What is <i>Polypodium vulgare</i> ? Discerning disparate origins of an iconic fern in Europe and Asia

Abstract Few fern taxa are as iconic as Polypodium vulgare, the type species of Polypodium, the namesake genus of the largest fern family, Polypodiaceae. Originally described by Linnaeus, P. vulgare was once considered a single circumboreal species exhibiting subtle morphological and ecological variation. Generations of researchers have revealed that P. vulgare is an allotetraploid belonging to a reticulation complex of mainly northern temperate species, but the P. vulgare complex continues to yield novel taxonomic diversity. Here we apply broad sampling of P. vulgare in Europe and Asia, maternally inherited plastome sequences, bi-parentally inherited nuclear target capture datasets, and the updated SORTER2-Toolkit bioinformatics pipeline to demonstrate that P. vulgare is restricted to Europe, whereas specimens from Asia represent a second allopolyploid species of distinct parentage. While both are of Pleistocene origin, our divergence dating estimates suggest that the Asian taxon, putatively named Polypodium ‘okiense’, probably formed prior to P. vulgare in Europe and support the hypothesis that European P. vulgare is a progenitor of the allohexaploid Polypodium interjectum. In addition, we infer the taxonomic identity of an enigmatic population from South Africa, and address implications for floristics and pharmaceutical applications of these conflated taxa.