Does geography, evolutionary history or ecology drive ploidy and genome size variation in the Minuartia verna group (Caryophyllaceae) across Europe?

Polyploidization, a key driver of plant diversification, is believed to have interacted with Pleistocene climatic oscillations and local ecological factors, leading to a complex spatio-ecological mosaic of diploid and polyploid populations. The typical ecogeographic pattern in European plants involves spatially restricted diploids growing in southern regions, interpreted as glacial refugia, and their widespread polyploid derivatives occupying larger and more northerly situated ranges with harsher environments. Whether this is true for individual ploidy-variable groups is, however, largely unknown because we lack sufficiently detailed investigations of ploidy-variable plant groups jointly applying cytological, ecological and genetic methods. We assessed ploidy and genome size variation, elevational and edaphic preferences, and plastid DNA variation within the Minuartia verna aggregate, a group of low-competitive heliophilous plants growing from the Mediterranean to Arctic Europe. Contrary to the expectations, tetraploids have a restricted distribution (Southern Europe) and inhabit a relatively narrow environmental niche. The distribution of diploids, on the other hand, spans the full range of conditions, including climatic (i.e. highest elevations and latitudes) and edaphic extremes (i.e. toxic serpentine and metalliferous substrates). The distribution pattern of the two ploidies could be explained by their distinct evolutionary histories, suggesting expansion of the diploid-dominated haplotype group accompanied by long-term persistence and local differentiation of tetraploids in refugia in the Balkan Peninsula. In summary, our study contradicts the prevailing view of polyploids as successful colonizers of novel and challenging habitats and points to the importance of combining ecological and genetic data when studying ploidy-variable species complexes.