Esterase gene amplification in Culex pipiens

In the mosquito Culex pipiens one of the major resistance mechanisms to organophosphorous pesticides (OPs) is increased detoxification of insecticide. This resistance is the consequence of overproduction of two types of esterases, esterases A and B, coded at two loci, Est-3 (A esterase) and Est-2 (B esterase). We have analysed the genomic structure of these genes in different strains resistant to OPs and have attempted to characterize the different types of mutations leading to the resistant phenotypes. It is shown that, concerning the more frequent resistant phenotypes, mutations leading to resistance are of two main types. First, overproduction of one A esterase present in Southern France results from a regulatory mechanism. The second type of mutation is gene amplification which involves events that have initially generated the duplication of both the A and B esterase or only the B esterase locus. We report the point that the most frequent esterase overproductions are the results of eight different mutations and that, given the range of distribution of these genotypes, mutation leading to an efficient resistance gene is one of the most limiting factors for the evolution toward resistance in Culex pipiens.