Unnoticed diversity within the disjunct moss Orthotrichum tenellum s.l. validated by morphological and molecular approaches

Abstract

Transoceanic disjunctions are thought to be common in species of bryophytes. While much effort focuses on identifying the causes of these disjunctions, few studies test the assumption that the disjunct populations are conspecific. The moss Orthotrichum tenellum is considered to be disjunct between western North America and the western Old World. A thorough morphological revision together with molecular analyses using four plastid (atpB-rbcL, rps4, trnG, trnL-F) and two nuclear loci (adk, ort-LFY) reveals that this putative taxon is composed of six well-supported species, each of them diagnosable by morphological and phylogenetic criteria. Furthermore, these species, which were previously treated as a single, morphologically variable taxon, belong to different lineages within the genus and are the result of separated evolutionary histories. Only two of them (O. tenellum s.str., O. norrisii) are sister species that may have resulted from speciation following a transatlantic disjunction. Today, all species are either restricted to the Old World (i.e., O. tenellum s.str., O. comosum sp. nov.) or endemic to North America (i.e., O. coulteri, O. norrisii, O cucullatum sp. nov., O. franciscanum sp. nov.). This work emphasizes the relevance of integrative approaches to solve taxonomic problems and to provide more solid bases for biodiversity assessment.