Evolutionary history of Castanea sativa in the Caucasus driven by Middle and Late Pleistocene paleoenvironmental changes

Abstract Due to global climate cooling and aridification since the Paleogene, members of the Neogene flora were extirpated from the Northern Hemisphere or were confined to a few refugial areas. For some species, the final reduction/extinction came in the Pleistocene, but some others have survived climatic transformations up to the present. This has occurred in Castanea sativa, a species of high commercial value in Europe and a significant component of the Caucasian forests’ biodiversity. In contrast to the European range, neither the historical biogeography nor the population genetic structure of the species in its isolated Caucasian range has been clarified. Here, based on a survey of 21 natural populations from the Caucasus and a single one from Europe, we provide a likely biogeographic reconstruction and genetic diversity details. By applying Bayesian inference, species distribution modelling and fossil pollen data, we estimated (i) the time of the Caucasian—European divergence during the Middle Pleistocene, (ii) the time of divergence among Caucasian lineages and (iii) outlined the glacial refugia for species. The climate changes related to the Early–Middle Pleistocene Transition are proposed as the major drivers of the intraspecific divergence and European–Caucasian disjunction for the species, while the impact of the last glacial cycle was of marginal importance. Sweet chestnut is a remnant of once widely spread ancient Neogene flora, which makes it a relic species. It is a highly economically and ecologically important tree distributed and cultivated mostly in the Mediterranean Basin. The species is also present in the Caucasus region, where it forms natural forests of high conservation value. In this work, we reconstructed the evolutionary history of this species in the Caucasus. We demonstrated that populations retained high genetic diversity and moderate differentiation, partly due to high gene flow and less human impact in the past, in comparison to the European populations. The split between European and Caucasian ranges occurred during the Middle Pleistocene. Based on genetic information, the Caucasian populations of the species were grouped into four genetic clusters which were formed long before the last glacial period.