Diversification within ploidy-variable Balkan endemic Cerastium decalvans (Caryophyllaceae) reconstructed based on genetic, morphological and ecological evidence

The Balkan Peninsula is a hotspot of European biodiversity, harbouring many endemic species. Cerastium decalvans is a heteroploid species endemic to mountainous areas throughout the Balkan Peninsula. It is morphologically extremely variable, and multiple infraspecific taxa have been described. Mostly tetra-, octo- and hexadecaploids have been reported, but their distribution and relationship to morphological differentiation remain unknown. To disentangle relationships among 62 populations sampled across the distribution of the species, we applied an integrative approach. Using flow cytometry, we show that tetraploids are the most widespread cytotype, whereas octoploids are limited to the south-east of the distribution area and hexadecaploids occur mostly in high mountains of the central-eastern Balkan Peninsula. The main genetic break revealed by AFLPs separates uniformly tetraploid populations from the Dinaric Mountains in the north-west of the distribution area from the south-eastern populations. Based on our data we hypothesize multiple (auto)polyploidization events leading to the origin of octoploid populations and two allopolyploidization events leading to the origin of hexadecaploids. By combining genome size, AFLP and morphological data, we suggest recognition of five subspecies, of which we here describe hexadecaploid C. decalvans subsp. doerfleri subsp. nov. endemic to the Albanian Alps. In addition, the presence of several phylogenetic lineages and multiple ploidies with pronounced morphological variability in the southern and eastern Balkan Peninsula points to pronounced environmental stability of this area throughout the Quaternary, enabling survival and divergence in situ, and further supports the role of the Balkans as a sanctuary of plant diversification.