Evolutionary relationships among the schistosomatidae (Platyhelminthes : Digenea) and an Asian origin for Schistosoma

Schistosome blood flukes parasitize birds, mammals, and crocodilians and ar e responsible for causing one of the great neglected diseases of humanity, schistosomiasis. A phylogenetic study of 10 schistosome genera using approx imately 1,100 bases of the large subunit of the nuclear ribosomal gene comp lex revealed 2 major clades. One clade is entirely mammalian and includes t he genera Schistosoma and Orientobilharzia. A close examination of relation ships in this group suggests that the medically important Schistosoma arose in Asia and not in Africa as generally presumed and is paraphyletic. The s econd clade is primarily avian, consisting of 6 genera of exclusively avian parasites and 2 genera of North American mammal flukes. These results indi cate a secondary host capture of mammals on the North American continent. T his study provides little evidence concerning the ancestral molluscan or ve rtebrate schistosome host but does demonstrate that host switching has been an important feature of schistosome evolution. Evidence also indicates tha t the reduced sexual dimorphism characteristic of some avian schistosomes i s derived evolutionarily.