Long-term genetic stability and population dynamics of laboratory strains of Schistosoma mansoni

Abstract

Measures of genetic differentiation between populations are useful tools for understanding the long-term dynamics of parasite communities. We followed the allele frequencies of microsatellite markers in samples taken over a period of 16 yr from the Case Western Reserve University-Naval Medical Research Institute (CWRU-NMRI) laboratory strain of Schistosoma mansoni. DNA was isolated from pooled samples of adults, eggs, or cercariae collected at 46 time points and genotyped for 14 tri- or tetranucleotide microsatellite markers. For comparison, 2 S. mansoni reference strains (Biomedical Research Institute-NMRI, which has a common origin with the CWRU line, and PR-1) were analyzed over shorter periods of time. We observed that the long-term allele frequencies are generally stable in large laboratory populations of this parasite, and a high degree of similarity was observed between the allele frequencies of consecutive samples from different developmental stages. The CWRU strain, however, showed 2 periods of marked deviation from stability as demonstrated using genetic differentiation measures. The first period corresponds to an admixture event with the BRI strain in which a new equilibrium was established as the "migrants" became blended into the existing CWRU population, consistent with 23% admixture from BRI. The second corresponds to a period of genetic drift when the CWRU population size was greatly reduced with an accompanying loss in genetic diversity. Having demonstrated the utility of pooled samples for the genetic analysis of population dynamics in laboratory strains of schistosomes, this approach will be useful for analyzing field samples to determine the impact of schistosomiasis control programs on parasite population structure. Accounting only for the intensity or prevalence of parasite populations may fail to register significant changes in population structure that could have implications for resistance, morbidity, and the design of control measures.

Figure 1

Schistosoma mansoni laboratory strain histories. The NMRI strain was isolated from Puerto Rican school children in Washington, D.C. The PR-1 strain was originally collected in Puerto Rico. Horizontal arrows indicate transfers or sample collections.

Figure 2

Major alleles at loci SMMS2 (a), SMMS13 (b), SMMS16 (c), SMMS18 (d), and SMMS21 (e) of the Schistosoma mansoni strain NMRI populations maintained at CWRU and BRI. Alleles with a frequency of greater than 5% at least once over the sampling period are presented.

Figure 3

Genetic diversity in the CWRU and BRI populations. Diversity values were averaged across loci for each sample.

Figure 4

Differentiation of Schistosoma mansoni NMRI populations at each sampling point. (a) Comparison of long-term intrapopulation differentiation with the indices D, F_ST, and F’_ST calculated pairwise against the first sample collected from the respective population (3/16/1984 for CWRU, 9/26/2001 for BRI). (b) Pairwise rate of differentiation. D/week between proximal pairs of samples using a sliding window approach. (c) Interpopulation differentiation using D calculated pairwise against the first sample collected from other populations (3/16/1984 for CWRU, 9/26/2001 for BRI, 4/15/2008 for PR-1).

Figure 5

D calculated pairwise between each sample collected from the CWRU population vs. the first sample from that population (3/16/1984). (a) All developmental stages. (b) Egg and adult worm samples. (c) Cercarial samples. Closed diamond – Adult worms; Open circle – Eggs; Open diamond – Cercariae.