Crops genetically engineered to make insecticidal proteins have allowed some farmers to apply less chemical pesticides. But insects might adapt to these toxins and render the crops useless. In the 3 August issue of Science, two teams of scientists announce they've discovered two genes that confer resistance to the toxin--a key to predicting the occurrence of such resistance.
Toxins from the bacterium Bacillus thuringiensis (Bt) kill insects by binding to cells in their guts and causing the cells to burst. A mutation that prevents this binding could protect insects from Bt damage. One place researchers have looked for such mutations is in a lab strain of the tobacco budworm that resists the Bt toxin. In 1997, a team led by evolutionary biologist David Heckel of the University of Melbourne in Australia discovered that a gene responsible for the tobacco budworm's Bt resistance is located on chromosome 9.
After narrowing the location of the putative gene, which they called BtR-4, the team checked that stretch of the chromosome for known genes that code for proteins that bind the Bt toxin. They isolated a fragment of a gene that mapped to the same neighborhood as BtR-4. That's "almost irrefutable evidence" that BtR-4 is a Bt resistance gene, says Bruce Tabashnik, an entomologist at the University of Arizona, Tucson.
Another gene was found by Raffi Aroian of the University of California, San Diego, and colleagues. The group studies Bt resistance in the roundworm Caenorhabditis elegans, which like insects suffers intestinal damage from Bt toxins. They cloned a gene, bre-5, and confirmed that blocking its activity, as a mutation might do, makes the worm resistant to two Bt toxins. "It's an important mechanism to understand," Aroian says, because losing the enzyme could be an effective way to gain resistance to many Bt toxins at once.
"It's a huge leap forward," Tabashnik says of the new genes. The most practical payoff may be an easy DNA test for detecting resistance in insect pests; this could help alert farmers to burgeoning resistance in time to stop planting Bt crops and switch to chemical pesticides for a while.