Phylogenetic relationships and patterns of character change in the tribe Lactuceae (Asteraceae) based on chloroplast DNA restriction site variation

Abstract

The Lactuceae is perhaps the most easily recognizable tribe in the Asteraceae, distinguished by the presence of milky latex and of ligulate florets in the inflorescence. Three existing taxonomic treatments of the tribe establish subtribal classifications but fail to resolve relationships among major lineages. Our study of chloroplast DNA restriction site variation sampled 60 Lactuceae taxa. We detected 1268 mutations, 612 of which are phylogenetically informative. Despite the large amount of variation detected, little resolution of relationships among major lineages was obtained from parsimony analyses, although the monophyly of many groups is strongly supported. These results, when considered along with data from morphological analyses of other workers, suggest that rapid diversification played an important role in early stages of the tribe's evolution. Our examination of character change further reveals that as noted by other workers, restriction site variation is not evenly distributed across the chloroplast genome and that regions with higher levels of variation do not necessarily have higher amounts of homoplasy. This is somewhat surprising, since we found that amounts of homoplasy along terminal branches of our phylogenetic tree are related to levels of divergence. Key words: Asteraceae, chloroplast DNA, homoplasy, Lactuceae, phylogeny, restriction site variation.