ARE THE PLATYHELMINTHES A MONOPHYLETIC PRIMITIVE GROUP - AN ASSESSMENT USING 18S RDNA SEQUENCES

In most zoological textbooks, Platyhelminthes are depicted as an early -emerging clade forming the likely sister group of all the other Bilat eria. Other phylogenetic proposals see them either as the sister group of most of the Protostomia or as group derived from protostome coelom ate ancestors by progenesis. The main difficulty in their correct phyl ogenetic placing is the lack of convincing synapomorphies for all Plat yhelminthes, which may indicate that they are polyphyletic. Moreover, their internal phylogenetic relationships are still uncertain. To test these hypotheses, new complete 18S rDNA sequences from 13 species of ''Turbellaria'' have been obtained and compared to published sequences of 2 other ''Turbellaria,'' 3 species of parasitic Platyhelminthes, a nd several diploblastic and deuterostome and protostome triploblastics . Maximum-parsimony, maximum-likelihood, and neighbour-joining methods were used to infer their phylogeny. The results show the order Catenu lida to form an independent early-branching clade and emerge as a pote ntial sister group of the rest of the Bilateria, while the rest of Pla tyhelminthes (Rhabditophora), which includes the parasites, form a cle ar monophyletic group closely related to the protostomes. The order Ac oela, morphologically considered as candidates to be ancestral, are sh own to be fast-clock organisms for the 18S rDNA gene. Hence, long-bran ching of acoels and insufficient sampling of catenulids and acoels lea ve their position still unresolved and call for further studies. Withi n the Rhabditophora, our analyses suggest (1) a close relationship bet ween orders Macrostomida and Polycladida, forming a clear sister group to the rest of orders; (2) that parasitic platyhelminthes appeared ea rly in the evolution of the group and form a sister group to a still-u nresolved clade made by Nemertodermatida, Lecithoepitheliata, Prolecit hophora, Proseriata, Tricladida, and Rhabdocoela; and (3) that Seriata is paraphyletic.