Genetic variation in Schistosoma mansoni: enzyme polymorphisms in populations from Africa, Southwest Asia, South America, and the West Indies

To assess the genetic differences underlying geographic variations in Schistosoma mansoni, individual adult worms from 22 populations, from East and South Africa, Southwest Asia, South America, and the West Indies, were processed for enzyme electrophoresis on starch gels. Fourteen enzyme systems were analyzed. An estimated 7 of 18 loci were polymorphic, and the most variable population was polymorphic at 6 of the loci (P = 0.33), with a heterozygosity H of 0.07. These results suggest that S. mansoni is as variable genetically as most other organisms. Most S. mansoni strains showed relatively low variability, however (P = 0.13 ± 0.02, H = 0.04 ± 0.005). This may be attributed to small founding populations and passage in the laboratory through low numbers of infected snails and through abnormal laboratory hosts, resulting in random fixation of alleles by the action of genetic drift and possibly in selection against particular alleles. This finding implies that geographic comparisons of any traits should be based on several isolates from each region compared, so as to adequately sample the total variation occurring within each region. Genetic distances between all strains were low (mean 0.052, range 0-0.275), suggesting that little intraspecific differentiation has occurred in S. mansoni, even between Old and New World populations. These results contrast with published electrophoretic evidence of significant divergence between geographic strains of S. japonicum. Most polymorphisms were consistent with a simple Mendelian interpretation, although formal genetic crosses were not performed. For those enzymes, the banding patterns of heterozygotes indicated that subunit structure is the same in S. mansoni as in many other organisms. Sexual differences in mobility and in number of bands were found in a few enzymes. The polymorphisms uncovered can eventually be used as genetic markers to map chromosomes and to study various traits, such as infectivity to snails and drug resistance.