Genetics of an esterase associated with resistance to organophosphorus insecticides in the sheep blowfly, Lucilia cuprina (Wiedemann) (Diptera: Calliphoridae)

Electrophoresis was used to characterize the esterases in Lucilia cuprina (Wiedemann). Of the 16 esterase bands visualized, only one was associated with resistance to organophosphorus insecticides. In laboratory reference strains, the esterase E₃ was consistently found in both susceptible and heterozygously resistant flies, but was absent from homozygously resistant flies. It was postulated that resistant flies possessed a non-staining form of E₃, designated E₃null. Genetic analyses mapped the locus for E₃null to a position in the same region as the gene for organophosphorus resistance. No recombinants between the genes for resistance and E₃null were detected. In field populations of L. cuprina from several areas in Australia, a close association was found between the frequency of E (0·97) and the proportion of flies resistant to organophosphorus insecticide (0·967). This association suggests that E₃null represents the product of a major resistance gene in terms of the ‘mutant ali-esterase’ theory. E₃ as found in susceptible blowflies, represents the ‘original’ ali-esterase, the gene for which mutated to one coding for E₃null. This mutant enzyme can hydrolyse organophosphates more efficiently than E₃ but has lost the ability to hydrolyse 1- and 2-naphthyl acetate (the substrates used to visualize the esterases after electrophoresis).