the phylogeographic pattern of a prominent plant steppe element. We genetically characterized 136 accessions of Sisymbrium polymorphum collected from field and herbarium surveys using a multi-locus approach. We dated the resulting phylogenetic trees and employed rooted TCS network methods to estimate the evolutionary history of individual genetic markers. The majority of haplo- and ribotypes showed restricted geographic distributions. Stem age of S. polymorphum was estimated to be in the early Pliocene, while its crown age was estimated to be in the late Pliocene. We hypothesize that this plant was introduced to the Eurasian steppe belt through the Kopet Dagh mountain chain over the Turanian lowlands via multiple migration events. Time frame along the migration route was limited by the retreat of the Paratethys at the end of the Pliocene and the Akchagyl transgression of the Caspian Sea at the beginning of the Pleistocene. No additional evidence was found to suggest any further migration events occurred. The Akchagyl transgression acted as a physiogeographical barrier in two ways. Firstly, it restricted the time window for immigration from the Iranian Plateau into the Eurasian steppe, and secondly, it temporarily blocked intra-steppe migration routes between the western and eastern parts of the Eurasian steppe.