Phylogenetic relationships among turbellarian orders inferred from 18S rDNA sequences.

The turbellarian flatworm is a key group to understand the origin and the early evolution of triploblastic, bilaterally symmetrical animals, but phylogenetic relationships among turbellarian orders have been a subject of debates for decades, especially on the position of the acoel turbellarians. Some workers have considered the acoel representing the most primitive turbellarian order but others have regarded them as regressive. We determined almost the entire lengths of the nucleotide sequences of 18S ribosomal RNA gene (rDNA) in 17 species from 9 turbellarian orders (the Acoela, Catenulida, Macrostomida, Lecithoepitheliata, Rhabdocoela, Prolecithophora, Proseriata, Tricladida, and Polycladida). After adding the sequences of a cestode, two trematodes and some diploblastic animals obtained from databases, we reconstructed phylogenetic trees using the neighbor-joining, maximum-likelihood and maximum-parsimony methods. All trees significantly indicated that the Acoela is the earliest divergent group among the turbellarian orders. The trees also suggested that the Tricladida evolved in the separate lineage from that of a cluster of the Catenulida, Macrostomida, Lecithoepitheliata, Rhabdocoela, Polycladida, Trematoda and Cestoda after the divergence of the Acoela.