Within the plant family Araliaceae, the genus Polyscias is thought to be closely related to Gastonia and Cuphocarpus, from which it may be distinguished morphologically by differences in carpel number, presence or absence of pedicel articulation, and leaf characteristics. Species of Polyscias from Madagascar and surrounding areas (Africa, the Comoro and Mascarene Islands) form a single clade within Araliaceae, and provide an ideal model to study evolution and biogeography. We present molecular evidence of relationships and compare the results with traditional taxonomic treatments, proposing a new biogeographic hypothesis to explain our results. Sequences from nuclear rDNA ITS and 5S, and from the cpDNA marker trnL-trnF were obtained from a highly representative sample of species (~40 spp.) found in this region. ITS, 5S, and trnL-trnF sequence lengths are 625, 320, and 850 base pairs, respectively; the 5S sequence has more variable and informative characters. Results of separate and combined analyses using parsimony methods suggest that Polyscias is paraphyletic, including within it Cuphocarpus and a polyphyletic Gastonia. Area cladograms based on the phylogenetic analysis show remarkable consistency with geographic distribution, which may have been the result of repeated and independent dispersal events from Madagascar to Africa, the Comoro and Mascarene Islands. An island stepping-stone model may explain the position of most African araliads, whose ancestors appear to have reached the continent relatively recently from the Comores via Madagascar. However, more detailed analysis of climate, geological history, ecological factors, and morphological studies are needed to assess these ideas further.

Key words: Araliaceae, biogeography, Gastonia, Madagascar, phylogenetic, Polyscias