Of the various tribes described for members of the cactus subfamily Cactoideae, the tribe Cacteae is perhaps one of the most morphologically distinguishable. The members of this tribe include the barrel cacti of North America, containing such familiar genera as Ferocactus, Astrophytum, Lophophora, and Mammillaria. One of the major phylogenetic questions that remains for this group is the assessment of the sister-group relationship, if any, to other tribes of the subfamily. Based on general stem morphology, the barrel cacti of South America, tribe Notocacteae (e.g. Parodia, Frailea, Blossfeldia, Copiapoa, and allied genera), have been implicated as the possible sister group to the Cacteae. Comparative sequence studies of representative members of both of these tribes, in addition to representatives from all other tribes of the subfamily Cactoideae were used to assess intertribal relationships, and to examine the phylogenetic affinities of tribe Cacteae. Parsimony analysis of sequence data from the plastid intron rpl16 supports the monophyly of the tribe Cacteae, places it as a distinct clade within the subfamily, and does not infer any direct relationship to members of the tribe Notocacteae. The rpl16 phylogeny suggests that the Cacteae represent an early discrete divergence from the cactoid ancestor, with no apparent directly-related sister clades. Furthermore, the data support the distinction of the Atacama endemic genus Copiapoa as a discrete lineage from the ‘core’ members of tribe Notocacteae. Additional sequence data are being accrued to evaluate phylogenetic information from other molecular markers and to examine the robustness of the relationships supported by the rpl16 intron phylogeny.

Key words: Cactaceae, Cacteae, classification, molecular systematics, phylogeny