Phylogenetic relationships in Pterostylidinae (Cranichideae: Orchidaceae): combined evidence from nuclear ribomsomal and plastid DNA sequences

A study to evaluate the relationships in subtribe Pterostylidinae (Cranichideae: Orchidaceae) was undertaken using DNA sequences from the nuclear ribosomal ITS region (256 taxa) and plastid matK (subset of 37 taxa). Parsimony analysis of nuclear, plastid and combined datasets revealed that there is strong support for the monophyly of Pterostylidinae, and three major groups therein. Clades A-C contain nine, possibly 10, identifiable groups supported by morphological synapomorphies. Clade A comprises the following two major, strongly supported groups that correlate with morphological synapomorphies: (1) Speculantha (including Petrorchis) and (2) Linguella and Eremorchis, sister to an unresolved polytomy containing Taurantha, a paraphyletic Crangonorchis and polyphyletic Diplodium. There is no support for continued recognition of Taurantha, Crangonorchis, Linguella and Eremorchis, all of which are embedded within the broader, strongly supported, monophyletic Diplodium. Clade B represents true Pterostylis. Clade C contains the morphologically disparate Bunochilus, Hymenochilus, Oligochaetochilus, Pharochilum, Plumatichilos, Stamnorchis and Urochilus (including Ranorchis) in a partially resolved tree. There is strong molecular and morphological synapomorphic internal support for the recognition of these taxa as genera. Our results revealed that none of the presently proposed classification systems for Pterostylidinae truly accounts for the underlying phylogenetic signal. A streamlined classification system, therefore, seems warranted, although further research based on a larger plastid DNA dataset is required to elucidate relationships in Clade C.