Phylogenetic position of Acoel turbellarians inferred from partial 18S rDNA sequences.

Primitive platyhelminths, especially Acoel turbellarians, are thought to be key to understanding the origin and evolution of metazoa. In order to infer their phylogenetic position within the phylum Platyhelminths, we determined and compared the complete nucleotide sequence of a region of about 750 base pairs in the central part of an 18S rDNA for ten turbellarians, including two species of the group Acoela, six species of the group Polycladida, and two species of the group Tricladida. The deduced phylogenetic tree suggests that the three groups examined form discrete and separate entities. In addition, the tree suggests an earlier emergence of the Acoel turbellarians than the other platyhelminths. This animal may not be derived by means of secondary reduction from advanced acoelomates but may be nearest to its metazoan ancestors.