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 nu cleotide sequences of the internal transcribed spacer regions (ITS) of the 18-26S nuclear ribosomal DNA. Sequences were determined for 15 po pulations of Dendroseris and one population for each of two outgroups from the genera Sonchus and Sventenia. Little length variation was det ected 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 diffe rences among the three subgenera. The molecular data also indicate tha t subg. Dendroseris and Phoenicoseris are holophyletic, but do not sup port holophyly of subg. Rea. The ITS sequences did not resolve relatio nships among subgenera, supporting the hypothesis of rapid adaptive ra diation of Dendroseris on the islands. Relative rate tests indicate th at rates of nucleotide substitutions in the ITS regions are not signif icantly different among the different lineages of Dendroseris followin g adaptive radiation. Comparisons of average pairwise sequence diverge nce of Dendroseris species in the ITS regions and chloroplast genome i ndicated that ITS sequences have evolved about 38 times faster than cp DNA in the genus. Rates of ITS sequence divergence of Dendroseris were estimated to be faster than (3.94 +/- 0.10) x 10(-9) per site per yea r, and likely (6.06 +/- 0.15) x 10(-9) per site per year.