Ficaria is a taxonomically intriguing polyploid complex with high morphological variability. Both hybridization and polyploidization have been suggested as the main evolutionary forces behind the high morphological variability in this genus; however, detailed studies are lacking. In Central Europe, two Ficaria taxa (diploid F. calthifolia and tetraploid F. verna subsp. verna) occasionally co-occur in local sympatry, which might result in hybridization. We investigated sympatric populations of the two Ficaria taxa using flow cytometry, chromosome counts, AFLP analysis and plastid DNA sequencing; we also performed experimental homoploid and heteroploid crosses to determine the frequency and direction of hybrid triploid formation, an alternative route of triploid origin (autopolyploidy) and the possibility of a one-step neoallotetraploid origin. Sympatric populations were composed of three genetic clusters corresponding to diploid F. calthifolia (2n = 16), tetraploid F. verna subsp. verna (2n = 32) and triploid plants (2n = 24). The holoploid genome size and AFLP data suggest a hybrid origin of the triploids, thereby making their formation via autopolyploidization in F. calthifolia unlikely. The triploid populations are monoclonal and of independent origin. In contrast, the parental populations exhibit high genotypic diversity and frequent sexual reproduction, including those of predominantly asexual F. verna subsp. verna. Experimental crossing confirmed that both parental taxa produce fertile seeds via a sexual pathway, but not by apomixis, and that both serve as pollen acceptors in heteroploid crosses, which is consistent with the plastid sequencing. However, hybridization is asymmetric, with maternal-excess crosses being significantly more successful. No signs of neoautotetraploidization or neoallotetraploidization were detected. In summary, recent gene flow between the studied Ficaria taxa is either limited or absent.

The genus Ficaria is a taxonomically intricate group in which polyploidization and hybridization contribute to taxa diversification. In central Europe, populations of diploid F. calthifolia and tetraploid F. verna subsp. verna occasionally come into contact, which results in an interspecific triploid hybrid, as recently demonstrated using molecular markers, genome size estimation and experimental crossing. In this study, we aimed to estimate the frequency and distribution of the triploid hybrid in central Europe, to identify those phenotypic traits that can be used to discriminate between hybrid and parental taxa and compare the phenology, pollen viability, sexual and asexual reproduction and niche differentiation of the triploid hybrid and parental taxa. Flowcytometry analyses of 1171 individuals sampled from 67 localities revealed that triploid hybrids were regularly found at 89% of the sites sampled where there were sympatric populations of both parental taxa, with a mean percentage of 19.4% occurring at sites where there was a mixture of cytotypes. No pure hybrid population was found. The hybrids were intermediate between the parental taxa in most morphological characters and did not show any novel morphological characters. The characters that best differentiate the taxonomic groups were the production of axillary bulbils, aborted and well-developed achenes, leaf shape and plant habit. Almost all hybrids were sexually sterile: the pollen viability was considerably reduced (mean 8.4%) and on average 98% of the achenes were aborted. Local dispersal of the hybrids is possible through the production of bulbils in the leaf axils, although the number of bulbils per node and the mean weight of one bulbil were two and three times lower in the hybrid than in F. verna subsp. verna. The hybrid niche was intermediate between those of the parental taxa but shifted slightly towards that of F. verna subsp. verna. In addition, the taxonomic and nomenclatural treatment of the hybrid, which is described as Ficaria ×sellii Duchoslav, Popelka et Trávn., as well as a key for identifying the central European taxa of Ficaria, are presented.