Rhynchostegiella (Brachytheciaceae): molecular re-circumscription of a convenient taxonomic repository

The moss genus Rhynchostegiella (Helicodontioideae, Brachytheciaceae) has long served as a convenient repository for small brachythecioid pleurocarps. Its circumscription is revised in the context of a chloroplast phylogeny of the Helicodontioideae employing trnL-trnF, atpB-rbcL, psbT-psbH, and psbA-trnH sequence data. The analysis resolves with full posterior probabilities a core Rhynchostegiella clade of eight species. Rhynchostegiella pumila and R. duriaei are both resolved outside that clade and accommodated in their own genera, Microeurhynchium gen. nov. and Pseudorhynchostegiella gen. nov., respectively. Rhynchostegiella leptoneura is sister to Aerolindigia capillacea and R. papuensis is closely related to Eurhynchiella zeyheri. One of the reasons why these unrelated species, together with other taxa, were traditionally included within Rhynchostegiella, is that the genus is morphologically poorly defined by only a single synapomorphic change followed by reversals in half of the species. The Madeiran endemic Brachythecium percurrens is resolved as sister to all the other genera of the Helicodontioideae and is transferred into a new monotypic genus, Hedenasiastrum gen. nov