Extensive hybridization in Ranunculus section Batrachium (Ranunculaceae) in rivers of two postglacial landscapes of East Europe

Abstract

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.

Origin, genetic structure and evolutionary potential of the natural hybrid Ranunculus circinatus × R. fluitans

“…Based on the direct sequencing of ITS and cpDNA regions rpl32-trnL and psbE-petL , the parentage of this hybrid was confirmed and R. fluitans was proved to be a donor of plastid DNA 23 . The same direction of crossing was confirmed by Bobrov et al 24 for the hybrid individuals occurring in the Lithuanian rivers.

Figure 1 ( a ) map of the sampled populations of the hybrid Ranunculus circinatus × R. fluitans (green circles) and its parental species: R. circinatus (orange triangles) and R. fluitans (violet circles).

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“…Ranunculus circinatus × R. fluitans has sterile seeds, therefore its effective propagation is achieved only vegetatively, mainly by stem fragments within one catchment area. Recently, thanks to molecular evidence, the hybrid has been confirmed in many localities in Central and East Europe 16 , 23 , 24 . Due to the frequent occurrence and persistence over time and space, the hybrid is an ecologically important element of river vegetation.…”

“…This finding indicates the recurrent formation of the studied hybrid in at least seven independent hybridization events in different catchment areas. The biological and ecological factors facilitating the formation of the R. circinatus × R. fluitans hybrid have been discussed more extensively in the paper by Bobrov et al 24 .…”

“…One of the Batrachium hybrids, namely Ranunculus circinatus × R. fluitans , has received special attention from taxonomists 16 , 23 , 24 . This taxon was first reported from Germany 25 , however, due to the lack of easily detectable diagnostic features, it was not later included in subsequent taxonomic studies 26 .…”

“…Recently, the hybrid was confirmed based on molecular evidence from Bavaria 16 , several sites in Poland 23 , and Lithuania, from where it was formally described as R. × redundans AA. Bobrov, Butkuvienė et Sinkevičienė 24 . It is possible that this hybrid is much more common in Europe than it appears from the published data, but it is overlooked due to its significant morphological similarity to other hybrids of R. fluitans, and the lack of common investigation of molecular features, which facilitates identification of morphologically ambiguous taxa.…”