Pink bollworm a potent threat to GM cotton

the quest to promote Bt (Bacillus thuringiensis) cotton may suffer a serious setback with scientists from the us-based University of Arizona discovering that the pink bollworm -- one of the most destructive agricultural pests -- harbours three genetic mutations that make it resistant to the transgenic variety of the plant.

Bollworms usually die when they eat genetically modified (gm) cotton that produces the Bt toxin. But according to the recent finding, more than 500 species of the insect are immune to Bt.

Each of the three mutations occurs in a gene encoding a protein called cadherin. In humans and other mammals, cadherin coordinates cell-to-cell interactions and may cause cancer if it does not function properly.

The Bt toxin usually attacks the cadherin present in the gut membranes of pests. But the three mutations disrupt instructions for producing cadherin, thus blocking the 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.

Combined with the existing evidence, the new results imply that mutations in the cadherin gene may be vital for pest resistance to Bt crops. The breakthrough paves the way for dna-based screening that could be 1,000 times more efficient in detecting pest resistance than the 'bioassays' that are currently used. Knowledge of the genetic basis of resistance also opens new avenues for designing novel toxins to overcome the insects' defences.

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