Striking diversity in Alpine lakes: genetic variation, hybridization and taxonomy of the aquatic Ranunculus trichophyllus group

The Ranunculus trichophyllus group has a complex evolutionary history and remains taxonomically unresolved. In central Europe outside the Alps, two tetraploid cytotypes distinguished as R. trichophyllus A and B have been recognized in previous studies. Another ecologically specialized taxon, R. confervoides (R. trichophyllus subsp. eradicatus), is reported to have a disjunct distribution in northern Europe, the Alps and Pyrenees, and mountains and northern regions of Asia and North America. However, the classification of the populations outside northern Europe as R. confervoides has recently been questioned. We studied the R. trichophyllus group in 35 lakes in the Alps and five in northern Europe, involving genome size determination, sequencing the nuclear ITS region and two non-coding plastid DNA regions, and recording 10 morphological characters. The populations referred to as R. confervoides in the Alps did not substantially differ from the northern European ones morphologically, had identical genome size and shared the plastid haplotypes and ITS genotypes with them. Consequently, we consider the northern and the Alpine populations to belong to the same species R. confervoides. Interestingly, we also identified plants that combine morphological characters of R. confervoides and R. trichophyllus A, even though the latter was not found in the Alps during our study. Their status remains unclear; they may represent hybrids, but partly they can be a manifestation of morphological plasticity of these taxa. Nine plastid haplotypes were observed in R. confervoides and the “intermediate” populations,which contrastswith only two haplotypes identified in each of the two R. trichophyllus cytotypes outside the Alps. This surprising haplotype diversity may result from faster fixation ofmutations in the Alpine populations due to prevailing isolation and clonality. Alternatively and not exclusively, this diversity may indicate higher age of the Alpine R. confervoides. Three hybrids unknown so far were recorded in lakes at the foothills of the Alps: triploid R. circinatus × R. confervoides, pentaploid hybrids of R. confervoides with an unknown species, and hybrids between R. trichophyllus B and the same unknown species. In addition, one putative allohexaploid population of R. confervoides and R. trichophyllus parentage was found. Given ecological distinctness and prevailing cleistogamy in R. confervoides, its involvement in multiple hybridizations is rather surprising. However, this species can produce open flowers when growing on exposed shores of lakes.We hypothesize that in such conditions hybridization can happen and the hybrids may survive by vegetative growth for a very long time.