Pinguicula vulgaris in central Europe: when does one species turn into another?

Pinguicula vulgaris is one of the most common carnivorous plants in the Northern Hemisphere. It is a member of the species complex (P. vulgaris agg.) of which in Europe six taxa are recognized at different taxonomic ranks. One of them is P. bohemica, a taxon considered endemic to the Czech Republic, for which there are varied reports on its chromosome number (ploidy level) and contrasting opinions on its taxonomic rank. Based on the persistent instability of the taxonomy of this group in central Europe, we investigated the ploidy level, genome size, seed structure and genetic differentiation of taxa of P. vulgaris agg. in order to resolve the questions connected with P. bohemica. We sampled all taxa currently recognized as members of P. vulgaris agg. in Europe. Karyological analysis supported the octoploid level for P. bohemica and all other taxa, and flow cytometry further supported cytotype uniformity. Although generally conservative, the absolute genome size differentiates the morphologically similar taxa in P. vulgaris agg. Molecular markers suggested the presence of distinct groups within the central European populations of P. vulgaris agg., where the groups correspond to already recognized or even unrecognized taxa at the infraspecific level. We identified another lineage from southern Poland within the P. vulgaris group (with provisional name P. “polonica”). Most probably it is a direct descendant of the P. bohemica lineage, which diversified under isolation. The taxa P. bohemica and P. “polonica” are genetically well differentiated from P. vulgaris s.s. and P. vulgaris var. bicolor. The molecular-genetic differentiation of P. bohemica is comparable to narrowly endemic taxa from central Italy. Spatial isolation seems to be an important force causing the gradual divergence of isolated lineages within the P. vulgaris agg. Considering all the distinctive features, we suggest a subspecific category for P. bohemica, thus P. vulgaris subsp. bohemica.