Restricted variation in plant barcoding markers limits identification in closely related bryophyte species

Species-level identification and delimitation of bryophytes using the proposed general barcode markers for land plants has been challenging. Bryophyta (mosses) is the second most species-rich group of land plants after angiosperms, and it is thus of great importance to find useful barcoding regions also for this group of plants. We investigated how the plastid regions atpF-atpH, rbcL and trnH-psbA and the nuclear ITS2 region performed as barcode markers on closely related bryophyte taxa of selected moss (Bartramia, Distichium, Fissidens, Meesia and Syntrichia) and liverwort (Blepharostoma) genera from boreal and arctic regions. We also evaluated how sequencing success of herbarium specimens is related to length of the sequenced fragment, specimen age and taxonomic group. Sequencing success was higher for shorter fragments and younger herbarium specimens, but was lower than expected in the genera Distichium and Fissidens, indicating imperfect universality of the primers used. None of the studied DNA barcode regions showed a consistent barcode gap across the studied genera. As a single locus, the region atpF-atpH performed slightly better than rbcL and ITS2 and much better than trnH-psbA in terms of grouping conspecific sequences in monophyletic groups. This marker also gave a higher percentage of correct hits when conducting blast searches on a local database of identified sequences. Concatenated data sets of two and three markers grouped more conspecific sequences in monophyletic groups, but the improvement was not great compared with atpF-atpH alone. A discussion of recent studies testing barcode regions for bryophytes is given. We conclude that atpF-atpH, rbcL and ITS2 are to be the most promising barcode markers for mosses.