A comparison of microsatellite polymorphism and heterozygosity among field and laboratory populations of Schistosoma mansoni

The genetic diversity of a field population (recently collected in Melquiades, Brazil) and two laboratory strains (LE and NMRI) of a human blood fluke, Schistosoma mansoni, were analysed using microsatellite markers. Data from the three groups showed an extreme and consistent discrepancy in the level of polymorphism at all microsatellite loci between the field population and laboratory populations. The numbers of alleles detected in LE and NMRI populations averaged only 14 and 10% of those found in the field population, respectively. Especially apparent was the abundance of rare alleles in the Melquiades population when compared with the laboratory strains. The reduction in allelic diversity in the laboratory strains is most likely due to the founder effect and potential bottlenecks that may have occurred during decades of laboratory maintenance. Surprisingly, a much less drastic difference was found when comparing the average heterozygosity of the field population with the laboratory strains. This apparent anomaly may be explained by observed population substructuring (and a potential resultant Wahlund effect) in the natural population. Our comparison of genetic diversity between laboratory and field populations of S. mansoni emphasizes the need for studies of representative populations in schistosome vaccine development.