ELECTROPHORETIC STUDIES OF ENZYME VARIATION IN THE HUMAN BLOOD FLUKE SCHISTOSOMA MANSONI

MADELEINE FLETCHER, Purdue University

Abstract

In "Part One," to assess the genetic differences underlying geographic variation in Schistosoma mansoni, individual adult worms from 22 populations, from East and South Africa, Southwest Asia, South America, and the Caribbean, were processed for enzyme electrophoresis on starch gels. Fourteen enzyme systems were analyzed. An estimated seven of 18 loci were polymorphic, and the most variable population was polymorphic at six 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 populations showed low variability, however (P = 0.13 (+OR-) 0.02, H = 0.04 (+OR-) 0.005). This can 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 a number of isolates from each region to adequately represent variation occurring within a region. Genetic distances between all strains were low (mean 0.052, range 0 to 0.275), suggesting that little intraspecific differentiation has occurred in S. mansoni, even between Old and New World strains. 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. As an example, a search was made in "Part Two" for electrophoretic markers associated with genes for infectivity to snails in Schistosoma mansoni. Individual schistosomes from strains selected for high or low infectivity were subjected to electrophoresis on starch gels and stained for 14 enzyme systems. The strains were selected from different isolates from Puerto Rico in several strains of Biomphalaria glabrata. Of an estimated 18 loci, three were polymorphic and the remainder monomorphic. For one of the three polymorphic enzyme loci, lactate dehydrogenase (Ldh), phenotype frequencies were correlated with infectivity to snails. In schistosome strains of low infectivity, frequencies of the Ldh-N phenotype ranged between 0.56 and 0.69, while in strains of high infectivity, Ldh-N frequencies were typically 0.91 to 1.00. Whether the correlation is accidental or due to some form of association, such as chromosomal linkage, between the locus responsible for variation in lactate dehydrogenase and a gene for infectivity to snails remains to be determined. In "Part Three," S. mansoni was compared by enzyme electrophoresis to a closely related species parasitic in rodents, S. rodhaini, in order to find diagnostic enzymes that could help in the identification of adults and cercariae. Seven of the 15 enzymes compared differed in mobility, with the two phosphoglucomutases providing the best diagnostic tool to differentiate S. mansoni from S. rodhaini. The mobility of cercarial phosphoglucomutases was identical to that of adult worms in both species. Thus, species identification by electrophoresis of phosphoglucomutasses may prove useful in field studies of the prevalence of S. mansoni and S. rodhaini in snails.

Degree

Ph.D.

Subject Area

Genetics

COinS