Linkage of a gene causing malaria refractoriness to Diphenol oxidase-A2 on chromosome 3 of Anopheles gambiae.

An inbred line of the African malaria vector Anopheles gambiae is refractory to development of malaria parasites. It is homozygous for a 4.3-kb Sal I restriction fragment at the Dox-A2 locus, whereas the parent population is polymorphic at this locus, and a susceptible line is homozygous for an alternate 3.85-kb fragment. The Dox-A2 locus is located in the middle of chromosome 3R, in division 33B, and is tightly linked to a cluster of genes including Dopa decarboxylase that are involved in the production of melanin. Because the refractoriness phenotype, melanotic encapsulation of ookinete/oocysts, might involve activation of or alteration in one or more of these genes, we performed genetic crosses to determine whether a previously identified Plasmodium cynomolgi Ceylon refractoriness gene, Pif-C, is linked to Dox-A2. Backcross mosquitoes fed on one infected monkey developed infections of < or = 100 oocysts. About 50% of these mosquitoes appeared phenotypically refractory, as expected for the backcross performed, but gave slight evidence of linkage between a refractoriness gene and Dox-A2. In contrast, females fed on a monkey that yielded higher infection levels, up to > 300 oocysts, showed clear evidence of linkage between a refractoriness gene and Dox-A2. We conclude that this Dox-A2-linked refractoriness gene is expressed under conditions particular to the higher infection levels, or that environmental factors obscured the genetic effect of this gene at lower infection levels.