Phylogenetic relationships within the polyopisthocotylean monogeneans (Platyhelminthes) inferred from partial 28S rDNA sequences

Recent studies based on molecular data (18S rDNA and partial 28S rDNA) and morphology did not resolve a terminal polytomy within the Polyopisthocotyle a. Here, we have used sequences from the full domain D2 of the 28S rDNA for 24 species (18 new sequences) with three phylogenetic methods, maximum par simony, neighbour-joining and maximum likelihood, to infer the relationship s among the Polyopistho cotylea. The analysis of the domain D2 of the 28S r DNA has been performed on two data sets. The first one, complete, included the Polystomatidae as the outgroup in order to infer general relationships, and the second one, reduced. excluded the Polystomatidae and the polyopist hocotylean parasites of chondrichthyans, but used the Mazocraeidae as the o utgroup in order to resolve the relationships between the terminal groups. The topology found, sustained by high bootstrap and decay index value, is: (outgroup (Chimaericolidae (Mazocraeidae (Gastrocotylinea, other Polyopisth ocotylea)))). The polyopisthocotylean parasites of chondrichthyans are the sister-group of the polyopisthocotylean parasites of teleosts. In the latte r, the Mazocraeidae, essentially parasites of Clupeidae, have a basal posit ion. The polytomy between Gastrocotylinea, Discocotylinea and Microcotyline a is partially resolved in this study for the first time: the Gastrocotylin ea are the sister-group of an unresolved group including the Microcotylinea , Discocotylinea rind Plectanocotylidae. Inclusion of the Plectanocotylidae in the suborder Mazocraeinea is rejected. Monophyly of the Microcotylinea and Plectanocotylidae is confirmed, but monophyly of the Discocotylinea is questioned by the exclusion of Diplozoon. (C) 2001 Australian Society for P arasitology Inc. Published by Elsevier Science Ltd. All rights reserved.