Enzyme electrophoresis, 18S rRNA sequences, and levels of phylogenetic resolution among several species of freshwater planarians (Platyhelminthes, Tricladida, Paludicola)

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Abstract
Enzyme polymorphism and 18S rRNA sequences have been used to measure genetic distances between several species of Platyhelminthes belonging to different taxa including freshwater and parasitic forms. We have used these data to address unresolved phylogenetic and taxonomic problems with this group at several different levels ranging from phylum to subgenus. The main conclusions supported by the data seem to be the following: (i) 18S rRNA data strongly suggest that the Platyhelminthes are monophyletic, being a sister-group to the other Eubilateria; a similar conclusion applies to the Paludicola as to the rest of Platyhelminthes studied; (ii) 18S rRNA and enzyme data indicate that the family Dugesiidae of the Paludicola is monophyletic with respect to the other two families, Planariidae and Dendrocoelidae; and (iii) the subgenus Schmidtea of the genus Dugesia is monophyletic with respect to the other two subgenera of Dugesia, Dugesia and Girardia. Other aspects of the relationships of subgenera and families could not be satisfactorily resolved, but point to new problems that should be addressed in future studies, namely the taxonomic status of the family Planariidae and the relationships between the genera and subgenera of the family Dugesiidae.

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