The Eurasian steppe belt is the vastest grassland region worldwide, stretching approximately 8000 km from the Pannonian Basin in the west to the Amur river in the east, altogether covering more than 8 million km2. Due to its size and location, the vegetation of this biome was under strong influence of past climatic fluctuations that reached their peak in the Pleistocene. Fossil record of different steppe-associated flora and fauna places the onset of Eurasian open grasslands into Central Asia from which the grasslands spread westwards around 20 MYA and reached the East European Plain first in the Late Miocene around 8 MYA. However, while useful as a proxy to infer past vegetation patterns, fossils suffer from low-resolution power and thus fail to elucidate more detailed picture of the onset and the development of the Eurasian steppe belt flora. The working hypothesis driving the whole project was that molecular signals in typical steppe plant species reflect the climate-landscape history of the steppe and the biogeographic dynamics of steppe taxa and thus allow for a much finer resolution of the history of the steppe belt in comparison to floristic and fossil-based methods. By applying a plethora of different phylogenetic, phylogeographic and biogeographic methods, I first investigated the evolutionary history of four unrelated Brassicaceae taxa that can be nowadays found in the Eurasian steppe belt. Second, I tried to draw parallels with the climate-landscape history of the Eurasian steppe belt as inferred from the fossil record to test the above-mentioned working hypothesis. The first case study dealt with the economically important Brassicaceae genus Camelina, with an emphasis on C. microcarpa, which can be found across the whole Eurasian steppe belt. I uncovered that this taxon’s contemporary uninterrupted distribution was split along the north coast of the Caspian Sea approximately 1 MYA, dating back to the Apsheron and Baku transgression events. During this time period, a polyploidisation event took place giving rise to a new hexaploid taxon and subsequently preventing gene flow after the regression of the Caspian Sea. The second case study investigated the evolutionary history of Schivereckia, which exhibits a highly disjunct distribution along the East European Plain and the Balkans and the whole Schivereckia clade that can be nowadays found either at higher latitudes in the subarctic zone or mountain ranges of the northern hemisphere. My analyses placed the radiation of this clade at the beginning of Pleistocene, when low temperatures promoted speciation and radiation of cold-adapted flora and fauna. The study showed that the highly disjunct distribution of Schivereckia podolica mirrors the Pleistocene refugial areas of different ages and points towards a close evolutionary relationship between contemporary steppe and tundra flora. A third case study focussed on Sisymbrium. I uncovered that while Sisymbrium did not evolve in the Eurasian steppe belt, it invaded this area as well as the Mediterranean several times independently throughout the Pleistocene in the south-north and east-west trajectory, respectively. This study proved yet again how variable the Eurasian steppe flora is in terms of evolutionary onset and that many of steppe elements originated in its adjacent areas in the south. These then subsequently migrated to the Eurasian steppe belt only after it was already fully developed either towards the west into the Euro-Siberian steppe or to the east into the Mongol-Chinese steppe. The fourth case study investigated the evolutionary history of Capsella. A GBS-based approach was proven successful for inferring biological history of young taxa. I showed that contemporary steppe element Capsella orientalis invaded the Eurasian steppe belt long after it was already developed in the south-north trajectory. I acquired further insights in the evolutionary history of its cosmopolitan sister tetraploid C. bursa-pastoris and placed the origin of the whole genus into the late Pliocene continental Asia. All studies showed the importance of a taxon sample and offered an alternative nesting dating approach for reliable calibration in cases where no fossil record could be obtained. In conclusion, evolutionary history of all four investigated taxa were shaped by the same environmental changes that played a major role in the biological history of the Eurasian steppe belt. Overall, our working hypothesis that molecular signals in typical steppe plant species reflect the climate-landscape history of the steppe and the biogeographic dynamics of steppe taxa, turned out to be correct.

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