High expression of Cyp6g1, a cytochrome P450 gene, does not necessarily confer DDT resistance in Drosophila melanogaster.

Cytochrome P450 monooxygenases, a family of detoxifying enzymes, are thought to confer resistance to various insecticides including DDT. Daborn et al. [Daborn, P., Yen, J.L., Bogwitz, M., Le Goff, G., Feil, et al. 2002. A single p450 allele associated with insecticide resistance in Drosophila. Science 297, 2253-2256.] suggested that the Accord transposable element causes overexpression of a Cyp6g1 allele, which has spread globally and is the basis of DDT resistance in Drosophila melanogaster populations. To determine whether the same phenomenon also operates in other Drosophila strains, we investigated 91-R, 91-C, ry(506), Wisconsin, Canton-SH and Hikone-RH strains. While the LC(50) values for the 91-R and Wisconsin strains are 8348 microg and 447 microg of DDT, respectively, values for the other four strains range between 0.74 to 20.9 microg. As expected, the susceptible ry(506) and 91-C strains have about 16-33-fold lower levels of CYP6G1 mRNA than the resistant 91-R and Wisconsin strains. Surprisingly, CYP6G1 mRNA and protein levels in the Canton-SH and Hikone-RH strains are as high as in the two resistant strains, yet they are as susceptible as the 91-C strain. The susceptible phenotype of the Canton-SH and Hikone-RH strains is not due to mutation in the Cyp6g1 gene; sequence analysis showed that Cyp6g1 alleles of resistant and susceptible strains are very similar and cannot be classified into resistant and susceptible alleles. As observed by others, we also found that only the 5'-upstream DNA of overexpressing alleles of Cyp6g1 has an insertional DNA, which is similar to Accord and Ninja elements. To examine the role of Cyp6g1 in DDT resistance, we substituted the Cyp6g1 allele of the 91-R strain with the allele from the susceptible 91-C strain via recombination and synthesized three recombinant lines. All three lines lacked Accord insertion and showed low expression of Cyp6g1 like the 91-C strain, yet they were as highly resistant as the 91-R strain. We conclude a strain may not have to have Accord insertion in the Cyp6g1 gene and the Cyp6g1 itself may not have to be overexpressed for DDT resistance to occur.