Insects may rapidly develop resistance to killing toxins produced by genetically modified (GM) crops that are inserted with only one insecticidal gene, US researchers reported on Friday. Genes of a soil bacterium called Bacillus thuringiensis (Bt) are inserted into crop plants such as maize and cotton, creating toxins deadly to insects but harmless to humans. Bt crops have been commercialized since 1996, but some scientists are concerned that use of Bt crops would create conditions for insects to evolve and develop resistance to the toxins. Until now, it has not been shown if neighboring plants producing a single Bt toxic protein might play a role in insect resistance to transgenic crops expressing two insecticidal proteins. But a research group from Cornell University found when crops engineered with just one of those toxins grow nearby, insects may more rapidly develop resistance to all the insect- killing plants. "Our findings suggest that concurrent use of single- and dual- gene Bt plants can put the dual-gene plants at risk if single-gene plants are deployed in the same area simultaneously," said Anthony Shelton, professor at Cornell University who led the study. Their paper was published in the latest issue of the Proceedings of the National Academy of Sciences (PNAS). "Single-gene plants really function as a steppingstone in resistance of two-gene plants if the single gene plants contain one of the same Bt proteins as in the two-gene plant," Shelton said. Cotton and maize are the only commercial crops engineered with Bt genes. In 2004 these crops were grown on 22.4 million hectares worldwide. After eight years of extensive use, there have been no reports of crop failure or insect resistance in the field to genetically modified Bt crops, Shelton said. Still, several insects have developed resistance to Bt toxins in the lab, and recently, cabbage loopers, a moth whose larvae feed on plants in the cabbage family, have shown resistance to Bt sprays in commercial greenhouses. Diamondback moths used in this study have also developed resistance to Bt toxins in the field. In greenhouse studies, the researchers used three types of GM broccoli plants: two types of plants each expressed a different Bt toxin, and a third, known as a pyramided plant, expressed both toxins. First, the researchers bred moth populations in which a low percent of the moths were resistant to a single Bt toxin. The insects were then released into caged growing areas with either single-gene plants, dual-gene plants or mixed populations and allowed to reproduce for two yea They found that the insects living in the greenhouse with single-gene and dual-gene plants housed together damaged all the plants after 26 generations, because more insects developed resistance to the plants' toxins overtime. However, in the same two-year time frame, all or almost all of the insects died when exposed to pyramided plants alone. "It's easier for an insect to develop resistance to a single toxin," said Shelton. "If an insect gets a jump on one toxin, then it becomes more rapidly resistant to that same toxin in a dual- gene plant." While single-gene Bt plants are most prevalent, industry trends suggest that pyramided plants may be favored in the future. In the United States, companies introduced dual-gene cotton in 2003, but single-gene varieties remain on the market. "Single-gene Bt plants have provided good economic and environmental benefits, but from a resistance management standpoint they are inferior to dual-gene plants. US regulatory agencies should consider discontinuing the use of those single- gene plants as soon as dual-gene plants become available," Shelton said. "And industries should be encouraged to create more dual- gene plants." Source: Xinhua
