Phylogenetic placement of the enigmatic fern genus Trichoneuron informs on the infra-familial relationship of Dryopteridaceae

The taxonomic status and phylogenetic placement of the monotypic genus Trichoneuron is studied using DNA sequences of three plastid-coding genes. Trichoneuron was originally assigned to Thelypteridaceae but has been considered to be a synonym of Lastreopsis in recent treatments. In this study, the proposed relationship of Trichoneuron to Lastreopsis was tested using a comprehensive taxa sampling of Dryopteridaceae. Our results strongly support the recognition of Trichoneuron as a distinct genus belonging to Dryopteridaceae. Trichoneuron does not group together with Lastreopsis. Instead the genus was found to be sister to the Neotropical genus Polystichopsis. This sister relationship is supported by the occurrence of long pluricellular hairs attached to the rachis of both genera. The morphological distinction of Trichoneuron from Lastreopsis is backed by the structure of the adaxial groove of rachis-costa junction and the hairs at abaxial side of leaf axes. The Polystichopsis–Trichoneuron clade was found to form the sister clade to an entirely Neotropical clade comprising Polybotrya and its relatives. The new subfamily Polybotryoideae is introduced based on the presented observations. In turn, the newly recovered sister relationship between Polystichopsis and Trichoneuron provides evidence for geographic disjunctions involving the Caribbean islands and the Indo–Burma biodiversity hotspot.