Evolution of insecticide resistance in the mosquito Culex pipiens: The migration hypothesis of amplified esterase genes

Resistance to organophosphorus insecticides has been studied at the gene and the population levels in Culex pipiens in various geographic areas. Only three loci have developed major resistance alleles in this species, including Est-2 (or esterase B), at which resistance occurs through gene amplification. Gene amplification involving a same particular haplotype has been found at the esterase B locus of mosquitoes from various continents. This situation, which has been explained by a unique amplification event followed by migration and selection by OP insecticides, has been sometimes questioned. A clarification of the hypotheses proposed is presented, and how it is possible to prove or disprove them Recent data on the extent of polymorphism at the esterase B locus in susceptible populations provide a strong support of the migration hypothesis.