THE SELECTION AND GENETIC-ANALYSIS OF ESTERASE ELECTROMORPHS IN AN ORGANOPHOSPHATE-RESISTANT STRAIN OF CULEX-PIPIENS FROM ITALY

An Italian organophosphate-resistant strain of Culex pipiens (Lucca) w as found to be polymorphic for elevated and nonelevated esterases. Sel ection for high esterase activity produced a strain homozygous for ele vated esterases A2 and B2. Selection for low activity produced a strai n homozygous for nonelevated esterases, A4(i) and B1(i) Crossing exper iments-showed that A2 and B2 are coded by separate but closely linked genes, as are A4(i) and B1(i). Results indicate that elevated A2 and n onelevated A4(i) ale alleles of a single gene (Est-3 locus), as are el evated B2 and nonelevated B1(i) (Est-2 locus). Selection for electromo rph variants gave four elevated A variants and three elevated B varian ts. These esterases were not selected in the field. In Lucca, A2 and B 2 replaced A1, suggesting a selective advantage to the former over the latter in the presence of chlorpyrifos. It is hypothesized that the d egree of amplification is an important factor in the selection of a pa rticular ester ase electromorph as a resistance mechanism and that mig rating individuals with amplified genes could have an advantage when i nvading a population under selection.