Asclepias is a large genus with about 120 species native to North America. The genus has been of great interest to systematists, ecologists, and evolutionary biologists because of complex floral morphology, specialized interactions with insect herbivores, and low fruit set suggestive of sexual selection. Current understanding of phylogenetic relationships within Asclepias is based on the last comprehensive monograph of the genus and cladistic analysis of morphological data. To further explore these relationships, DNA sequences from two non-coding regions of the chloroplast genome were analyzed. Sequences were obtained from the rpl16 intron (c. 1200 bp) and the trnC-rpoB spacer (c. 1175 bp) regions for samples representing 100 species. Analyses of these sequences resulted in phylogenetic trees in which several small clades were strongly supported, but for which relationships among these clades and remaining species were supported quite weakly. The strongly supported clades correspond to several of the smaller infrageneric groupings (e.g., subg. Podostemma, ser. Incarnatae) identified in the most recent generic revision. Well supported relationships conflicting with that revision suggest that earlier classification relied too strongly on a small set of floral characters (e.g., shape of corona segments), to the exclusion of vegetative characters (e.g., growth form). Our results are consistent with a rapid radiation of Asclepias in North America.

Key words: Asclepias, chloroplast, phylogeny, rapid radiation, rpl16, trnC-rpoB