Radiation of the endemic genus Dendroseris (Asteraceae) on the Juan Fernandez Islands: evidence from sequences of the its regions of nuclear ribosomal DNA

Phylogenetic relationships among nine of the 11 species of the endemic genus Dendroseris on the Juan Fernandez Islands were inferred from nucleotide sequences of the internal transcribed spacer regions (ITS) of the 1 8-26S nuclear ribosomal DNA. Sequences were determined for 15 populations of Dendroseris and one population for each of two outgroups from the genera Sonchus and Sventenia. Little length variation was detected in the ITS regions of Dendroseris, with ITS 1 253 or 254 bp long and ITS 2 224 or 225 bp. The sequence data provide strong support for the holophyly of Dendroseris despite the distinct morphological differences among the three subgenera. The molecular data also indicate that subg. Dendroseris and Phoenicoseris are holophyletic, but do not support holophyly of subg. Rea. The ITS sequences did not resolve relationships among subgenera, supporting the hypothesis of rapid adaptive radiation of Dendroseris on the islands. Relative rate tests indicate that rates of nucleotide substitutions in the ITS regions are not significantly different among the different lineages of Dendroseris following adaptive radiation. Comparisons of average pairwise sequence divergence of Dendroseris species in the ITS regions and chloroplast genome indicated that ITS sequences have evolved about 38 times faster than cpDNA in the genus. Rates of ITS sequence divergence of Dendroseris were estimated to be faster than (3.94 ? 0.10) x 1 0-9 per site per year, and likely (6.06 ? 0.15) x 10-9 per site per year. The Juan Fernandez Islands lie 580 km west of mainland Chile at 330 S latitude. The archipelago consists of two major islands-Masatierra and Masafuera-which are approximately 4 million and 1-2 million years old, respectively (Stuessy et al., 1984). The islands, with an unusually high level of endemic flora (60% at the specific level), offer an excellent model system for studying plant phylogeny (Stuessy, Crawford, and Marticorena, 1990). Further, these islands with known ages and rich endemism should serve as a simplified system for studying molecular evolution. The endemic genus Dendroseris D. Don. (Asteraceae: Lactuceae) is the most speciose genus on the Juan Fernandez Islands and consists of 11 species in three subgenera: subg. Dendroseris Skottsb. (D. litoralis Skottsb., D. macrantha [Bertero & Dcne.] Skottsb., D. macrophylla D. Don, and D. marginata [Bertero & Dcne.] Hook. & Am.); subg. Phoenicoseris Skottsb. (D. berteroana [Dcne] Hook. & Am., D. pinnata [Berteroi ex Dcne] Hook. & Am., and D. regia Skottsb.); and subg. Rea (Bertero ex Dcne.) Skottsb. (D. gigantea Johow, D. micrantha [Bertero ex Dcne.] Hook. & Am., D. neriifolia [Dcne.] Hook. & Am., and D. pruinata [Johow] Skottsb.). The three subgenera are quite distinct morphologically, ranging from palmiform and rosette trees to rosette shrubs and sparsely branched trees, which represent the typical habits often encountered in plants on oceanic islands (Carlquist, 1974). Eight of the species are endemic to the older island of Masatierra, and one species in each subgenus (D. macro