Unit 7: Agriculture // Section 7: Genetic Improvement and Food Production
Farmers have manipulated the genetic makeup of plants and animals since the dawn of agriculture. Initially they used selective breeding to promote qualities that made breeds readily usable for agriculture, such as animals that domesticated well and plants that were easy to harvest. Next, breeders focused on varieties that could be grown outside of their native geographic range—for example, overcoming natural photoperiod requirements (the amount of daylight that plants need to flower). In the twentieth century, plant geneticists selected for traits that would allow plants to use high levels of water and nitrogen to increase yields. Similarly, animal breeders worked to increase the amount of meat or milk that various domestic animal lines produced.
Today classical agricultural breeding is a highly quantitative science that uses genetic markers (specific DNA sequences) to select for desired characteristics. This approach enables scientists to manipulate the genetic makeup of crops with substantial precision, as long as genetic variation exists for particular traits. Agricultural breeders also use biotechnology to move genes across taxonomic barriers, combining genetic material from species that would not cross-breed naturally. For example, Bt corn has been modified by inserting a gene from the bacterium Bacillus thuringiensis that kills harmful insects so that farmers do not need to use insecticide.
Since the mid-1990s, the U.S. Department of Agriculture has approved 63 genetically engineered (GE) crops for unrestricted sale, including strains of corn, soybeans, cotton, potatoes, wheat, canola, and papaya. Most of these crops have been developed to tolerate herbicides or resist insects or fungi, while others have been engineered for specific product qualities such as longer shelf life. Products under development include grains, field crops, fruits, vegetables, trees, and flowers designed to achieve desirable growing properties such as cold or drought resistance or efficient use of nitrogen (footnote 11). The extent to which such strategies will be able to enhance agricultural productivity, however, remains to be seen.
An alternative use of biotechnology that some supporters advocate is to develop crops with improved nutritional content to combat nutritional disorders. One widely-publicized product is golden rice, a rice variety into which several "trans" or foreign genes have been added so that the plant produces beta-carotene (vitamin A) in its grains (Fig. 11). Vitamin A deficiencies are widespread in societies that consume rice-based diets, causing thousands of cases of blindness and premature deaths among children in developing countries every year. Researchers are currently working to produce golden rice that contains the recommended daily allowance of vitamin A in a 100 to 200 gram serving, as well as to ensure the bioavailability of the beta-carotene contained within the modified rice grains. But not everyone is convinced by this approach: some experts argue that the same goals could be met more cheaply by promoting balanced, diverse diets in the target countries.
Figure 11. Conventional and golden rice
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Source: © 2007. Golden Rice Humanitarian Board.
In addition to questioning whether agricultural and nutritional goals might be more effectively met using more traditional approaches, critics have raised many concerns about GE foods, including potential harm to nearby ecosystems and the possibility that GE crops or animals will hybridize with and alter the genetic makeup of wild species. For example, over-planting Bt-resistant crops could promote increased Bt resistance among pests, while genes from GE crops could give wild plants qualities that make them more weedy and invasive. Although most of these effects will probably be benign, it is hard to predict when and where GE species could have harmful effects on surrounding ecosystems.
A 2002 National Research Council report concluded that genetically modified plants posed the same broad types of environmental risks as conventionally-produced hybrids, like the strains introduced during the Green Revolution. For example, both kinds of plants could spread into surrounding ecosystems and compete with local species. But the report noted that either type of plant could have specific traits that posed unique threats and accordingly called for case-by-case regulation of new GE strains. The committee also observed that future generations of GE plants are likely to have multiple introduced traits and forecast that these products will raise issues that cannot be predicted based on experience with early herbicide- and pest-resistant crops (footnote 12).