Phylogeny of the monopisthocotylea and Polyopisthocotylea (Platyhelminthes) inferred from 28S rDNA sequences.

This study focuses on the phylogenetic relationships within the Polyopisthocotylea and Monopisthocotylea, two groups that are often grouped within the monogeneans, a group of disputed paraphyly. Phylogenetic analyses were conducted with multiple outgroups chosen according to two hypotheses, a paraphyletic Monogenea or a monophyletic Monogenea, and with three methods, namely maximum parsimony, neighbour joining and maximum likelihood. Sequences used were from the partial domain C1, full domain D1, and partial domain C2 (550 nucleotides, 209 unambiguously aligned sites) from the 28S ribosomal RNA gene for 16 species of monopisthocotyleans, 26 polyopisthocotyleans including six polystomatids, and other Platyhelminthes (61 species in total, 27 new sequences). Results were similar with outgroups corresponding to the two hypotheses. Within the Monopisthocotylea, relationships were: ¿[(Udonella, capsalids), monocotylids], (diplectanids, ancyrocephalids)¿; each of these families was found to be monophyletic and their monophyly was supported by high bootstrap values in neighbour joining and maximum parsimony. Within the Polyopisthocotylea, the polystomatids were the sister-group of all others. Among the latter, Hexabothrium, parasite of chondrichthyans, was the most basal, and the mazocraeids, mainly parasites of clupeomorph teleosts, were the sister-groups of all other studied polyopisthocotyleans, these, mainly parasites of euteleosts, being polytomous.