Bt crops are plants that have been genetically-engineered to produce proteins that are harmless to humans but are toxic to some devastating insect pests. The proteins are produced by genes from the bacterium Bacillus thuringiensis (Bt) that have been inserted into the crops. These same Bt proteins have been used by organic farmers for more than 50 years in a spray formulation.
Bt crops were introduced in 1996 and have helped reduce the use of insecticides in corn and cotton fields around the world. In Arizona alone, cotton growers have reduced spraying broad-spectrum insecticides, which kill beneficial insects along with pests, by 80 percent.
The Bt protein works by binding to receptors in certain insects’ guts — which makes it harmful to targeted insects but safe for others.
As Dr. Bruce Tabashnik (University of Arizona) once explained it, the idea behind Bt crops “can be explained with a lock-and-key analogy … The lock on the door is the receptor protein in the insect’s gut, and the key is the Bt toxin that binds to that receptor. To be able to kill the insect, the toxin must fit the lock to open the door and get inside.”
Several insect pests have evolved resistance to Bt crops, one of which is called the pink bollworm (Pectinophora gossypiella), and Dr. Jeff Fabrick, a USDA-ARS entomologist, and his collaborators from the University of Arizona have unraveled the genetic mechanism by which it occurs. Their findings are described in the journal PLOS ONE.
Fabrick and his colleagues have spent more than a decade studying how insects adapt to Bt crops. They produced Bt resistance in pink bollworm in the laboratory and determined that the resistance is caused by changes to a gut protein called cadherin. In susceptible insects, cadherin binds to the Bt toxin, eventually leading to the death of the insect. When mutations in the gene encoding cadherin block this binding, the insect becomes resistant.
The scientists compared the cadherin gene in their lab-raised resistant insects with that same gene in resistant pink bollworm found in India. They discovered that the resistant pink bollworm from India had different changes and many more changes to that gene. In total, 19 unique mutations were found in just eight resistant pink bollworms from India. By comparison, the scientists found just four cadherin mutations in several laboratory-raised resistant strains from the U.S.
The researchers found that the pink bollworm from India uses a novel genetic mechanism to develop resistance. Known as alternative splicing, it enables a single DNA sequence to produce many variants of a protein. The diversity of mutations and the novel mechanism that gives rise to that diversity show that a variety of molecular mechanisms could be important in how insects develop Bt resistance.
More work is still needed to determine how widespread resistance due to alternative splicing is in other pests.
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