18S RIBOSOMAL-RNA SEQUENCES AND PHYLOGENY OF PLATYHELMINTHES

The phyletic relationships, and hence the systematic position, of freshwater planarians (Tricladida, Turbellaria, Platyhelminthes) within the Turbellaria, and of Platyhelminthes within the Metazoa are uncertain. This stems partially from the shortcomings of the characters and techniques clasically used in the systematic study of these groups. It is, therefore, necessary to apply new methodologies and to propose new characters (basically proteins and nucleic acids) which offer the advantages of supplying a large amount of data and of allowing quantitative estimates. This study presents 18S rRNA partial sequences from several species belonging to the phylum Platyhelminthes, including parasitic groups and free-living forms. Sequence data were compared with a large number of animal and protist sequences using both most parsimonious and additive methods and phylogenies inferred. The main conclusions supported by the data can be summarized as follows: (1) Tricladida Paludicola are monophyletic to the rest of Platyhelminthes (100% bootstrapping). (2) The Trematode and the Neorhabdocoel do not group together. This does not support the hypothesis of the origin of trematodes from rabdocoels. (3) Platyhelminthes are monophyletic (97% of bootstrapping), being a sister group of the other Eubilateria. This supports the basal position of the Platyhelminthes in the radiation of Eubilateria, and rejects hypotheses of a secondary derivation of Platyhelminthes from the Eucoelomates.