CHLOROPLAST DNA VARIATION AND THE RECENT RADIATION OF THE GIANT SENECIOS (ASTERACEAE) ON THE TALL MOUNTAINS OF EASTERN AFRICA

Chloroplast DNA restriction-site variation was surveyed among 40 acces sions representing all 11 species of giant senecios (Dendrosenecio, As teraceae) at all but one known location, plus three outgroup species. Remarkably little variation (only 9 variable sites out of roughly 1000 sites examined) was found among the 40 giant senecio accessions, yet as a group they differ significantly (at 18 sites) from Cineraria delt oidea, the closest known relative. This pattern indicates that the gia nt senecios underwent a recent dramatic radiation in eastern Africa an d evolved from a relatively isolated lineage within the Senecioneae. B iogeographic interpretation of the molecular phylogeny suggests that t he giant senecios originated high on Mt. Kilimanjaro, with subsequent dispersion to the Aberdares, Mt. Kenya, and the Cherangani Hills, foll owed by dispersion westward to the Ruwenzori Mountains, and then south to the Virunga Mountains, Mt. Kahuzi, and Mt. Muhi, but with dispersi on back to Mt. Elgon. Geographic radiation was an important antecedent to the diversification in eastern Africa, which primarily involved re peated altitudinal radiation, both up and down the mountains, leading to morphological parallelism in both directions. In general, the plant s on a given mountain are more closely related to each other than they are to plants on other mountains, and plants on nearby mountains are more closely related to each other than they are to plants on more dis tant mountains. The individual steps of the geographic radiation have occurred at various altitudes, some clearly the result of intermountai n dispersal, The molecular evidence suggests that two species are exta nt ancestors to other species on the same or nearby mountains.