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    16. Pest Resistance to Genetically Modified Crops Decoded

    (Above) Genetically modified cotton is protected from pest attack, but pink bollworm caterpillars with mutations conferring resistance devour the seeds inside cotton bolls. (Front) Since 1997, genetically modified cotton that produces Bt toxin has accounted for more than half of Arizona's cotton. Photos by Timothy Dennehy, department of entomology, University of Arizona.

    By Susan McGinley
    Apr 16, 2003

    A team of scientists centered at the University of Arizona has discovered that field populations of pink bollworm, the longtime scourge of Arizona farmers, harbor three genetic mutations that confer resistance to genetically modified cotton. This breakthrough paves the way for DNA-based screens that could be 1,000 times more efficient in detecting pest resistance than the bioassays that are currently used. The report appears in the online edition of the Proceedings of the National Academy of Sciences (see sidebar).

    Normal pink bollworm caterpillars die when they eat the bolls of genetically modified cotton plants that produce Bt (Bacillus thuringiensis) toxin, but resistant caterpillars survive. So far in cotton fields, resistance remains rare in pink bollworm and other targeted insects. This enables Bt cotton to control some major pests and has helped farmers reduce insecticide applications.

    But it may only be a matter of time before the pink bollworm and other pests adapt to Bt cotton. Already, more than 500 species of insects have evolved resistance after repeated exposure to natural and synthetic toxins. Pest populations also harbor rare genes that confer resistance to Bt. Scientists are trying to delay pest resistance to Bt crops, but progress has been stymied by lack of information about the genetic basis of such resistance.

    An international research team, led by Shai Morin in the UA College of Agriculture and Life Sciences, found that each of the three mutations in pink bollworm occurs in a gene encoding a protein called cadherin. In people and other mammals, cadherins mediate cell-cell interactions and cancer may result if they do not function properly.

    Harmless to people and animals, Bt toxins attack cadherin in gut membranes of insects. The Morin team reports that each of the three resistance mutations disrupts instructions for producing cadherin, thus blocking toxicity of Bt.

    This resistance is inherited as a recessive trait, so caterpillars with two mutant versions of the cadherin gene are resistant, but those with one or none are susceptible. Together with previous evidence, the new results imply that mutations in the cadherin gene may be central in pest resistance to Bt crops.

    This discovery will speed development of fast and precise DNA-based tests for resistance. Unlike bioassays, DNA-based screening can detect individuals with single copies of resistance genes, and work with either live or dead insects. Knowledge of the genetic basis of resistance also opens new avenues for designing novel toxins to overcome the insects' defenses.

    SIDEBAR: Mutations in a key insect pest confer resistance to genetically modified cotton, according to article #1036 of the April 29 issue of the Proceedings of the National Academy of Sciences, online now in the Early Edition - Updated: June 30, 2003

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