Quantitative variation and selection of esterase gene ampli®cation in Culex pipiens

Although descriptions of evolutionary mechanisms are common in the literature, very few studies focus on the possible evolution of the adaptive genes themselves, i.e. their quantitative and qualitative changes. Evolution of insecticide resistance in Culex pipiens is a suitable model for studying such processes. In this species, organophosphorous insecticide resistance can be achieved through the overproduction of esterases that sequester the insecticide, and this overproduction can be caused by gene ampli®cation. It is generally assumed, but never veri®ed, that esterase activity, and therefore resistance, is monotonically related to gene ampli®cation. We have analysed resistance, esterase activity and gene ampli®cation in di€erent laboratory strains and natural populations in order to detect variability and to infer e€ects of selection on these factors. We have shown that resistance, esterase activity and ampli®cation covary, that insecticide selection is able to increase ampli®cation levels, and that a ®tness cost is probably attached to the ampli®cation in laboratory strains, related to the level of ampli®cation. The importance of variation in gene ampli®cation level is discussed and some evolutionary implications are proposed.