Melastomeae come full circle: Biogeographic reconstruction and molecular clock dating

Rhexia, with 11 species in the Coastal Plain province of North America, is the only temperate zone endemic of the tropical eudicot family Melastomatac eae. It is a member of the only pantropical tribe of that family, Melastome ae. Based on the chloroplast gene ndhF, we use a fossil-calibrated molecula r clock to address the question of the geographic origin and age of Rhexia. Sequences from 37 species in 21 genera representing the tribe's geographic al range were analyzed together with five outgroups. To obtain better clade support, another chloroplast region, the rpl16 intron, was added for 24 of the species. Parsimony analysis of the combined data and maximum-likelihoo d analysis of ndhF alone indicate that the deepest split is between Rhexia plus its sister group, a small Central American genus, and all other Melast omeae. Old World Melastomeae are monophyletic and nested within New World M elastomeae. Although likelihood-ratio tests of clock and nonclock substitut ion models for the full or moderately pruned datasets rejected the clock, t hese models yielded identical topologies (for 30 taxa) with few significant ly different branch lengths as assessed by a Student's t-test. Age estimate s obtained were 22 million years ago (Mya) for the divergence of Rhexia fro m its sister group, 12 Mya for the dispersal of Melastomeae from the New Wo rld to West Africa, and 1 Mya for the diversification of Melastoma in South east Asia. The only other genus of Melastomeae to have reached Southeast As ia from Africa or Madagascar is Osbeckia. The age and geographic distributi on of fossils, which come from Miocene sites throughout Eurasia, suggest th at Melastomeae once ranged from Eurasia across Beringia to North America fr om whence they reached South America and subsequently Africa and Southeast Asia. Climate deterioration led to their extinction in the Northern Hemisph ere, with Rhexia possibly surviving in Coastal Plain refugia.