The evolution of Araliaceae : a phylogenetic analysis based on ITS sequences of nuclear ribosomal DNA

Abstract

Phylogenetic analyses of ITS sequence data from 70 species and 40 genera of Araliaceae (representing all major lineages within the "core group" of the family) do not support the widely used traditional division of Araliaceae into three tribes. Tribe Aralieae (characterized by imbricate petals) is found nested within a paraphyletic Schefflerieae (whose taxa have valvate petals). There are, however, two large monophyletic groups comprising most araliad genera: the "Aralia-Polyscias-Pseudopanax group" (which includes Aralia, Meryta, Munroidendron, Panax, Pentapanax, Polyscias, Pseudopanax, Reynoldsia, Sciadodendron, Tetraplasandra, and their close allies), and the "Eleutherococcus-Dendropanax-Schefflera group" (including Brassaiopsis, Dendropanax, Eleutherococcus, Fatsia, Hedera, Oreopanax, Schefflera, Sinopanax, and their close allies). The ITS trees also permit a re-evaluation of several taxonomically important morphological characters (e.g., petal aestivation, leaf architecture, carpel number, and habit), and provide the opportunity to assess traditional generic delimitations in the family. Four of the largest genera appear to be either polyphyletic (Schefflera, Pseudopanax) or paraphyletic (Aralia, Polyscias), but further studies will be needed to fully re-define these complex taxa. Outgroup comparisons and the placement of Astrotricha and Osmoxylon (in basally-branching lineages in Araliaceae) help to confirm a paleotropical origin of the family. The ITS topologies suggest that biogeographic radiations into different tropical/subtropical regions and into the north and south temperate regions occurred early in the history of core Araliaceae. Temperate taxa have arisen several times independently from tropical and subtropical relatives, although a few subtropical taxa may be found nested within temperate clades (e.g., Pentapanax within Aralia). Migrations between the Old and New Worlds are also suggested for several taxa, including Aralia, Panax, Oplopanax, and the Sinopanax-Oreopanax generic pair.