Pinpointing DDT resistance. (Genetic Research).

Insecticide resistance is a major global health issue, with 40% of the world's population at risk for malarial diseases carried by Anopheles mosquitoes and these disease vectors showing increasing resistance to DDT. Now research published in the 27 September 2002 issue of Science shows that, at least in some strains of the fruit fly Drosophila melanogaster, overtranscription of a single gene may confer low to moderate DDT resistance.

Using microarray analysis, the researchers examined the activity of all 90 P450 genes, which control metabolic functions including chemical detoxification. Focusing on two strains of D. melanogaster, they found that not only did DDT resistance coincide with over-activity of the P450 gene Cyp6g1, this upregulation also conferred resistance to two other types of insecticides that mapped to the same genetic region as Cyp6g1.

The researchers sequenced the two DDT-resistant strains and found the same mutation in each: the insertion of the Accord transposable element just upstream of Cyp6g1 (such elements are small pieces of DNA that can jump to new genome locations). This upstream insertion could affect how often the gene was transcribed, and thus the amount of a detoxifying enzyme produced. They also showed that the insertion was present in 20 DDT-resistant fruit fly strains of diverse geographic origins. Most likely, the resistance developed "as a single mutagenic event some years ago" and has since spread globally, explains research team member Tom Wilson, an entomologist at Colorado State University.

The team produced DDT resistance in a line of susceptible fruit flies by inserting a copy of the Cyp6g1 gene that was controlled by a heat shock driver. After exposure to heat, these flies transcribed the gene at 100 times the rate of nontransgenic fly strains and were resistant to DDT. The team interpreted these results to mean that "overtranscription of Cyp6g1 alone is both necessary and sufficient for P450-mediated DDT resistance."

But David Begun, an assistant professor of evolution and ecology at the University of California, Davis, comments that for the particular transgenic flies created, the upregulation of Cyp6g1 was sufficient; he disagrees that the experiment demonstrates that upregulation is necessary for DDT resistance. This research likely reveals only part of the DDT-resistance story, he says.

In a highly resistant D. melanogaster strain investigated by Amy H. Tang, an assistant professor at the Mayo Clinic Cancer Center, a retrotransposon was found to be inserted in the same upstream location as the Accord element described earlier, and it too upregulated the Cyp6g1 gene. However, when Tang produced recombinant lines lacking the retrotransposon, she found that, despite the expected lack of Cyp6g1 upregulation, the strains were still highly DDT-resistant.

Tang agrees that upregulation of the Cyp6g1 gene is able to confer low-to-moderate levels of DDT resistance. However, she says, upregulation of Cyp6g1 clearly is not necessary to ensure DDT resistance at high levels in the strain she investigated. "Most likely, upregulation of detoxification enzymes alone is not responsible for conferring the high level of DDT resistance we observed," Tang says. "Our results are pointing to multiple mechanisms of DDT resistance besides detoxification."