ITS SEQUENCES AND THE PHYLOGENY OF THE GENUS ROBINSONIA (ASTERACEAE)

Sequences from the internal transcribed spacer region (ITS) of nuclear ribosomal DNA were used to produce a hypothesis of phylogenetic relat ionships of six of the seven known species of Robinsonia, the second l argest genus endemic to the Juan Fernandez Islands. Sequence divergenc e between species ranges from 0.00 to 6.77% [mean (3.65 +/- 2.15)%], a nd all sequences are the same length. One most parsimonious tree was p roduced from the 70 variable nucleotide sites, including the species o f Senecio as outgroups; this had a consistency index of 0.92 excluding uninformative sites. The cladogram is fully concordant with one gener ated from morphology, with R. berteroi, the only species of subg. Rhet inodendron, as the sister taxon to the remaining species in subg. Robi nsonia. Within subg. Robinsonia, sects. Eleutherolepis and Robinsonia are monophyletic. Within the former section, R. masafuerae, the only s pecies of Robinsonia on the younger island of Masafuera, is the sister species to R., evenia, as it is in the phylogeny based on morphology. ITS sequences also provide strong support for the monophyly of Robins ania. The average rate of ITS sequence divergence within the genus was estimated to be at least (7.83 +/- 0.74) x 10(-9) per site per year. Relative rate tests indicate that the molecular clock cannot be reject ed for ITS sequence evolution in Robinsonia. The mode and tempo of ITS and cpDNA evolution were compared in Robinsonia and Dendroseris, the two largest endemic genera on the Juan Fernandez Islands. In both gene ra, mean sequence divergence between species was higher in ITS than in cpDNA. The distribution of mutations in ITS and cpDNA differ between the two genera. In Dendroseris, ITS sequences produced the same phylog eny as cpDNA, whereas in Robinsonia, cpDNA restriction site mutations did not resolve phylogenetic relationships among the studied species w hile ITS sequences generated a highly resolved phylogeny.