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Limits of Traditional Breeding |
Farmers have noticed for centuries that some individual plants in a given species manage to survive disease or epidemics of insects relatively unscathed, while their neighbors succumb to infection or insect predation. In 1905, Sir Roland Biffen of Cambridge, England, wondered whether healthy plants inherited pest resistance, just as they might inherit the tendency to be tall or short. His experiments on two varieties of wheat showed that the ability to resist infection by a rust fungus was indeed inherited, a discovery that intensified attempts by farmers and plant breeders to produce varieties of pest-resistant crop plants.
Similar attempts continue today and primarily involve screening a large number of plant varieties to identify those that are resistant to particular pests. Resistant varieties are then crossed with those desirable for other reasons--for example, because they produce more grain per acre. Careful selection and repetitive crossing of progeny can eventually generate varieties that are both high yielding and resistant to particular pests. But the process is extremely time consuming--it can take more than 15 years to bring a new variety to market.
One challenge encountered in traditional breeding is that generally only closely related species of plants can be cross-bred. If no varieties are naturally resistant to a particular fungus or insect, traditional breeders have no way to create resistance to that fungus or insect. Furthermore, breeders frequently face a situation in which a resistance gene is closely linked to a gene that adversely affects the quality of a crop, that is, where the two traits are always inherited together. For example, insect resistance in lettuce plants might be inherited along with a tendency for the lettuce to taste bitter. In the early 1990s, despite plant breeders' best attempts to improve the pest and disease resistance of cotton, corn, rice, and other crops, farmers worldwide lost about one-fourth of their crops to pests and diseases. |
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