| This study addresses a number of questions related to the systematics and evolution of the subfamily Opuntioideae of the Cactaceae. Five interrelated topics are addressed: (1) a broad view of the systematics, biogeography, and evolution of the opuntioid cacti; (2) the systematics and taxonomy of the grusonioid cacti, a group of opuntioids found in North America; (3) reticulate evolution in Opuntia; (4) the evolution of a crop species, Opuntia ficus-indica; (5) an investigation into the early diversification and evolution of the Cactaceae, and (6) a study of the contextual interpretation of early cactus evolution. Primary data types gathered here include DNA sequences of the nrITS and trnL-F regions, RAPD banding pattern data, comparative morphological data, primary and secondary historical sources, and field-collected data. Various phylogenetic and statistical analyses (including Bayesian, bootstrap, factor, jackknife, maximum parsimony, median network, and neighbor joining analyses) were performed on these data, yielding a number of inferences: (a) the Opuntioideae are monophyletic; (b) the subfamily is composed of 4 lineages (Maihueniopsis-like plants, the genus Pterocactus , and one clade each marked by terete or flat stemmed morphology); (c) many currently accepted genera are para- or polyphyletic in origin; (d) the Opuntioideae first evolved in west-central South America; (e) increased surface area may have been a factor in opuntioid evolution; (f) the early morphotype of the Opuntioideae may have been most similar to the extant representatives of Maihueniopsis; (g) the modern Grusonia concept is inadequate to classify the morphological variation and lineages within this group; (h) various morphological and molecular data support the hypothesis that reticulate evolution readily occurs in the genus Opuntia; (i) Opuntia ficus-indica originated in south-central Mexico; (j) the taxon O. ficus-indica may circumscribe a group of plants derived from multiple lineages, and this is likely obscured by artificial selection of this species; (k) early cacti may have been diminutive, possibly geophytic succulents, and not broadleaved woody shrubs like Pereskia; (l) interpretation of early cactus evolution may be influenced by the context of the specific author. |
