Identification of a point mutation in the voltage-gated sodium channel gene of Kenyan Anopheles gambiae associated with resistance to DDT and pyrethroids

A field trial of permethrin-impregnated bednets and curtains was initiated in Western Kenya in 1990, and a strain of Anopheles gambiae showing reduced susceptibility to permethrin was colonized from this site in 1992. A leuci ne-phenylalanine substitution at position 1014 of the voltage-gated sodium channel is associated with resistance to permethrin and DDT in many insect species, including Anopheles gambiae from West Africa. We cloned and sequen ced a partial sodium channel cDNA from the Kenyan permethrin-resistant stra in and we identified an alternative substitution (leucine to serine) at the same position, which is linked to the inheritance of permethrin resistance in the F-2 progeny of genetic crosses between susceptible and resistant in dividuals. The diagnostic polymerase chain reaction (PCR) developed by Mart inez-Torres et al. [(1998) Insect Mol Biol 7: 179-184] to detect kdr allele s in field populations of An. gambiae will not detect the Kenyan allele and hence reliance on this assay may lead to an underestimate of the prevalenc e of pyrethroid resistance in this species. We adapted the diagnostic PCR t o detect the leucine-serine mutation and with this diagnostic we were able to demonstrate that this kdr allele was present in individuals collected fr om the Kenyan trial site in 1986, prior to the introduction of pyrethroid-i mpregnated bednets. The An. gambiae sodium channel was physically mapped to chromosome 2L, division 20C. This position corresponds to the location of a major quantitative trait locus determining resistance to permethrin in th e Kenyan strain of An. gambiae.