Note to Editors: Copies of the article appearing in this month’s issue of the Proceedings of the National Academy of Sciences where the Colorado State study appears are available by calling Professor Tom Wilson, (970) 491-2542.
A team of Colorado State University researchers have identified and cloned a gene in fruit flies responsible for developing resistance to a common insecticide used on a wide variety of pest insects.
Findings from the 10-year study led by biology Professor Tom Wilson are important because they will help researchers identify similar resistance genes in other common insect pests, such as flies and mosquitoes. These insects have genetic structures so complex it would have taken years to isolate the same genes without the Colorado State study on fruit flies.
"We now can use the resistance gene found in the fruit fly to isolate similar genes in other insects, which we could not have done before," Wilson said. "We’d like to be able to use this discovery to forecast how insect populations develop resistance to insecticides and find a way to combat resistance."
The study by Wilson and colleagues Mudgapalli Ashok and Christopher Turner appears in this month’s issue of the Proceedings of the National Academy of Sciences. The study focused on the fruit fly’s genetic resistance to methoprene, an insecticide based on the insect juvenile hormone, which plays a key role in development. Since vertebrates don’t have this hormone, methoprene is nontoxic to humans and other vertebrates.
When methoprene is sprayed right before the critical stage of metamorphosis, it wreaks biological havoc on fruit flies and other insects and causes them to die. Insects exposed to methoprene during any other stage of development experience no ill effects, thus making the timing of exposure to the insecticide critical.
The Colorado State team exposed fruit flies to a toxic dose of methoprene and selected those insects that showed resistance to concentrations approximately 50 times higher than normally used to control pests.
Using a process that Wilson likens to "searching for a needle in a haystack," the researchers began looking for the single gene responsible for resistance to methoprene out of the fruit fly’s 10,000 genes. In their search, they used a smaller group of genetic elements in the fruit fly known as P elements, which migrate slowly throughout the organism. When these P elements insert themselves into genes, they disrupt the gene’s normal activity.
For example, the fruit fly’s normal resistance gene actually makes the insects susceptible to poisoning by the insecticide. Resistance to the insecticide occurs when the gene becomes mutated, generally by a single amino acid change in the protein encoded by the gene. When a P element inserts itself, the normal protein is not made.
Wilson’s team used these P elements–about 40 in all–to "tag" regions of DNA where the resistance gene was suspected to be located. Once they found the gene they believed was responsible for resistance, the researchers cloned and sequenced the gene to prove it was a match.
The discovery led to a surprising find. When Wilson entered the fruit fly resistance gene into a genetic database, he found that a similar gene had been cloned in other insects but had not been associated with insecticide resistance. The gene is a member of a family of genes known as transcription factors, which are responsible for turning other genes on and off. Normally, methoprene exposure causes certain genes to be activated at the wrong time in development, killing the insect. Wilson and his team believe that when resistant insects are exposed to methoprene, the resistance gene may protect the insect when these genes fail to become activated.
Wilson said these mutant flies are rare in nature, but they become more numerous after insecticides kill susceptible insects. As more and more of these mutant flies survive insecticide exposures, they become a larger segment of the insect population and pass on their resistance genes to new generations.
How fast resistance to methoprene in pest insects is occurring is not known, but Wilson said further studies could help researchers quickly identify mutant resistant insects and track the speed of resistance evolution.