Evolution of monogenean parasites across vertebrate hosts illuminated by the phylogenetic position of Euzetrema Combes, 1965 within the Monopisthocotylea

The Monogenea, which is divided into two clades, namely the Monopisthocotylea and Polyopisthocotylea, is a highly diversified group of platyhelminth parasites that infest mainly actinopterygian and chondrichthyan fishes but also, to a lesser extent, freshwater sarcopterygian hosts. Euzetrema knoepffleri Combes, 1965 (Monogenea: Iagotrematidae), which is specific to the salamander Euproctus montanus Savi, 1838 is among the rare monopisthocotylean parasites infesting tetrapod hosts. We sequenced the complete 18S rRNA gene of this parasite to infer its phylogenetic position within the Monopisthocotylea. Our results provide a new insight for coevolutionary scenarios between monopisthocotyleans and gnathostomatan hosts. Indeed, the basal position of E. knoepffleri within a subgroup of the Monopisthocotylea which comprises two clusters that both include parasites of the Actinopterygii and Chondrichthyes, suggests a very old association between the Iagotrematidae and tetrapods. Furthermore, if we take into account a recent view of Gnathostomata evolution where bony and cartilaginous fishes are regarded as a monophyletic group, it could be argued that the Iagotrematidae arose very early, during the fish-tetrapod transition, as did the Polystomatidae, the only monogenean family of the Polyopisthocotylea that infests sarcopterygian hosts. (C) 2003 The Linnean Society of London.