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Scientists find genetic basis of insect's resistance to engineered crops

August 02, 2001

Genetically engineered crops with built-in insecticides are an increasingly popular tool for controlling agricultural pests. Some experts, however, believe that using those modified crops could backfire by forcing the development of genetically resistant pests.

Now, a team of geneticists has identified a gene that confers high levels of resistance in a common agricultural pest - a discovery which will allow farmers and government officials to take early steps to prevent uncontrollable outbreaks.

The scientists published their findings in the Aug. 3 issue of the journal Science.

The geneticists, from North Carolina State University, Clemson University and the University of Melbourne, studied the DNA of the tobacco budworm moth (Heliothis virescens), which feeds on a variety of crops and has developed resistance to most conventional chemical insecticides.

"Not only will knowledge about this gene enable us to detect the early signs of pests evolving resistance to the current engineered plants, it may also allow us to modify the plants so they will be defended against the new pest strains," said Dr. Fred L. Gould, the William Neal Reynolds Professor of entomology at NC State and a co-author on the Science paper.

Specifically, the researchers located the recessive gene (BtR-4) that confers much of the resistance in the moth to natural toxin from the soil bacterium called Bacillus thuringiensis (Bt). Several crops - including cotton, which is a host plant for the moth's larvae - have been genetically encoded with the insecticidal Bt toxin, which kills all budworm moths except rare individuals that contain a pair of the recessive genes.

The popular Bt crops give farmers a tool for controlling pests like the tobacco budworm moth while reducing the need for potentially dangerous chemical pesticides. But some people, including organic farmers who have long used naturally produced Bt bacteria for controlling pests, worry that the new, genetically altered crops could cause pests to rapidly develop resistance to naturally produced Bt toxins as well as the transgenic Bt toxins, leaving farmers without a reliable organic pest-control agent.

To address these concerns, the Environmental Protection Agency requires that cotton farmers plant at least 4 percent of their fields with non-modified cotton to ensure the dominant genes of susceptible moths remain common in moth populations.

While resistant budworm moth strains have not yet caused damage in the field, previous research by Gould and his colleagues established that 1.5 of every 1,000 moths carry one of the genes for resistance to the Bt toxin. Based on this frequency of resistance, the researchers predicted that it would likely take about 10 years for Bt resistance in budworm moths to become a problem if Bt cotton was widely planted. Those results assume that cotton farmers are complying with the EPA's "high-dose/refuge" mandate.

Researchers and government regulators have had difficulty verifying whether the EPA's strategy is slowing the spread of resistance, however, because of the difficulty in measuring the frequency of moths with a pair of the resistant genes.

Conventional bioassay-based monitoring methods, which count the number of moths that are resistant to the Bt toxin, are not sensitive enough because resistant individuals are quite rare. Instead, Gould and his colleagues recommend using a DNA-based method of identifying moths that have only one of the genes (moths that are heterozygous for the gene) as well as those that have both (those that are homozygous).

"Monitoring resistance allele frequencies in field populations will enable a direct test of whether the high-dose/refuge strategy is succeeding," the researchers write in Science. "If it starts to fail, tracking the increasing heterozygote frequencies will sound a warning well before resistant homozygotes become frequent enough to cause uncontrollable outbreaks."

Such a strategy, they say, could give researchers and government regulators enough time to adjust the resistance management strategy - by increasing the percent of fields left as "refuges," for example - to reverse the increase in resistant moths. At the least, they say, current bioassay-based monitoring programs should preserve DNA samples from moths, so that researchers can have a DNA bank to analyze other resistance genes that are discovered in the moths.

But, the authors add, "any delay in initiating BtR-4 allele monitoring erodes the opportunity to make informed modifications to the high-dose/refuge strategy, that could sustain use of Bt-transgenics and prolong the environmental benefits they bring by reducing the use of conventional insecticides."
-end-
The co-authors on the Science paper were Gould, Dr. Linda J. Gahan of Clemson University and Dr. David G. Heckel of the University of Melbourne in Australia, who was project leader. The research was funded by the National Science Foundation, and it builds upon earlier research by Gould and Heckel that was funded by the USDA Competitive Research Grants Initiative.

Editor's note: A copy of the Science paper is available before 2 p.m. Aug. 2 by calling Science at 202-326-6440. After that, the paper is available by contacting Kevin Potter or Tim Lucas, NC State News Services, at 919-515-3470 or kevin_potter@ncsu.edu or tim_lucas@ncsu.edu, or from Dr. Fred Gould, at 919-515-1647 or fred_gould@ncsu.edu.
An abstract of the paper follows.

"Identification of a gene associated with Bt resistance in Heliothis virescens" Authors: Linda J. Gahan, Clemson University; Fred Gould, North Carolina State University; and David G. Heckel, University of Melbourne, Australia Published: Aug. 3, 2001, in Science.

Abstract: Transgenic crops producing insecticidal toxins from Bacillus thuringiensis (Bt) are widely used for pest control. Bt-resistant insect strains have been studied but the molecular basis of resistance has remained elusive. Here we show that disruption of a cadherin-superfamily gene by retrotransposon-mediated insertion was linked to high levels of Cry1Ac toxic resistance in the cotton pest Heliothis virescens. Monitoring the early phases of Bt resistance evolution in the field has been viewed as crucial but extremely difficult, especially when resistance is recessive. Our findings enable efficient DNA-based screening for resistant heterozygotes by directly detecting the recessive allele.

North Carolina State University

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