On the systematic position of Tortella arctica and Trichostomum arcticum (Bryophyta, Pottiaceae)

A molecular-taxonomic study of Tortella artica and Trichostomum arcticum has been conducted using nrITS and cptrnL-F sequence data obtained from multiple specimens across most of their geographic range. Two taxa are recognized within the traditional concept of Trichostomum arcticum and both of them are included in the genus Tortella under the names T spitsbergensis and Tortella x cuspidatissima. The evidence presented suggests that the latter is a hybrid between Tortella arctica and Tortella spitsbergensis. Tortella spitsbergensis and Tortella x cuspidatissima can be easily distinguished by the stem central strand, which is very well developed in T spitsbergensis and weakly developed (or rarely absent) in T x cuspidatissima. The generic boundaries of Tortella are discussed in connection with the closely related genera Oxystegus, Chionoloma, Pseudosymblepharis and Trichostomum. A key for these three arctic taxa is presented and their distribution and ecology are discussed.