GEOGRAPHIC COMPATIBILITY OF THE FRESH-WATER SNAIL BULINUS-GLOBOSUS AND SCHISTOSOMES FROM THE ZIMBABWE HIGHVELD

Populations of Bulinus globosus were sampled from two rivers, 60 km ap art, in nothern Zimbabwe, and offspring from each geographic strain re ared in the laboratory. Schistosomes were harvested from each strain a nd 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 re lated to: (i) the geographic origin of the snail, and (ii) an interact ion between the geographic origin of snail and parasite, such that sym patric combinations of snails and parasites produced a greater proport ion of compatible infections than allopatric combinations. Cercarial o utput of infected snails was found to be related to the geographic ori gin of the parasite, but not of that of the snail. It is suggested tha t, as the snails used were laboratory bred and naive to schistosome in fection, differences in snail - schistosome compatibility were genetic ally 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 e ither increased infectivity by the parasite, or decreased resistance b y the snail.