Natural hybridization between diploid Ficaria calthifolia and tetraploid Ficaria verna subsp. verna in central Europe

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.