Piecing Together the Evolutionary History of Schistosomes
Schistosomes as digenetic trematodes are members of the Phylum Platyhelminthes (Schistosome Basics). They have complex life cycles that involve sexually reproducing adult worms in a vertebrate definitive host, and asexually reproducing larval stages in a snail intermediate host. Schistosomes achieve notoriety for causing schistosomiasis, a disease that afflicts 200 million people, mostly in tropical Africa. There are about 100 recognized species of schistosomes in 13 recognized genera. Most species are found in birds or mammals, though one enigmatic species, Griphobilharzia amoena, has been described from the Australian freshwater crocodile.
One of our goals in recent years has been to use the methods of molecular phylogenetics to provide modern hypotheses for how the genera within the family Schistosomatidae are related to one another. This will help us better understand how schistosomes have evolved and how humans were colonized as hosts. Our studies have involved collecting schistosomes or their snail hosts from North and South America, Europe, Africa, Asia and Australia. A major, long-standing collaborator in this work is Dr. Gerald Mkoji of the Kenya Medical Research Institute in Nairobi, Kenya (web page). We have now acquired specimens from all the commonly recognized genera, including Griphobilharzia. Starting with the work of Dr. Scott Snyder (web page), we provided the first molecular overview of the relationships among most schistosome genera (Abstract). This paper also suggested that the medically important genus Schistosoma arose in Asia, not Africa, as generally assumed. This "Out of Asia" hypothesis has since been supported by other studies. Dr. Jess Morgan and Dr. Randy DeJong have provided additional information about schistosomes infecting the hippopotamus (Abstract) and the widespread human-infecting species, Schistosoma mansoni (Abstract). We are proud to say that these studies have involved important contributions from parasitologists from around the globe. Most recently, Dr. Sara Brant (web page) has worked with both G. amoena (and provided molecular evidence to suggest this species is not a schistosome) (Article), and also has been actively collecting schistosomes from birds from diverse locations. One of her goals is to provide a reliable data base that can help us identify the particular species of avian schistosomes that every year cause many dermatitis outbreaks (swimmer's itch) in bathers in both freshwater and marine habitats. It is very likely that this body of work has identified new species and even genera of schistosomes (Article), and thus has changed our overall perspectives on global schistosome diversity. It also has provided new insights into the evolution of dioecy, the presence of separate male and female worms in schistosomes, and the relative role of co-speciation and host switching in schistosome evolution. We can conclude that the human-infecting schistosomes do not form a monophyletic group, suggesting schistosomes have colonized humans on several separate occasions (Loker, E.S. and Mkoji, G.M. 2005. Schistosomes and their snail hosts: The present and future of reconstructing their past. Chapter 1 in Schistosomiasis, edited by W. Evan Secor and Daniel G. Colley, World Class Parasites Series, Volume 10, Springer).
We sincerely thank the NIH for grants IP20RR18754 from the Institutional Development Award (IDeA) Program of the National Center for Research Resources and AI24340 and AI44913 that enable many of the studies reported here.
Updated June 2006
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