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Aquatic Ranunculus (sect. Batrachium) include homophyllous and heterophyllous plants. The development of floating leaves may be induced by genetic mechanisms or/and environmental conditions and this fact complicates the morphologically based identification of species. DNA-based studies provide the opportunity to expand the knowledge of this complicated group. We studied heterophyllous Ranunculus with well-developed capillary and intermediate leaves and visually homophyllous plants with capillary leaves from a single river basin, with the aim to evaluate their genetic polymorphism and taxonomic status—whether the plants with well-developed and weakly expressed intermediate leaves belong to different forms (taxa) or if they just express morphological variation of one or two taxa in a specific, very variable river environment. The studied heterophyllous and homophyllous plants from different rivers showed high genetic differentiation and a low level of genetic diversity within these groups. The molecular analysis did not reveal any inter simple sequence repeat (ISSR) polymorphism associated with the development of intermediate leaves. An analysis of nuclear ribosomal internal transcribed spacers ITS1–2 sequences revealed several ribotypes, which indicated the genetic heterogeneity of studied plants and indirectly confirmed the hybrid origin of some of them. Sterile plants originated from crossing of R. circinatus and R. penicillatus were discovered in the Skroblus River; however, identification of the parental species was impeded by the polymorphism detected. For this reason, cytological studies were performed and allowed confirmation of the hybrid origin of these plants.

We demonstrate a wide distribution and abundance of hybrids between the river species Ranunculus aquatilis, R. fluitans and R. kauffmannii with the still water species R. circinatus (Batrachium, Ranunculaceae) in rivers of two postglacial landscapes of East Europe, i.e., Lithuania and Central European Russia. The Batrachium species and hybrid diversity is higher in the rivers of Lithuania (4 species and 3 hybrids vs. 2 and 1) and represented mainly by western R. aquatilis, R. fluitans and their hybrids whereas in Central European Russia, the East European species R. kauffmannii and its hybrid are the only dominant forms. Hybrids make up about 3/4 of the studied individuals found in 3/4 of the studied river localities in Lithuania and 1/3 of the individuals found in 1/3 of the localities in Central European Russia. Such extensive hybridization in river Batrachium may have arisen due to the specificity of rivers as open-type ecosystems. It may have been intensified by the transformation of river ecosystems by human activities and the postglacial character of the studied landscapes combined with ongoing climate change. Almost all hybrids of R. aquatilis, R. fluitans and R. kauffmannii originated from unidirectional crossings in which R. circinatus acted as a pollen donor. Such crossings could be driven by higher frequency and abundance of R. circinatus populations as well as by some biological mechanisms. Two hybrids, R. circinatus × R. fluitans and R. circinatus × R. kauffmannii, were formally described as R. × redundans and R. × absconditus. We found a hybrid which most likely originated from additional crossing between R. aquatilis and R. circinatus × R. fluitans.

River riparian ecosystems deteriorate because of anthropogenic factors that are related to industry, intensive agriculture and tourism. Two native species, Bidens tripartita and B. cernua, are weeds widespread in Lithuania on river banks and other habitats. An alien species, B. frondosa, is also widespread in the same habitats. We investigated the genetic diversity of B. frondosa, compared its population structure to that of related species and assessed the possibility of genetic interaction between B. frondosa and native congeners. Genetic analysis of populations was based on inter-simple sequence repeats (ISSR) polymorphism. Our results revealed high genetic differentiation in the B. frondosa populations (48.2%) and high genotypic diversity (i = 0.99). Bayesian cluster analysis using the program Structure revealed signs of gene flow between B. frondosa and the native species; however, principal coordinate analysis did not support this result. Our results do not indicate the role of interspecific hybridization in the invasiveness of B. frondosa.