A phylogenetic study of Ferocactus Britton and Rose (Cactaceae: Cactoideae)
Jonathan F. Wendel
Ferocactus is the third largest genus in the tribe Cacteae of the Cactoideae, and the number of species is still matter of taxonomic discussion. The different interpretation of morphological characters has led to discrepancies in species boundaries. This dissertation includes four papers describing the taxonomic history of Ferocactus to elucidate its evolutionary history and phylogenetic relationships with related taxa of the tribe Cacteae. The first paper is a general review of Ferocactus and summarizes the knowledge of the genus and discusses the relevance of understanding its phylogenetic relationships at the specific, generic and tribal levels. The second paper is a survey to study chromosome numbers in 14 taxa of Ferocactus, all of which have the base number of x=11. Analyses of meiotic figures failed to provide evidence of hybridity, at least for the populations investigated. The third and fourth papers are general accounts of molecular studies in Ferocactus to investigate its putatively monophyletic origin. These two papers approach the problems using molecular data generated from restriction site analysis (third paper) of the chloroplast DNA (cpDNA) and sequence data from two non-coding regions (rpl16 and intergenic spacer between the trnL and trnF genes) (fourth paper) of the cpDNA. Parsimony analyses of the restriction site variants and the two DNA sequence data sets have provided insight into the phylogenetic relationships between Ferocactus and Echinocactus grusonii. The three molecular phylogenies are consistent in identifying a clade in which E. grusonii is sister to F. histrix and F. glaucescens. This clade suggests that Ferocactus, as currently circumscribed, might be paraphyletic; the taxonomic implications of this relationship are discussed. In addition, E. grusonii is basal to F. histrix and F. glaucescens in the phylogeny of the restriction site variants further indicating that Ferocactus might have shared a common ancestor with Echinocactus. The phylogenetic relationships at the interspecific level remain unresolved, perhaps due to a rapid radiation of Ferocactus and other members of the tribe Cacteae which involved morphological differentiation accompanied by little genetic divergence. Some species groups form small clades which in turn correlate with the geographic distribution (northern and southern) of Ferocactus throughout its extant geographic range.