Welcome to the Czech Republic again! Rare northern mosses Calliergon megalophyllum and Drepanocladus sordidus (Amblystegiaceae) in South Bohemia in light of their European distribution and habitat preferences.

Two aquatic moss species, Calliergon megalophyllum and Drepanocladus sordidus (Amblystegiaceae, Bryophyta), which had been considered extinct in the Czech Republic, were found in the Třeboň Basin, South Bohemia, in 2016-2017. They co-occurred in extensive reed- and sedge-dominated fen pools with humic water on the shore of an old fishpond and the former species was also discovered in a small humic pool in an old shallow sand-pit. The new Czech sites of these rare boreal species represent one of the southernmost known outposts within their entire European range. Previously, the two species were only known from single records in the Czech Republic from the late 19th and early 20th centuries. To confirm our morphological observations, we used phylogenetic analyses of DNA sequence variation in four chloroplast loci (atpB-rbcL, trnL-trnF, rpl16, trnG) and one nuclear region, the internal transcribed spacers of ribosomal DNA (ITS). We found (1) monophyly of all Calliergon megalophyllum specimens tested; (2) based on chloroplast DNA sequences, monophyly among all Drepanocladus sordidus specimens and representatives of Pseudocalliergon turgescens and P. lycopodioides moss species; (3) based on nuclear ITS sequences, monophyly of all original D. sordidus specimens. These results corroborate morphological studies and thus confirm the existence of natural sites for the studied moss species in the Třeboň Basin, South Bohemia, Czech Republic.