Phylogenetic placement of the enigmatic orchid genera Thaia and Tangtsinia

Abstract

The phylogenetic position of two enigmatic Asian orchid genera, Thaia and Tangtsinia, were inferred from molecular data and morphological evidence. An analysis of combined plastid data (rbcL+matK+psaB) using Bayesian and parsimony methods revealed that Thaia is a sister group to the higher epidendroids, and tribe Neottieae is polyphyletic unless Thula is removed. Morphological evidence, such as plicate leaves and corms, the structure of the gynostemium and the micrornorphology of pollinia, also indicates that Thaia should be excluded from Neottieae. Thaieae, a new tribe, is therefore tentatively established. Using Bayesian and parsimony methods, analyses of combined plastid and nuclear datasets (rbcL, matK, psaB, trnL-F, ITS, Xdh) confirmed that the monotypic genus Tangtsinia was nested within and is synonymous with the genus Cephalanthera, in which an apical stigma has evolved independently at least twice. The tribe Neottieae (excluding Thaia) is strongly supported as monophyletic based on molecular analyses and divided into three clades as follows: Palmorchis diverged earliest; Cephalanthera (including the former Tangtsinia) is sister to the remaining of the genera; and Aphyllorchis is immediate sister to Limodorum, forming a third clade with Neottia Guett. (including Listera) and Epipactis.