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 relationships of six of the seven known species of Robinsonia, the second largest genus endemic to the Juan Fernandez Islands. Sequence divergence between species ranges from 0.00 to 6.77% (mean (3.65 ? 2.15)%), and all sequences are the same length. One most parsimonious tree was produced from the 70 variable nucleotide sites, including the species of Senecio as outgroups; this had a consistency index of 0.92 excluding un- informative sites. The cladogram is fully concordant with one generated from morphology, with R. berteroi, the only species of subg. Rhetinodendron, as the sister taxon to the remaining species in subg. Robinsonia. Within subg. Robinsonia, sects. Eleutherolepis and Robinsonia are monophyletic. Within the former section, R. masafuerae, the only species 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 Robinsonia. 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 rejected 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 genera, 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 phylogeny as cpDNA, whereas in Robinsonia, cpDNA restriction site mutations did not resolve phylogenetic relationships among the studied species while ITS sequences generated a highly resolved phylogeny.