Testing the "malaria hypothesis" for the case of Thailand: a genetic appraisal

This study provides statistical analyses of allele frequencies for populations of Thailand, with an attempt to trace the roles of differential malarial selection and genetic admixtures on the observed frequency variation of certain red cell genetic abnormalities (the two beta-globin variants--hemoglobin E and beta-thalassemia--and G-6PD deficiency), probably evolving under malarial endemicity. It is found that frequencies of hemoglobin E vary accordingly with those of G-6PD deficiency, and with diverse malarial ecology. The levels of genetic diversity are greater for hemoglobin E and G-6PD deficiency than for most other nonmalarial related genetic markers, suggesting the evolution of these two genetic abnormalities under differential selection. Results of the Mantel's statistical test for correspondence between distance matrices suggest distinctive patterns of allele frequency differentiation between malarial-related and nonmalarial-related genetic loci. Correlations between beta-globin and G-6PD genetic distances, as well as those between both sets of distances and the malarial distances, are statistically significant. On the other hand, a correlation between malarial distances and the genetic distances for nonmalarial-related genetic loci is not significant statistically. A correlation between the beta-globin genetic distances and the genetic distances for nonmalarial-related genetic loci is, however, statistically significant. The latter result could be attributed largely to the clustering of relatively high hemoglobin E frequencies among genetically closely related populations of northeastern Thailand, whose recent homeland was Laos. The consistently low frequencies of beta-thalassemia observed in most studied populations are explained as a result of the replacement of this genetic variant by hemoglobin E, under long-term malarial selection.