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

This study focuses on the phylogenetic relationships within the Polyopistho cotylea and Monopisthocotylea, two groups that are often grouped within the monogeneans, a group of disputed paraphyly. Phylogenetic analyses were con ducted with multiple outgroups chosen according to two hypotheses, a paraph yletic Monogenea or a monophyletic Monogenea, and with three methods, namel y maximum parsimony, neighbour joining and maximum likelihood. Sequences us ed were from the partial domain C1, full doXmain D1, and partial domain C2 (550 nucleotides, 209 unambiguously aligned sites) from the 28S ribosomal R NA gene for 16 species of monopisthocotyleans, 26 polyopisthocotyleans incl uding six polystomatids, and other Platyhelminthes (61 species in total, 27 new sequences). Results were similar with outgroups corresponding to the t wo hypotheses. Within the Monopisthocotylea, relationships were: (((Udonell a, capsalids), monocotylids), (diplectanids, ancyrocephalids)); each of the se 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 oth ers. Among the latter, Hexabothrium, parasite of chondrichthyans, was the m ost basal, and the mazocraeids, mainly parasites of clupeomorph teleosts, w ere the sister-groups of all other studied polyopisthocotyleans, these, mai nly parasites of euteleosts, being polytomous. (C) 2000 Australian Society for Parasitology Inc. Published by Elsevier Science. All rights reserved.