Three species for the price of one within the moss Homalothecium sericeum s.l.

Phylogenetic analyses within the moss Homalothecium sericeum resolved two clades and four haplotypes in a polytomy. Because the clades and the four haplotypes as one group exhibit comparable levels of genetic divergence to those observed among sister species in the genus, significant morphological differences, and distinct geographic distributions, they are recognised as three distinct species. Discriminant analysis was employed to assign the types of “forgotten” taxa previously recognized within H. sericeum s.l. to those three species based on their morphology. While a growing number of “cryptic species” has been reported in the literature, the results suggest that thorough morpho-anatomical investigations are likely to reveal morphological discontinuities among such taxa and trigger their formal description at the appropriate taxonomic level. Homalothecium sericeum s.str., H. mandonii (Mitt.) Geh. and H. meridionale (M. Fleisch. & Warnst) Hedenäs comb. & stat. nov. clearly differ in sporophytic traits but the identification of sterile specimens is challenged by the overlap in gametophytic characters. As a consequence, 8%–37% of the specimens were mis-classified in discriminant analyses in an attempt to find the best combination of gametophytic traits to identify specimens that were assigned to one of the three species on the basis of their genotype. The three species differ in rpl16, a region that distinguishes species also in other moss genera and should be further explored as a candidate for DNA barcoding marker among mosses. Homalothecium mandonii is the second case of an endemic Macaronesian bryophyte species whose range encompasses the Cape Verde Islands, the Canary Islands, Madeira, and the Azores. Homalothecium meridionale is circum-Mediterranean and was reported from one locality in Lanzarote in the Canary Islands, while H. sericeum occurs across Europe, eastern North America, and western Asia. The distributions of the segregate species within H. sericeum s.l. reinforce the notion that many disjunctions typically observed in moss distribution ranges are due to taxonomic shortcomings and call for the need of substantial taxonomic revisions of previously broadly defined bryophyte species.