VU, NINH VIET1*, GREGORY MICHAEL PLUNKETT1, and PORTER P. II LOWRY2. 1Department of Biology, Life Sciences Building, 816 Park Avenue, P.O. Box 842012, Richmond Virginia 23284-2012; 2Missouri Botanical Garden, St. Louis MO 63166-0299. - Phylogenetic study of Madagascan Polyscias and close relatives in the Ginseng family (Araliaceae).
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