Molecular phylogeny of the Annelida

Traditionally, the Annelida has been classified as a group comprising the Polychaeta and the Clitellata. Recent phylogenetic analyses have led to profound changes in the view that the Annelida, as traditionally formulated, is a natural, monophyletic group. Both molecular and morphological analyses support placement of the Siboglinidae (formerly the Pogonophora) as a derived group within the Annelida; there is also evidence, based on molecular analysis of the nuclear gene elongation factor-1 alpha, that the unsegmented echiurids are derived annelids. While monophyly of the Clitellata is well-supported by both molecular and morphological analyses, there is no molecular evidence to support monophyly of the polychaete annelids; the Clitellata fall within a paraphyletic polychaete grade. Relationships among groups of polychaete annelids have not yet been resolved by molecular analysis. Within the Clitellata, paraphyly of the Oligochaeta was indicated in a phylogenetic analysis of cytochrome c oxidase I, which supported a sister relationship between the leeches, including an acanthobdellid and a branchiobdellid, and two of the four oligochaetes in the analysis. There is some evidence from analysis of 18S rRNA sequences for a sister-group relationship between the clitellates and the taxon Aeolosoma. There is no agreement regarding the body form of the basal annelid, and while molecular analyses provide strong support for the Eutrochozoa, the identity of sister-group to the Annelida among the Eutrochozoa remains enigmatic. It is recommended that future investigations include additional conserved gene sequences and expanded taxon sampling. It is likely that the most productive approach to resolving annelid phylogeny, and thus increasing our understanding of annelid evolution, will come from combined analyses of several gene sequences.