Placing the regionally threatened moss Orthodontium gracile in the big picture – Phylogeny, genome incongruence and anthropogenic dispersal in the order Orthodontiales

[Display omitted] •Orthodontium gracile is not close to O. lineare but is sister to Orthodontopsis.•This Holarctic-Asian clade diverged from the austral Orthodontium lineare ∼53 Ma.•Orthodontopsis consists of just one species, now named Orthodontium lignicola.•An austral origin for introduced European populations of O. gracile is supported.•Some American O. gracile populations are introgressive hybrids or products of HGT. The Orthodontiaceae is a small family of predominantly Southern Hemisphere temperate and South East Asian mosses that has a key phylogenetic position for research into the evolution of pleurocarpy. In the United Kingdom it is represented by the rare conservation priority species Orthodontium gracile and the abundant exotic O. lineare, introduced from the Southern Hemisphere around a century ago. Although the two species are superficially very similar and difficult to tell apart in the field, very little is known about how closely they are related or about the phylogeny, biogeography and evolutionary history of the genus Orthodontium as a whole. Phylogenetic inference and divergence time estimation were used to explore relationships within the genus globally, date major lineage splits, detect reticulate evolutionary processes and test monophyly of taxa. It was shown that Orthodontium gracile belongs to a Holarctic and Asian clade that diverged from the exclusively southern temperate lineage of O. lineare approximately 53 Ma and that it is sister to the Himalayan and South Siberian bispecific genus Orthodontopsis, which we now recognise as a single species within Orthodontium, O. lignicola. Orthodontium lignicola is quite distinct from O. gracile morphologically but may have a closely overlapping centre of extant diversity in the Himalaya, in contrast to O. lineare which is morphologically similar but biogeographically dissimilar. The introduced European populations of Orthodontium lineare were shown to share plastid and nuclear haplotypes with four collections from Tasmania and Southern Chile, but to be distinct from other Chilean and South African haplotypes. Finally, well-supported incongruence between nuclear and plastid sequences in some Western North American populations of Orthodontium gracile strongly implies one or more chloroplast capture or horizontal genome transfer events involving this species and the regionally sympatric O. pellucens. An appeal is made for targeting phylogenetic research at the intersection points of practica