A population of Schistosoma mansoni from Kenya was isolated in 1968 and subsequently passaged simultaneously through 2 different vertebrate hosts: baboons and mice. Recent electrophoretic studies demonstrated that genetic differences in the degree of polymorphism and in allele frequencies of polymorphic loci existed between S. mansoni populations from the 2 hosts. The present study was undertaken to assess the importance of vertebrate host-induced selection against particular alleles as mechanism to account for the observed differences. A population of S. mansoni which had originally been passaged through baboons and subsequently passaged through murine hosts for 4 generations was studied. At least 20 infected snails served as the source of parasite for each mouse passage. Allele frequencies of 4 polymorphic loci were assessed for each generation using horizontal starch gel electrophoresis. All 4 polymorphic loci (PGM-2, MDH-2, MDH-1, PGI) showed a selective trend towards allele frequencies identical with that of a strain (from the same isolate) maintained in mice for 12 yr. These data suggest that vertebrate host-induced selection results in a decrease in parasite variability due to loss of alleles as field isolates of S. mansoni are passaged in murine hosts. The use of non-human primate hosts, on the other hand, maintains a higher level of parasite variability.

Genetic studies on the human blood fluke Schistosoma mansoni were undertaken using starch gel electrophoresis to detect new gene loci and allelic variation. The number of enzyme staining systems useful with S. mansoni was increased from 14 to 34. It was found that unmated female worms stained as well as male worms. Three new polymorphic loci, fructose biphosphatase (FBP), gly-leu dipeptide peptidase (PEP-4), and glyceraldehyde-3-phosphate dehydrogenase (G3PDH) were detected. This brings the known number of polymorphic loci to 10 for this species. One locus (FBP) was found to be polymorphic in the PR-1 strain of S. mansoni. This strain was previously reported to be invariant.

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