Evidence for Host-Induced Selection in Schistosoma mansoni

Abstract

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.

Experimental models of Schistosoma mansoni infection

“…a higher proportion of eggs in the liver) in mice (Anderson & Cheever 1972) and a different pattern in monkeys (Powers & Cheever 1972). Rapid changes in isoenzyme patterns occur during successive generations of newly isolated S. mansoni strains in mice so that it is clear that genetic selection may occur rapidly in the laboratory (LoVerde et al 1985, Bremond et al 1993. Pinto et al (1997) recently documented the greater genetic variability in worms from field isolates compared to the LE strain long maintained in the laboratory.…”

Experimental host-induced selection in Schistosoma mansoni strains from Guadeloupe and comparison with natural observations

“…Bottlenecks occurring at the extraction phase, or at each experimental infection step, have been proposed as an explanation for the low genetic variability observed between the S. mansoni strains. As schistosome strains are maintained experimentally on laboratory vertebrate hosts different from the natural host, a differential host-induced selection may be also responsible for a modification in the genomic structure (Fletcher et a!., 1981;LoVerde et al, 1985).…”

The evolution of the Th2 immune responses and its relationships with parasitic diseases and allergy

“…Como mencionamos anteriormente, una gran virulencia del parásito se opone a la relación de parasitismo, cuya existencia depende de la supervivencia del huésped (49). De esta manera, su evolución presupone distintas adaptaciones que la mantengan y es posible que el huésped ejerza mayores presiones selectivas sobre el parásito que viceversa, lo que es muy difícil de demostrar experimentalmente, pero podría ser el caso de los helmintos (50).…”

“…La especiación de los helmintos guarda una estrecha relación con su huésped y las adaptaciones se dan en forma relativamente rápida (50,59). Es posible que la IgE tuviera algún impacto evolutivo sobre los helmintos, seleccionando aquellos que resistieran sus efectos.…”