Pests' genetic resistance to GM cotton discovered
Crops genetically engineered to produce proteins from the bacterium Bacillus thuringiensis – or Bt – were introduced in 1996 and planted on more than 180 million acres worldwide during 2013. Organic growers have used Bt proteins in sprays for decades because they kill certain pests but are not toxic to people and most other organisms. Pest control with Bt proteins – either in sprays or genetically engineered crops – reduces reliance on chemical insecticides. Although Bt proteins provide environmental and economic benefits, these benefits are cut short when pests evolve resistance.
The emergence of resistant pink bollworm in India provided the researchers an opportunity to test the hypothesis that insects in the field would evolve resistance to Bt toxin by the same genetic mechanism found previously in the lab. In the lab strains, the scientists had identified mutations in a gene encoding a protein called cadherin. Binding of Bt toxin to cadherin is an essential step in the intoxication process. Mutations that disrupt cadherin block this binding, which leaves the insect unscathed by the Bt toxin.
"We wanted to see if field-resistant pink bollworm from India harbored these same changes in the cadherin gene," Fabrick said. He said that by collaborating with Indian scientists, "we discovered that the same cadherin gene is associated with the resistance in India, but the mutations are different and much more numerous than the ones we found in lab-selected pink bollworm from Arizona."
Tabashnik added: "In 17 years of research and screening more than 10,000 individuals from Arizona, we identified four cadherin-based resistance mutations. And in just eight individuals from India, we found 19 different cadherin variants that confer resistance. It blew our minds."
Sequencing the DNA of resistant pink bollworm collected from the field in India, the team found that the insects produce remarkably diverse disrupted variants of cadherin. The researchers learned that the astonishing diversity of cadherin in pink bollworm from India is caused by alternative splicing, a novel mechanism of resistance that allows a single DNA sequence to code for many variants of a protein. "Our findings represent the first example of alternative splicing associated with Bt resistance that evolved in the field," said Fabrick, who is also an adjunct scientist in the Department of Entomology at UA.
Mario Soberón, a Bt expert at the Universidad Nacional Autónoma de México in Cuernavaca who was not an author of the study, commented, "This is a neat example of the diverse mechanisms insect possess to evolve resistance. An important implication is that DNA screening would not be efficient for monitoring resistance of pink bollworm to Bt toxins."
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