A reappraisal of the phylogenetic and taxonomic position of land planarians (Platyhelminthes, Turbellaria, Tricaldida) inferred from 18S rDNA sequences

摘要

Land planarians belong to the suborder Tricladida within which four infraorders are currently recognized: the Maricola, the Cavernicola, the Paludicola and the Terricola (land planarians). Phylogenetic analyses using morphological characters support the monophyly of all infraorders with the Maricola as the primitive sister group to the rest of the Tricladida, and Terricola and Paludicola being derived sister groups. In view of the biological importance of land planarians, a robust phylogenetic scheme of the Tricladida is required for both taxonomy and comparative purposes. Using nucleotide-sequence data of the 18S ribosomal genes it has recently been reported that the Terricola and a family of the Paludicola, the Dugesiidae, share an 18S gene duplication (types I and II) that can be explained only by accepting a common origin for the Terricola+Dugesiidae (Carranza et al. 1998). In addition, both distance and maximum parsimony analyses also support that Terricola and Dugesiidae form a monophyletic group. Here, we extend our analysis up to 18 species of paludicolans and 11 species of terricolans. Distance-based and maximum-likelihood analyses resulted in phytogenetic trees strongly supporting the Terricola and the Dugesiidae as forming a monophyletic group, though the internal phylogeny of each clade remains unresolved. These results strongly supports that landplanarians derive from freshwater ancestors and, therefore, that its taxonomic rank and position within the Tricladida has to be reconsidered.