Several genera within the subfamily Cactoideae of the Cactaceae have historically been taxonomically problematic, due to the difficulty of confidently classifying them within the tribal groups of Buxbaum and the subsequent recent variations on his classification for the subfamily. Parallelisms, character loss, and nebulously-defined tribal boundaries have contributed to this confusion, thereby rendering modern intergeneric classification schemes less robust. Forthcoming taxonomic revisions of the cactus family require the assignment of these unusual genera within a defined tribal classification. Specifically, these studies have been conducted to evaluate the evolutionary affinities of the genera Deamia Britton and Rose, Eulychnia Philippi, Harrisia Britton, Stetsonia Britton and Rose, and Uebelmannia Buining among others. To provide additional data to assist with tribal placement of these problematic genera, variation in several molecular markers from the plastid genome were used. Maximum parsimony estimates of phylogenetic relationships for representatives of all major tribal lineages within the subfamily Cactoideae were determined and used to elucidate the position of the problematic genera. Phylogenies inferred from these new data are used in concert with morphological characters to determine the closest evolutionary relationships for these genera. These studies will ultimately suggest whether new morphological definitions of evolutionarily-related generic groups need to be established to reflect a revised tribal classification.

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