Circumscriptions of Arethuseae have varied since the tribe was first described by John Lindley in 1840, containing over ninety genera among the different authors. The latest system of Arethuseae defined by Dressler, including around thirty genera, is the most commonly accepted today. The goals of this study are to assess whether Arethuseae sensu Dressler and component subtribes are monophyletic and evaluate the position(s) of Arethuseae within Orchidaceae. Sequences of two plastid genes, matK and rbcL, have been obtained for 24 representative genera of Arethuseae in Dressler's latest two taxonomic systems for the tribe, plus 46 other genera throughout Orchidaceae. Both separate and combined analyses of the matK and rbcL data indicate that the tribe may not be monophyletic, which is also true for most subtribes within Arethuseae. Furthermore, matK data suggest that this gene may be non-functional within Orchidaceae.
Communicating Editor: Kathleen A. Kron