Many plants, including some genetically altered ones, rely on a biologically produced insecticide called Bacillus thuringiensis(Bt) for protection against pests. But as the use of Bt toxins increases, so does the fear of developing Bt-resistant strains of insects. Two reports in this weeks Science explore the mechanisms and genetics behind Bt resistance in worms and moths, providing insight into how to avoid the problem.
Biologists at the University of California at San Diego investigated the mechanism of Bt toxin in roundworms. Comparing proteins produced by normal genes to those produced by mutant Bt resistant ones, the team discovered that enzymes play a key role in determining whether or not a worm succumbs to the toxin. The Bt-resistant worms, they found, lacked the enzyme galactosyltransferase, which modifies carbohydrates. The scientists speculate that crystalline Bt toxins recognize the outer surface of intestinal cells through carbohydrates and sugars. They then attack and dissolve their hosts intestines, killing them. If a host lacks galactosyltransferase, however, the Bt toxin cant locate the intestines and the worm sustains no damage to its internal composition. Whether all Bt toxins operate in the same manner and require this enzyme remains unclear. But the new findings do shed light on earlier, seemingly contradictory evidence from Bt-resistant insects, the scientists say.
In a second article appearing in the same journal, a team of geneticists studying the tobacco budworm moth discovered a recessive gene (BtR-4) that provides resistance to Bt toxin. "Not only will knowledge about this gene enable us to detect the early signs of pests evolving resistance to the current engineered plants," co-author Fred Gould of North Carolina State University notes, "it may also allow us to modify the plants so they will be defended against the new pest strains."
The scientists suggest using DNA-based methods to identify and monitor both resistant moths, which have two recessive genes, and moths that have only one of the genes. As the number of susceptible but heterozygous moths increases, so, too, does the chance that they will breed with another heterozygous individual, eventually leading to larger numbers of resistant insects. Such a monitoring program, the researchers write, would "sound a warning bell before resistant homozygotes become frequent enough to cause uncontrollable outbreaks."