A Paedomorphic Parasite Associated with a Neotenic Amphibian Host: Phylogenetic Evidence Suggests a Revised Systematic Position for Sphyranuridae within Anuran and Turtle Polystomatoineans

The phylogenetic relationships of the families Polystomatidae and Sphyranuridae (subclass Polystomatoinea) within tetrapod monogenean parasites were investigated using partial 18S rDNA sequences. About 600 nucleotides of 11 species were sequenced, including 7 species of the most common subfamilies of Polystomatidae found in anurans and turtles, 1 species of the family Sphyranuridae parasitizing exclusively urodelans, and 3 species of the subclass Oligonchoinea infesting teleostean fishes. The phylogenetic analyses were performed using three reconstruction methods: neighbor-joining, maximum-parsimony, and maximum-likelihood. Polystomatoineans but not polystomatids were shown to be monophyletic. Within the polystomatoineans there are two clades: one includes the amphibian monogeneans (anuran polystomatids and urodelan sphyranurids) and the other includes the turtle polystomatids. Polystomatoineans may have coevolved with amphibian hosts, and an ancestral "polystome" dispersed at least 200 million years ago, either from the basal stem of lissamphibians or from an anuran ancestral stock, to freshwater turtles. Furthermore, the urodelan genus 'Sphyranura', initially assigned to the family Sphyranuridae on the basis of morphological and ontogenetic evidence, is clearly nested within polystomatids, suggesting that its systematic status must be revised. This supports recent findings which argue that species of the family Sphyranuridae may be paedomorphic parasites exclusively infesting neotenic mudpuppies.