Population genetic structure of Plasmodium falciparum across a region of diverse endemicity in West Africa

Abstract

Background: Malaria parasite population genetic structure varies among areas of differing endemicity, but this has not been systematically studied across Plasmodium falciparum populations in Africa where most infections occur.

Methods: Ten polymorphic P. falciparum microsatellite loci were genotyped in 268 infections from eight locations in four West African countries (Republic of Guinea, Guinea Bissau, The Gambia and Senegal), spanning a highly endemic forested region in the south to a low endemic Sahelian region in the north. Analysis was performed on proportions of mixed genotype infections, genotypic diversity among isolates, multilocus standardized index of association, and inter-population differentiation.

Results: Each location had similar levels of pairwise genotypic diversity among isolates, although there were many more mixed parasite genotype infections in the south. Apart from a few isolates that were virtually identical, the multilocus index of association was not significant in any population. Genetic differentiation between populations was low (most pairwise F(ST) values < 0.03), and an overall test for isolation by distance was not significant.

Conclusions: Although proportions of mixed genotype infections varied with endemicity as expected, population genetic structure was similar across the diverse sites. Very substantial reduction in transmission would be needed to cause fragmented or epidemic sub-structure in this region.

Figure 1

Location of 8 West African sites sampled for study ofP. falciparumpopulation structure, with analysis of 10 microsatellite loci in a total of 268 isolates (sample sizes at each of the sites are given in parentheses). In this region, transmission shows a gradient from low levels in the north to higher levels in the south [5].

Figure 2

Distribution of similarity indices of the 10 locus genotype profiles among all pairs of isolates within each of the sampled populations. Median similarity in each population is only 2 out of 10 loci with matching alleles. Hardly any isolates match at more than 5 loci, except for very few pairs that are virtually identical.

Figure 3

Scatterplot of pairwise genetic divergence between populations (F_STmean for all 10 microsatellite loci) plotted by geographical distance in kilometers between them. Comparisons that include the northernmost population (Richard Toll) are shown with square points, while all others are shown with circular points. An overall test for isolation by distance was not significant (Mantel test of matrix correlation, P = 0.304).