Callaghan_1993_Biochem.Genet_31_459

An Italian organophosphate-resistant strain of Culex pipiens (Lucca) was found to be polymorphic for elevated and nonelevated esterases. Selection for high esterase activity produced a strain homozygous for elevated esterases A2 and B2. Selection for low activity produced a strain homozygous for nonelevated esterases, A4i and B1i. Crossing experiments showed that A2 and B2 are coded by separate but closely linked genes, as are A4i and B1i. Results indicate that elevated A2 and nonelevated A4i are alleles of a single gene (Est-3 locus), as are elevated B2 and nonelevated B1i (Est-2 locus). Selection for electromorph variants gave four elevated A variants and three elevated B variants. These esterases were not selected in the field. In Lucca, A2 and B2 replaced A1, suggesting a selective advantage to the former over the latter in the presence of chlorpyrifos. It is hypothesized that the degree of amplification is an important factor in the selection of a particular esterase electromorph as a resistance mechanism and that migrating individuals with amplified genes could have an advantage when invading a population under selection.