Quantitative variation and selection of esterase gene amplification in Culex pipiens

Although descriptions of evolutionary mechanisms are common in the literatu re, very few studies focus on the possible evolution of the adaptive genes themselves, i.e. their quantitative and qualitative changes. Evolution of i nsecticide resistance in Culex pipiens is a suitable model for studying suc h processes. In this species, organophosphorous insecticide resistance can be achieved through the overproduction of esterases that sequester the inse cticide, and this overproduction can be caused by gent amplification. It is generally assumed, but never verified, that esterase activity, and therefo re resistance, is monotonically related to gene amplification. We have anal ysed resistance, esterase activity and gene amplification in different labo ratory strains and natural populations in order to detect variability and t o infer effects of selection on these factors. We have shown that resistanc e, esterase activity and amplification covary, that insecticide selection i s able to increase amplification levels, and that a fitness cost is probabl y attached to the amplification in laboratory strains, related to the level of amplification. The importance of variation in gene amplification level is discussed and some evolutionary implications are proposed.