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The Quest for Desirable Genes |
A tool for introducing genes into plants is useful only if scientists have found genes that they want to transfer. Part of the hunt for desirable genes began inadvertently in Japan in 1901, when bacteriologist Ishiwata Shigetane was asked to investigate the cause of a disease outbreak that was killing large numbers of silkworms. Shigetane discovered that the cause of the outbreak was a previously unidentified species of spore-forming bacteria, later named Bacillus thuringiensis, or Bt.
Researchers were quick to see the value of the insect-killing bacteria. By 1938 the first commercial insecticide containing Bt hit the market in France, where it was mainly used to kill flour moths. During the next 50 years, other insecticide sprays were developed that contained Bt. But the products had several limitations--rain washed away the insecticide, for example, and sunlight rapidly broke it down. Furthermore, many pests were not susceptible to the Bt spray, and some that were susceptible were able to avoid contact with it because they fed mainly on roots, inside the stems, or on other parts of crop plants that are inaccessible to sprays. Given those limitations and the availability of more effective chemical insecticides, Bt insecticides were used only by niche markets in agriculture and forestry.
That situation changed in the 1980s, as many insects grew increasingly resistant to the commonly used insecticides, and as scientists and the public became aware that many of these chemicals are harmful to the environment. Bt insecticides affect only specific insect pests and do not persist in the soil or on leaves. As a result, they are generally considered environmentally benign. Thus, a number of commercial, government, and academic research laboratories embarked on research aimed at increasing the effective uses of Bt insecticides.
A critical piece of missing information was how Bt kills insects. When the first Bt insecticides were commercialized in the 1930s and 1940s, researchers knew only that they killed insects, not what the mechanism was. By the 1950s, a series of experiments by several research groups revealed that proteins produced by Bt bacteria were lethal to particular insect species. Over the next 20 years several different strains of Bt bacteria were discovered, and each strain was found to produce specific proteins toxic to different groups of insects. By 1980, dozens of studies had made it clear that the different proteins produced by different strains of Bt bacteria determined which groups of insects would be killed.
Researchers then zeroed in on identifying the genes associated with the production of Bt proteins. Information about the genes was gathered by a pair of microbiologists looking into why the Bt genes triggered production of their toxic protein only when Bt bacteria started to produce spores. In 1981, Helen Whiteley and Ernest Schnepf, then at the University of Washington, discovered that the insecticidal proteins were found in a crystal-like body that was produced by the bacteria. They used recombinant DNA techniques to isolate a gene that encodes for an insecticidal protein. By 1989, more than 40 Bt genes, each responsible for a protein toxic to specific groups of insects, had been pinpointed and cloned by various researchers. |
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