Molecular phylogeny of the Annelida

Traditionally, the Annelida has been classified as a group comprising the P olychaeta and the Clitellata. Recent phylogenetic analyses have led to prof ound changes in the view that the Annelida, as traditionally formulated, is a natural, monophyletic group. Both molecular and morphological analyses s upport placement of the Siboglinidae (formerly the Pogonophora) as a derive d group within the Annelida; there is also evidence, based on molecular ana lysis of the nuclear gene elongation factor-1 alpha, that the unsegmented e chiurids are derived annelids. While monophyly of the Clitellata is well-su pported by both molecular and morphological analyses, there is no molecular evidence to support monophyly of the polychaete annelids; the Clitellata f all within a paraphyletic polychaete grade. Relationships among groups of p olychaete annelids have not yet been resolved by molecular analysis. Within the Clitellata, paraphyly of the Oligochaeta was indicated in a phylogenet ic analysis of cytochrome c oxidase I, which supported a sister relationshi p between the leeches, including an acanthobdellid and a branchiobdellid, a nd two of the four oligochaetes in the analysis. There is some evidence fro m analysis of 18S rRNA sequences for a sister-group relationship between th e 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 inve stigations include additional conserved gene sequences and expanded taxon s ampling. It is likely that the most productive approach to resolving anneli d phylogeny, and thus increasing our understanding of annelid evolution, wi ll come from combined analyses of several gene sequences.