ESTERASE GENE AMPLIFICATION IN CULEX-PIPIENS

In the mosquito Culex pipiens one of the major resistance mechanisms t o organophosphorous pesticides (OPs) is increased detoxification of in secticide, This resistance is the consequence of overproduction of two types of esterases, esterases A and a, coded at two loci, Est-3 (A es terase) and Est-2 (B esterase). We have analysed the genomic structure of these genes in different strains resistant to OPs and have attempt ed to characterize the different types of mutations leading to the res istant phenotypes. It is shown that, concerning the more frequent resi stant phenotypes, mutations leading to resistance are of two main type s, First, overproduction of one A esterase present in Southern France results from a regulatory mechanism, The second type of mutation is ge ne amplification which Involves events that have initially generated t he duplication of both the A and B esterase or only the B esterase loc us, We report the point that the most frequent esterase overproduction s are the results of eight different mutations and that, given the ran ge of distribution of these genotypes, mutation leading to an efficien t resistance gene is one of the most limiting factors for the evolutio n toward resistance in Culex pipiens.