Abstract: The recent development of genetic methods has facilitated the identification of cryptic species across different groups of organisms, including plants. However, next-generation sequencing has rarely been used to study cryptic speciation in plants, especially in bryophytes, organisms with a dominant haploid life phase. The ability to capture variation across the whole genome makes this method an effective tool for distinguishing cryptic lineages. We have focused on the genetic structure of the moss Meesia triquetra along the Alps-to-Scandinavia transect. We detected the presence of the two genetically critically different lineages of M. triquetra in Europe. These lineages overlap in both morphological characters of the gametophyte and distribution ranges. However, they considerably differ in ecological preferences to groundwater pH. While lineage 1 occupied alkaline to subneutral fens, lineage 2 occurred in fens saturated with neutral to acidic water. We consider the entities cryptic species with respect to genetic and ecological differences but the absence of morphological features necessary for determining the entities. We hypothesize that fragmentation of the ancestral population of the moss in geographically isolated refugia differing in the commonness of acidic and alkaline substrates led to consequent long-term adaptation to different environmental conditions, then drove diversification in M. triquetra.

Key words: bryophyta, cryptic speciation, endangered moss, Europe, morphometry, north–south gradient, RAD-seq, rich fen