Insect Resistance in Genetically-Modified Crops
A team of scientists centered at the University of Arizona has discovered that field populations of pink bollworm, the longtime scourge of Arizona cotton farmers, harbor three genetic mutations that confer resistance to genetically-modified cotton. This discovery 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.
Normal pink bollworm caterpillars die when they eat the bolls of genetically modified cotton plants that produce B.t. (Bacillus thuringiensis) toxin, but resistant caterpillars survive. So far in cotton fields, resistance remains rare in pink bollworm and other targeted insects. This enables B.t. 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 B.t. 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 B.t. Scientists are trying to delay pest resistance to B.t. crops, but progress has been stymied by lack of information about the genetic basis of such resistance.
An international research team 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, B.t. toxins attack cadherin in gut membranes of insects. The team reports that each of the three resistance mutations disrupts instructions for producing cadherin, thus blocking toxicity of B.t. 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 B.t. 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 works 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. (Proceedings of the National Academy of Sciences, 4/29/03).
