Unnoticed diversity in the Riccia glauca-bifurca group (Ricciaceae, Marchantiales): morphological differentiation and phylogeny of R. gothica and R. pusilla in Europe

The genus Riccia is the most species-rich genus among complex thalloid liverworts in Europe. High intraspecific variability in presumed diagnostic thallus characters leads to problems in species delimitation. We started a morphological and phylogenetic study of 105 specimens assigned to R. bifurca, R. glauca, R. gothica, and an unknown entity morphologically related to these species. Selected characters were statistically analyzed and compared to molecular results obtained by sequencing three nuclear marker regions: the commonly used ITS2 and two single-copy markers, RPB1 and an anonymous locus, both specifically targeted for the first time in bryophytes. These data indicate, that R. bifurca, R. glauca, and R. pusilla (the unknown entity) display distinct linages reflected in spore morphology. Riccia gothica forms a subclade within R. pusilla, thus we interpret the taxon as a subspecies of. R. pusilla. Lectotypification of R. pusilla and R. gothica is carried out. Both subspecies are more widespread in Central Europe than previously assumed, but precise distribution is still unknown. We show that only spore characteristics are useful to distinguish the four taxa, which we present in detail. Similarity in spore morphology does not reflect phylogenetic relationships. Overinterpretation of thallus characteristics could lead to failure in determination of Riccia-species.