Rapidly evolving lineages impede the resolution of phylogenetic relationships among Clitellata (Annelida)

The phylogenetic relationships of the Clitellata were investigated using a data set with published and new complete or partial 18S rRNA and mtCOI gene sequences of 13 and 49 taxa representing 8 and 14 families, respectively. Three different alignments were considered for 18S, and the possible influence of departures from rate constancy among sites was evaluated by analyses using a Gamma model of rate heterogeneity. Maximum-likelihood estimates of the shape parameter alpha of the Gamma distribution were very low, whatever the alignment or the gene considered, suggesting that phylogenetic reconstructions taking into account the rate heterogeneity among sites are likely to be the most reliable. Analyzed separately, the two genes did not resolve the relationships among the Clitellata, but the consensus tree was congruent with the morphology-based relationships. Our data suggest the inclusion of the Euhirudinea, Acanthobdellida, and Branchiobdellida in the Oligochaeta and suggest the Lumbriculidae as the link between both assemblages. Although separate analyses of both genes, as well as different alignments for the 18S rRNA sequences, yielded conflicting results concerning the phylogenetic position of leeches and leech-like worms vis-à-vis the Oligochaeta, subsequent analyses using the Gamma model greatly reduced the observed inconsistencies. Our analyses show that among the Clitellata, the leeches and the leech-like and gutless worms represent significantly faster evolving lineages. It is suggested that the observed higher mutation rates may be explained by the fact that these lineages contain almost exclusively commensal and/or parasitic taxa.