Premise of the study: Genome duplication and interspecific hybridization are important evolutionary processes that significantly influence phenotypic variation, ecological behavior, and reproductive biology of land plants. These processes played a major role in the evolution of the Dryopteris carthusiana complex. This taxonomically intricate group composed of one diploid (D. expansa) and two allotetraploid (D. carthusiana and D. dilatata) species in Central Europe. Overall phenotypic similarity, great plasticity, and the incidence of interspecific hybrids have led to a continuous dispute concerning species circumscription and delimitation. •
Methods: We used flow cytometry and multivariate morphometrics to assess the level of phenotypic variation and the frequency of hybridization in a representative set covering all recognized species and hybrids. •
Key results: Flow cytometric measurements revealed unique genome sizes in all species and hybrids, allowing their easy and reliable identification for subsequent morphometric analyses. Different species often formed mixed populations, providing the opportunity for interspecific hybridization. Different frequencies of particular hybrid combinations depended primarily on evolutionary relationships, reproductive biology, and co-occurrence of progenitors. •
Conclusions: Our study shows that genome size is a powerful marker for taxonomic decisions about the D. carthusiana complex and that genome size data may help to resolve taxonomic complexities in this important component of the temperate fern flora.