Evolution and taxonomy of aquatic species in the genus Rhynchostegium (Brachytheciaceae, Bryophyta)

Phylogenetic relationships of the aquatic moss genus Platyhypnidium and terrestrial species of Rhynchostegium were explored using a phylogeny based on three DNA sequence regions, the nuclear ITS1-5.8S-ITS2, and the plastid trnL-F and trnD-T. This is the first study using trnD-T in the phyogeny of mosses. Platyhypnidium appeared polyphyletic and most of its species were found within a Rhynchostegium clade including species from four different genera: Rhynchostegium, Eriodon, Eurhynchiella and Platyhypnidium. Use of the name Rhynchostegium requires conservation against Eriodon. Within Rhynchostegium s.l., aquatic Rhynchostegium species (i.e., Platyhypnidium) were resolved in four geographically delimited groups: an Afro-European group, an American—East Asiatic group, a Malesian-Australian group and a group comprising the South American P. fuegianum together with Eriodon and Eurhynchiella. Topologies from independent analyses of plastid and nuclear data did not show conflict, and the occurrence of hybridzation events in the clade was therefore not supported. The phylogeny indicates numerous habitat shifts between terrestrial and aquatic habitats, as well as between different terrestrial (epiphytic and epigeic) habitats, which may have affected taxonomic complexity in Rhynchostegium. New morphological characters, such as a striate leaf lamina, axillary hairs composed of relatively short cells, and clearly differentiated cells of the costa, separate Platyhypnidium species within Eurhynchioideae from those in the Oxyrrhynchium-Donrichardsia clade (Helicodontioideae). Two morphologically unique specimens, Platyhypnidium muellerii from Hawaii and Platyhypnidium sp. from Northern Australia, are described as Donrichardsia bartramii and Rhynchostegium brevinerve. The new genus Hedenaesia (Helicodontioideae) is established to accommodate Platyhypnidium austrinum.