Colorado State University Study Identifies Gene In Fruit Fly Resistant To Insecticide
Tuesday, March 17, 1998
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.
FORT COLLINS--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.
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