Diversification and reticulation in the circumboreal fern genus Cryptogramma

► Cryptogramma is monophyletic based on plastid sequence data. ► Most described Cryptogramma species are taxonomically valid monophyletic lineages. ► European Cr. crispa is an ancient autotetraploid. ► New World tetraploid Cr. sitchensis formed by Cr. acrostichoides and Cr. raddeana. ► Nuclear sequence data in Cr. acrostichoides complex exhibit signs of introgression.