Subtribe Astephaninae (Apocynaceae-Asclepiadoideae) Reconsidered: New Evidence Based on cpDNA Spacers

The circumscription of the subtribe Astephaninae has a complex and convoluted history due in part to a scarcity of readily observed distinctive morphological characters. In this study the subtribe Astephaninae, as hitherto defined by morphological characters, is reexamined using sequences of the tmT-L and trnL-F spacer, as well as the trnL intron for 12 of its 15 genera. Eustegia is found to occupy an isolated basal position in the tribe Asclepiadeae, and Schizostephanus is transferred to the Metastelminae. The remaining 10 genera fall into two only distantly related clades of 3 and 7 genera, respectively. The Astephaninae s. str. comprise only 3 South African genera, Astephanus s. str., Microloma, and Oncinema. The other 7 genera form a well-supported clade, the subtribe Tylophorinae. Both groups comprise only Old World genera. All New World taxa studied, even those that have never been formally excluded from Astephanus, are found in a distinct, well-supported New World Metastelminae clade. Based on the results presented here one of the key characters of the asclepiads-the corona-has been lost independently at least twice: once in the south African Astephaninae, and once in the New World Metastelminae. This once again underscores the extreme lability of this morphologically complex organ, as well as the difficulties in assessing homology in the asclepiads. A corollary classification of the Asclepiadeae is provided as an Appendix.