Laboratory of Parasitic Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Malaria Branch, Division of Parasitic Diseases, Centers for Disease Control, Bethesda, Maryland
In a genetically selected strain of the malaria vector Anopheles gambiae, most species of Plasmodium parasites are surrounded by melanized capsules and killed in the wall of the mosquito midgut. Genetic studies demonstrate a significant association between the refractory response to Plasmodium cynomolgi B strain and the Est A allele at an autosomal esterase locus. Increased susceptibility to parasites is associated with an alternate Est C allele. Lines selected to be homozygous for the Est A and Est C alleles show enhanced levels of refractoriness and susceptibility when compared to the unselected parental stock. Expression of the refractory phenotype is reduced much more than the susceptible phenotype at high parasite density, suggesting that refractoriness may be due to a positive effector.