Molecular phylogeny of the Platyhelminthes

The phylum Platyhelminthes has traditionally been considered the most basal bilaterian taxon. The main difficulty with this placement is the lack of convincing synapomorphies for all Platyhelminthes, which suggest that they are polyphyletic. Recent molecular findings based on 18S rDNA sequence data and number and type of Hox genes strongly suggest that the majority of Platyhelminthes are members of the lophotrochozoan protostomes, whereas the Acoelomorpha (Acoela + Nemertodermatida) fall outside of the Platyhelminthes as the most basal bilaterian taxon. Here we review phylum-wide analyses based on complete ribosomal and other nuclear genes addressed to answer the main issues facing systematics and phylogeny of Platyhelminthes. We present and discuss (i) new corroborative evidence for the polyphyly of the Platyhelminthes and the basal position of Acoelomorpha; (ii) a new consensus internal tree of the phylum; (iii) the nature of the sister group to the Neodermata and the hypotheses on the origin of parasitism; and (iv) the internal phylogeny of some rhabditophoran orders. Some methodological caveats are also introduced. The need to erect a new phylum, the Acoelomorpha, separate from the Platyhelminthes (now Catenulida + Rhabditophora) and based on present and new morphological and molecular characters is highlighted, and a proposal made.