Genetic diversity Schistosoma mansoni: Evidence and implications of population structure
Abstract
Blood flukes in the genus Schistosoma are important human parasites in tropical regions. Current disease control programs focus on drug treatment of human patients, but this imposes strong selection for resistant parasites. Mathematical models suggest that drug treatment of patients with schistosomiasis may help to maintain genetic polymorphism in the parasite population. Analyses of these models show that whether resistant strains increase in frequency depends on the interplay between their relative fitness, the cost of resistance, and the degree of selection pressure exerted by drug treatments. The goal of this research is to describe and attempt to explain more thoroughly the extent and distribution of genetic diversity in a schistosome population from a Brazilian village. In the course of these investigations, two different types of molecular markers were employed, and their utility was compared. When examined with a mitochondrial repeat element (mtVNTR), schistosome isolates from 21 different patients—representing 7 different “boroughs” of the village—demonstrate substantial genetic polymorphism, which is more readily detected because of high levels of heteroplasmy. Due to a high number of common haplotypes in the population, a large proportion of its genetic variation at this repeat region is described by differences among mitochondrial genomes within individual worms. However, when only rare haplotypes are considered, population structure can be detected. In order to further resolve the schistosome population structure in this village (and other schistosome populations), a set of polymorphic microsatellite loci were characterized. The five loci revealed extensive polymorphism in preliminary studies. A subset of these microsatellite markers (along with three other previously published loci) were used to reanalyze the Melquiades schistosome population. Estimates of genetic subdivision among patient isolates from Melquiades at each of seven microsatellite loci were used to quantify the extent of schistosome population structure. These estimates, in combination with previous estimates based on mtVNTR data, yield a comprehensive picture of parasite genetic structure within this cluster of infection foci. Substantial genetic polymorphism exists within the schistosome population of this village, and is significantly subdivided among the various patients and boroughs. This may have important implications for future understanding of schistosome epidemiology.
Degree
Ph.D.
Advisors
Minchella, Purdue University.
Subject Area
Ecology|Molecular biology
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