A molecular test of platyhelminth phylogeny: inferences from partial 28S rDNA sequences

Nucleotide sequences of the region extending from the D3 to the D6 expansion segments of the 28S rDNA gene were used to reconstruct evolutionary relationships within the Platyhelminthes. Neighbor-joining and parsimony analyses of representatives of most major platyhelminth taxa revealed a basal Catenulida, a monophyletic Acoelomorpha, a sister-group relationship of Macrostomorpha and Polycladida (59% of bootstrap replications), and monophyletic Tricladida. We found no evidence of a taxon Seriata (p=.0001); however, the paraphyletic status of the Proseriata needs further investigation. Although Neodermata appeared as a monophyletic group, Monogenea was paraphyletic (p<.0001), thus not supporting the taxon Cercomeromorpha. Monopisthocotylean Monogenea was the most basal taxon among the Neodermata, and Udonella sp. consistently grouped with the Monopisthocotylea. Other relationships within the Neodermata showed Trematoda as a monophyletic taxon and, among its members, Rugogaster hydrolagi was identified as an aspidogastrean. The taxa Cestodaria (Amphilinidae and Gyrocotylidae) and Eucestoda were both supported; however, constraining these 2 clades into a monophyletic Cestoda resulted in a significantly longer tree (p=.0303). We tentatively conclude that the immediate sister group of Neodermata is Fecampiida. But because of contradictory results of the constraint analyses, we cannot exclude the possibility that the sister group to Neodermata is a taxon containing many neoophoran turbellarians (e.g., Rhabdocoela and Tricladida).