Geographic compatibility of the freshwater snail Bulinus globosus and schistosomes from the Zimbabwe highveld.

Populations of Bulinus globosus were sampled from two rivers, 60 km apart, in northern Zimbabwe, and offspring from each geographic strain reared in the laboratory. Schistosomes were harvested from each strain and also maintained in the laboratory. Cross-infection experiments were conducted, whereby snails of both geographic strains were exposed to parasites of both geographic strains. Compatibility was found to be related to: (i) the geographic origin of the snail, and (ii) an interaction between the geographic origin of snail and parasite, such that sympatric combinations of snails and parasites produced a greater proportion of compatible infections than allopatric combinations. Cercarial output of infected snails was found to be related to the geographic origin of the parasite, but not of that of the snail. It is suggested that, as the snails used were laboratory bred and naive to schistosome infection, differences in snail-schistosome compatibility were genetically determined, rather than an adaptive response. This study supports earlier work which suggests the existence of geographic compatibility amongst natural populations of snails and trematodes, indicative of either increased infectivity by the parasite, or decreased resistance by the snail.