Evidence for host-induced selection in Schistosoma mansoni.

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.