Evolutionary history of the Eurasian steppe plant Schivereckia podolica (Brassicaceae) and its close relatives ?

Abstract

The genus Schivereckia (Brassicaceae) - presently included in the genus Draba - comprises two species, S. podolica and S. doerfleri. Schivereckia podolica is an eastern European species with disjunct distribution in Russia, Ukraine, and Podolian-Galician outposts. Schivereckia doerfleri is reported from alpine fields of the Balkan Peninsula and from mountain chains of northern Anatolia, Turkey. Schivereckia podolica shares its ecological and biogeographic characteristics with a number of other steppe plants. The `relic hypothesis' interprets the present locations of Schivereckia and its associates as relics of a formerly continuous distribution belt of a cold periglacial steppe vegetation. We employed nuclear ITS and chloroplast trnL-trnF region, and DNA fingerprinting (RAPD) to elucidate the evolutionary history and historical biogeography of Schivereckia. RAPD data prove isolation by distance and mirror the present disjunct geographical distribution pattern of Schivereckia podolica. Phylogenetic analyses revealed that Schivereckia might be polyphyletic and nested within a highly supported clade of central Asian, Caucasian and North American Draba species. Time divergence estimation and ancestral area reconstruction support the interpretation of species of the Schivereckia subclade as periglacial relic. Our results argue for a continuous persistence of certain steppe species since the Pleistocene, and support the view that the modern steppes of Europe have derived partly from Pleistocene glacial steppes.