Biogeography of the Eurasian steppe belt

The Eurasian steppe, consisting of the Euro-Siberian part in the west and the Mongol-Chinese part in the east, is the largest steppe region in the Northern Hemisphere. It stretches across 8000 km between the Hungarian basin and the Amur and has some smaller exclaves in Middle Europe and northeastern Asia. Based mainly on macrofossils of vertebrates it is assumed that the formation of the steppe started in the Lower Miocene in Asia and extended westwards in the Middle to Upper Miocene, so that a continuous steppe belt was probably present at the Miocene/Pliocene boundary, latest at the end of the Pliocene. Pristine steppe habitats were widely transformed into agricultural landscapes, while secondary steppe originated in Europe after opening of the dominating forests due to human influence. We propose to apply phylogeographic and biogeographic methods on typical and widespread steppe indicator species to understand the geographic origin and the climate-landscape history of the Eurasian steppe belt. Our focal point is the western Eurasian steppe belt, which is the Euro-Siberian steppe. Especially during the Quaternary climate macro-cycles, the steppe faced extensive range shift, range contractions and expansions. We intend to comparatively analyze 13 diverse plant species/species groups with molecular- systematics methods (sequencing of nuclear genes, and genome-wide SNP analysis through GBS) and analyze the data with phylogenetic and phylogeographic methods (dated phylogenies, Bayesian and maximum-likelihood methods on biogeography reconstruction). Thus we want to trace past and ongoing in situ evolution in expanding and/or subdividing steppe habitats, as well as identify times of intense speciation (or lack thereof) due to climate or geographical changes or stasis, influencing western Eurasia from Miocene till present.

Our project provided four case studies of the evolutionary history of characteristic Euro-Siberian steppe plants, which differ in their ecological requirements (e.g., they grow in desert steppe, short grass steppe, long grass steppe or forest-steppe), with the ultimate goal to increase our understanding of steppe evolution as a whole. We applied high-resolution sequencing to assess patterns of relatedness in approximately 200 individuals in each of four species and we used DNA sequences for molecular dating. The temporal sequence proved to be important for the interpretation of the differences in the evolutionary histories of the species. The first case study was concerned with Krascheninnikovia ceratoides. The desert-like dry areas of Asia are the home of this plant, which expanded with the spread of the steppe from Asia to Europe in the later Tertiary and Quaternary. Krascheninnikovia ceratoides tolerates great drought and has a wide temperature amplitude. This allowed it to spread in the dry-cold open vegetation types of the ice ages, which makes it a "winner" of the ice ages. The range extension of K. ceratoides from its area of diversification in the Altai Mountains region occurred in various waves. The second case study was concerned with Onobrychis arenaria, which shows some similarities with K. ceratoides but differs in other aspects. Similarities are its age of diversification in the later Tertiary/early Quaternary, the extension of its distribution to the east, i.e., across the Altai Mountains region to Central Asia (Mongolia), and the multiple incidences of polyploidisation events. Onobrychis arenaria differs from K. ceratoides by its ecological requirements (it is growing in meadow steppes and mountain steppes) and its area of origin in the region surrounding the Black Sea. The third and fourth case studies were concerned with Adonis vernalis and Astragalus austriacus, two characteristic steppe and forest-steppe species with European-Western Siberian distribution (but not across the Altai Mountains to the east). The patterns in both of these species turned out to be very similar and very different from those in K. ceratoides and O. arenaria. First, molecular dating revealed a younger age of the eastern lineage of the two species, most probably after the Mid-Pleistocene Transition at around 0.9 million of years ago. Second and maybe related to their younger age, both species are entirely diploid. In both species, the eastern lineage started its range expansion from the region north and/or west of the Black Sea (including the Carpathian region) and seems to have (re-)colonized the Western Siberian region east of the Ural Mountains postglacially. The four species investigated by us differ in age, areas of origin, and ecological requirements. These factors can be used to explain their different behavior across cold and warm periods of the Pleistocene.