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Basal cactus phylogeny: implications of Pereskia (Cactaceae) paraphyly for the transition to the cactus life form¹

²Department of Ecology and Evolutionary Biology and the Peabody Museum of Natural History, Yale University, P.O. Box 208105, New Haven, Connecticut 06520-8105 USA; ³Institut für Systematische Botanik, University of Zurich, Zollikerstrasse 107, CH-8008 Zurich, Switzerland

ABSTRACT

The cacti are well-known desert plants, widely recognized by their specialized growth form and essentially leafless condition. Pereskia, a group of 17 species with regular leaf development and function, is generally viewed as representing the "ancestral cactus," although its placement within Cactaceae has remained uncertain. Here we present a new hypothesis of phylogenetic relationships at the base of the Cactaceae, inferred from DNA sequence data from five gene regions representing all three plant genomes. Our data support a basal split in Cactaceae between a clade of eight Pereskia species, centered around the Caribbean basin, and all other cacti. Two other Pereskia clades, distributed mostly in the southern half of South America, are part of a major clade comprising Maihuenia plus Cactoideae, and Opuntioideae. This result highlights several events in the early evolution of the cacti. First, during the transition to stem-based photosynthesis, the evolution of stem stomata and delayed bark formation preceded the evolution of the stem cortex into a specialized photosynthetic tissue system. Second, the basal split in cacti separates a northern from an initially southern cactus clade, and the major cactus lineages probably originated in southern or west-central South America.

Key Words: biogeography • Cactaceae • inferior ovary • Pereskia • phylogeny • phytochrome C • stem-based photosynthesis • trnK/ matK

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