Multilocus DNA analysis supports Didymodon gelidus (Musci, Pottiaceae) as a distinct endemic of the austral polar region

The taxonomic position of the Antarctic subendemic species Didymodon gelidus Cardot is controversial, notably because of its notorious sterile condition. Considering the overall appearance and the reddish coloration of the plants, the leaf areolation, reaction of the leaf lamina with KOH, and the presence of multicellular axillary gemmae, this species was considered to be conspecific with the Holarctic D. brachyphyllus (Sull.) R. H. Zander. As a result, the latter was established as a bipolar species. Recent detailed morphological and anatomical studies have revealed a number of features which enable recognition of D. gelidus and D. brachyphyllus, including the shape of leaves, leaf apices and basal leaf cells, as well as costal anatomy. Here, within a larger-scale project focused on the evolution and biogeographical connections of Antarctic endemic mosses, we analyzed the genetic relationships of D. gelidus and D. brachyphyllus to confront the morphology-based conclusions. We selected five geographically distinct collections per species and applied a multilocus DNA analysis based on nuclear (ITS) and plastid (atpIH, trnLF, trnG, rps4) sequences to assess the genetic differentiation of these two taxa. We also placed their lineages in a wider phylogenetic context using an extended sampling of Didymodon taxa and select other representatives of Pottiaceae. Our results showed a clear genetic differentiation of the Southern Hemisphere (D. gelidus) and Northern Hemisphere (D. brachyphyllus) plants. Moreover, the phylogenetic analysis showed that D. gelidus formed a strongly supported clade on its own which was distantly related to D. brachyphyllus. Accordingly, the two taxa do not represent geographical vicariants. Didymodon gelidus must be considered a distinct endemic species of the austral polar region, having its optimal occurrence in the Antarctic and weakly penetrating northward to South Georgia and Îles Kerguelen in the Subantarctic. The relatively isolated phylogenetic position of D. gelidus likely suggests its old age.