Didymodon acutus

Didimodon acutus View in CoL

The presence of D. acutus in Russia was already confirmed by Jiménez (2006), who cited one specimen of this species from North Ossetia collected by Brotherus in 1877. However, its distribution in the country remained insufficiently known, since small-sized plants with unistratose, narrowly recurved leaf margins and smooth laminal cells collected in Russia possessed a greater variability than it was provided in the description of D. acutus ; furthermore, some of them grew in habitats which did not suite the latter species. Thus, several specimens from different regions of Russia which possessed morphological characters of D. acutus were sampled. Four sequences were obtained (isolates OK3311, OK3320, OK3329, and OK4106); they were resolved within the clade 4 and formed a fully supported clade with the grade of three specimens of another unknown species, contrastingly different in morphology, at its base, with low support (PP 0.83, BS 65) for their mutual clade. This clade appeared to be sister to the clade of three species, D. tibeticus , D. vulcanius and D. icmadophilus . A the same time, GenBank accessions of D. acutus (specimens from Greece, Italy and Spain) were found within the clade 3, in a fully supported clade sister to the clade of D. glaucus + D. japonicus + D. jimenezii + D. tomaculosus . A closer morphological comparison of specimens from Russia with the description of D. acutus and specimens of this species from herbaria convinced us that they likely represent another species (see Fig. 2 View Fig showing leaves, leaf transverse sections and upper laminal cells of D. acutus from Spain and specimens in question from Russia). We also failed to find an appropriate name for our specimens among similar species recently described from China. Considering the differences between specimens from Russia and D. acutus in ecological preferences, were decided to describe them as a new species, D. borealis (see below in Taxonomy section). However, an expanded search for D. acutus in herbarium collections from Russia is needed, taking into account characters of the newly described species, as well as of D. mongolicus (see below); molecular barcoding can be also helpful to understand better their morphological distinctions.