Long-branch attraction and the rDNA model of early eukaryotic evolution

Phylogenetic analyses of ribosomal RNA genes have become widely accepted as a framework for understanding broad-scale eukaryotic evolution. Nevertheless, conflicts exist between the phylogenetic placement of certain taxa in rDNA trees and their expected position based on fossils, cytology, or protein-encoding gene sequences. For example, pelobiont amoebae appear to be an ancient group based on cytologic features, but they are not among the early eukaryotic branches in rDNA analyses. In this report, the derived position of pelobionts in rDNA trees is shown to be unreliable and likely due to long-branch attraction among more deeply branching sequences. All sequences that branch near the base of the tree suffer from relatively high apparent substitution rates and exhibit greater variation in ssu rDNA sequence length. Moreover, the order of the branches leading from the root of the eukaryotic tree to the base of the so-called "crown taxa" is consistent with a sequential attachment, due to "long-branch" effects, of sequences with increasing rates of evolution. These results suggest that the basal eukaryotic topology drawn from rDNA analyses may be, in reality, an artifact of variation in the rate of molecular evolution among eukaryotic taxa.