COEVOLUTION OF THE MONOGENOIDEA (PLATYHELMINTHES) BASED ON A REVISED HYPOTHESIS OF PARASITE PHYLOGENY

A revised hypothesis for the phylogeny of the Subclass Polyonchoinea ( Monogenoidea) was constructed employing phylogenetic systematics. The Acanthocotylidae (formerly of the Order Capsalidea) is transferred to the Order Gyrodactylidea based on this analysis. The new phylogeny is used to determine coevolutionary relationships of the familial taxa of Monogenoidea with their hosts. The coevolutionary analysis suggests t hat the Monogenoidea apparently underwent sympatric speciation or disp ersal while parasitic on ancestral Gnathostomata, resulting in two pri mary clades: the Polyonchoinea and the Oligonchoinea+Polystomatoinea. The two parasite clades apparently cospeciated independently with dive rgence of the Chondrichthyes and Osteichthyes. In the Polyonchoinea, t he clade associated with Chondrichthyes experienced primary extinction within the Holocephala, but coevolved into the Loimoidae and Monocoty lidae in the Galeomorphii and Squalea (Elasmobranchii), respectively. Within the Osteichthyes, polyonchoineans experienced primary extinctio n with the divergence of Sarcopterygii, Polypteriformes and Acipenseri formes. They demonstrate primary dispersal from the Neopterygii into t he Squalea (as Amphibdellatinea), Actinistia (as Neodactylodiscinea) a nd Urodela (as Lagarocotylidea). Secondary dispersals of polyonchoinea ns occurred in the Gyrodactylidae to the Polypteriformes, Urodela and Anura; in the Acanthocotylidae to the Myxinoidea and Squalea; in the C apsalidae to the Acipenseriformes and Elasmobranchii; and in the Monoc otylidae to the Holocephala. The Oligonchoinea and Polystomatoinea dev eloped upon divergence of the Chondrichthyes and Osteichthyes. Oligonc hoineans cospeciated within the Chondrichthyes, with the Chimaericolid ea developing within the Holocephala and the ancestor of the Diclyboth riidea+Mazocraeidea within the Elasmobranchii. Two cases of primary di spersal occurred within this clade: the Diclybothriidae to the Acipens eriformes and the ancestor of mazocraeidean families to the Neopterygi i (both Osteichthyes). Secondary dispersal within the Oligonchoinea in cludes host switching of the common ancestor of Callorhynchocotyle (He xabothriidae) to the Holocephala. Polystomatoineans coevolved within t he Osteichthyes, but experienced primary extinctions in the Actinopter ygii, Actinistia, Dipnoi and Amniota. Coevolution of the Sphyranuridae and Polystomatidae occurred with divergence of the Urodela and Anura, respectively. Secondary dispersal of polystomatids to the Urodela, Di pnoi and Amniota is suggested. A preliminary phylogenetic analysis of the Polystomatoinea suggests that primary extinction with secondary di spersal of polystomatids to the Dipnoi may not be necessary to explain extant parasite distributions, since Concinnocotyla (Concinnocotylina e) appears to represent the sister taxon of the remaining Polystomatid ae+Sphyranuridae. (C) 1997 Australian Society for Parasitology. Publis hed by Elsevier Science Ltd.