Use of a P-element mediated germline transformant to study the effect of gene dosage in cyclodiene insecticide-resistant Drosophila melanogaster (Meigen)

Susceptibility to cyclodiene insecticides in Drosophila melanogaster (Meigen) has been rescued (restored) by P-element mediated germline transformation of a 40-kilobase cosmid carrying a functional copy of the susceptible gene. The inserted cosmid is carried on chromosome two and the native gene is on chromosome three; thus, up to four alleles can now be crossed into one insect. In this study, we examined the effect of varying the number and ratio of susceptible and resistant alleles of this gene to examine whether resistance was conferred by alteration of the number or affinity of cyclodiene receptors. These experiments show that the heterozygous phenotype is restored in individuals with an even proportion of resistant and susceptible alleles and that susceptibility is independent of allele number. These results are consistent with susceptibility being related to the ratio of sensitive and insensitive receptors, rather than variation in total receptor number. The significance of altered receptor number or affinity in insecticide resistance is discussed.