Anatomical variation in Cactaceae sensu lato

by Ogburn, R. Matthew

Matthew Ogburn, University of Missouri-St. Louis, mro8r4@umsl.edu, Department of Biology, University of Missouri-St. Louis, 1 University Blvd., St. Louis, MO 63121 Anatomical variation in Cactaceae sensu lato Interpretation of the evolution of Cactaceae, a speciose and physiologically, ecologically, and morphologically distinctive angiosperm group, depends on a solid understanding of phylogenetic relationships both within and outside of the clade. Molecular approaches have begun to resolve these relationships and reveal 1) that Pereskia, the leafy genus long interpreted as the sister group of all other cacti, is likely paraphyletic, and 2) that Cactaceae is nested within a paraphyletic Portulacaceae as a member of the ‘ACPT clade’ (Anacampseroteae, Cactaceae, Portulaca, and Talinum). This information provides a framework for asking questions about important steps in the evolution and radiation of cacti. I examined vegetative anatomy in the ACPT clade, focusing on the outgroups of Cactaceae, with the goals of identifying potential synapomorphies at varying hierarchical levels, comparing conflicting phylogenetic hypotheses, and examining hypotheses about cactus evolution. Results indicate that Talinum retains many plesiomorphic characters supporting its position as sister to the rest of ACPT. Relationships between Ogburn, Matthew, 2007, UMSL, p. 4 Cactaceae, Portulaca, and Anacampseroteae are as yet unclear, with no morphological characters unambiguously favoring one arrangement over the other, although a sister-group relationship between Cactaceae and Portulaca appears unlikely. These results also indicate that the gain of stem stomata and delayed periderm in Pereskia, although important precursors for the shift to stem photosynthesis in Cactaceae, are common also in the outgroups of Cactaceae, and were thus not likely in themselves to have been key traits that facilitated the radiation of cacti. Other stem characters were identified here that correlate with the two Pereskia clades, uniting the Andean/southern group with core cacti. These characters include a greatly thickened stem epidermis, hypodermal druses, large and persistent mucilage cells, and radial layering of outer cortical cells. They potentially have important physiological implications for the evolution of stem-based photosynthesis and stem succulence, interacting with features like delayed periderm and stem stomata.