Genetically Modified Organisms

A genetically modified organism (GMO) contains a genome that has been engineered in the laboratory in order to favour the expression of desired physiological traits or the production of desired biological products. In conventional livestock production, crop farming, and even pet breeding, it has long been the practice to breed select individuals of a species in order to produce offspring that have desirable traits. In genetic modification, however, recombinant genetic technologies are employed to produce organisms whose genomes have been precisely altered at the molecular level, usually by the inclusion of genes from unrelated species of organisms that code for traits that would not be obtained easily through conventional selective breeding.

GMOs are produced using scientific methods that include recombinant DNA technology and reproductive cloning. Reproductive cloning technology generates offspring that are genetically identical to the parent by the transfer of an entire donor nucleus into the enucleated cytoplasm of a host egg. The first animal produced using this cloning technique was a sheep named Dolly, born in 1996. Since then a number of other animals, including pigs, horses, and dogs, have been generated using reproductive cloning technology. Recombinant DNA technology, on the other hand, involves the insertion of one or more individual genes from an organism of one species into the DNA of another. Whole-genome replacement, involving the transplantation of one bacterial genome into the “cell body,” or cytoplasm, of another microorganism, has been reported, although this technology is still limited to basic scientific applications.

G MOs produced through genetic technologies have become a part of everyday life, entering into society through agriculture, medicine, research, and environmental management. However, while GMOs have benefited human society in many ways, some disadvantages exist; therefore, the production of GMOs remains a highly controversial topic in many parts of the world.

Genetically modified (GM) foods were first approved for human consumption in the United States in 1995, and by 1999 almost 50 percent of the corn, cotton, and soybeans planted in the United States were GM. The introduction of these crops dramatically increased per area crop yields and, in some cases, reduced the use of chemical insecticides. For example, the application of wide-spectrum insecticides declined in many areas growing plants, such as potatoes, cotton, and corn, that were endowed with a gene from the bacterium Bacillus thuringiensis, which produces a natural insecticide called Bt toxin.

Field studies conducted in India in which Bt cotton was compared with non-Bt cotton demonstrated a 30–80 percent increase in yield from the GM crop. This increase was attributed to marked improvement in the GM plants’ ability to overcome bollworm infestation, which was otherwise common. Studies of Bt cotton production in Arizona, U.S., demonstrated only small gains in yield—about 5 percent—with an estimated cost reduction of $25–65 (USD) per acre due to decreased pesticide applications. In China, a seven-year study of farms planting Bt cotton demonstrated initial success of the GM crop, with farmers who had planted Bt cotton reducing their pesticide use by 70 percent and increasing their earnings by 36 percent. However, after four years, the benefits of Bt cotton eroded as populations of insect pests other than bollworm increased, and farmers once again were forced to spray broad-spectrum pesticides. While the problem was not Bt-resistant bollworms, as had been feared initially, it nonetheless became clear that much more research was needed for communities to realize sustainable and environmentally responsible benefits from planting GM crops.

Other GM plants were engineered for resistance to a specific chemical herbicide, rather than resistance to a natural predator or pest. Herbicide-resistant crops (HRC) have been available since the mid-1980s; these crops enable effective chemical control of weeds, since only the HRC plants can survive in fields treated with the corresponding herbicide. However, because these crops encourage increased application of chemicals to the soil, rather than decreased application, they remain controversial with regard to their environmental impact.

By 2002 more than 60 percent of processed foods consumed in the United States contained at least some GM ingredients. Despite the concerns of some consumer groups, especially in Europe, numerous scientific panels, including the U.S. Food and Drug Administration, have concluded that consumption of GM foods is safe, even in cases involving GM foods with genetic material from very distantly related organisms. Indeed, foods containing GM ingredients do not require special labeling in the United States, although some groups have continued to lobby to change this ruling. By 2006, although the majority of GM crops were still grown in the Americas, GM plants tailored for production and consumption in other parts of the world were in field tests. For example, sweet potatoes intended for Africa were modified for resistance to sweet potato feathery mottle virus (SPFMV) by inserting into the sweet potato genome a gene encoding a viral coat protein from the strain of virus that causes SPFMV. The premise for this modification was based on earlier studies in other plants such as tobacco in which introduction of viral coat proteins rendered plants resistant to the virus.

The so-called “golden” rice intended for Asia was genetically modified to produce almost 20 times the betacarotene of previous varieties. Golden rice was created by modifying the rice genome to include a gene from the daffodil Narcissus pseudonarcissus that produces an enzyme known as phyotene synthase and a gene from the bacterium Erwinia uredovora that produces an enzyme called phyotene desaturase. The introduction of these genes enabled beta-carotene, which is converted to vitamin A in the human liver, to accumulate in the rice endosperm—the edible part of the rice plant—thereby increasing the amount of beta-carotene available for vitamin A synthesis in the body.

Another form of modified rice was generated to help combat iron deficiency, which impacts close to 30 percent of the world population. This GM crop was engineered by introducing into the rice genome a ferritin gene from the common bean, Phaseolus vulgaris, that produces a protein capable of binding iron, as well as a gene from the fungus Aspergillus fumigatus that produces an enzyme capable of digesting compounds that increase iron bioavailability via digestion of phytate (an inhibitor of iron absorption). The iron-fortified GM rice was engineered to overexpress an existing rice gene that produces a cysteine-rich metallothioneinlike (metal-binding) protein that enhances iron absorption.

A variety of other crops modified to endure the weather extremes common in other parts of the globe are also in production.

Fig. 1. Genetically modified organisms are produced using scientific methods that include recombinant DNA technology. Encyclopedia Britannica, Inc.