Population genetics of insecticide resistance in the mosquito Culex pipiens

Thirty years of control of the mosquito Culex pipiens using organophosphate insecticides (OP) has selected for OP-resistance alleles on a world-wide s cale. As reviewed here, studies at the levels of gene and population allow identification of the main forces driving this process of adaptation. Three loci are involved in OP-resistance in C. pipiens. For two of these, adapti ve mutations were found to be rare events, such that the ubiquitous distrib ution of certain resistance alleles could only be explained as deriving fro m a single origin by mutation followed by extensive migration. Population s tructure analyses confirmed that long-distance migration is frequent. Thus, different resistance alleles could accumulate and compete within populatio ns soon after their origin by mutation. The different selection pressures a cting on these alleles, i.e. their selective advantage ill the presence of OP and their disadvantage (resistance cost) in absence of OP, were also ana lysed. Substantial differences ill resistance cost among alleles present wi thin the Mediterranean area were discovered. Long-term surveys of Mediterra nean populations confirmed the pivotal importance of resistance cost in sha ping the evolution of this adaptive polymorphism. Some hypotheses on the fu nctional links between the nature of the initial mutation events and the su bsequent evolution of polymorphism are discussed. (C) 1999 The Linnean Soci ety of London.