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Transgenic maize (corn) has been deliberately genetically modified to have agronomically desirable traits. Traits that have been engineered into corn are resistance to herbicides and incorporation of a gene that codes for the Bacillus thuringiensis (Bt) toxin, protecting plants from insect pests. Hybrids with both herbicide and pest resistance have also been produced. Transgenic maize is currently grown commercially in the United States.
Additional recommended knowledge
Herbicide resistant corn
Corn varieties resistant to glufosinate (Liberty) herbicides and Roundup have been produced. There are also corn hybrids with tolerance to imidazoline herbicides marketed by Pioneer Hi-Bred under the trade mark Clearfield, but in these the herbicide tolerance trait was bred without the use of genetic engineering. Consequently the regulatory framework governing the approval, use, trade and consumption of transgenic crops does not apply for imidazoline tolerant corn.
Herbicide resistant GM corn is grown in the United States. A variation of herbicide resistant GM corn was approved for import into the European Union in 2004. Such imports remain highly controversial (The Independent, 2005).
Expressing the toxin was achieved by inserting a gene from the soil-dwelling microorganism Bacillus thuringiensis into the corn genome. This gene codes for a toxin that will crystallize in the digestive tract of insect larvae, leading to its starvation.
In 2001, Bt176 varieties were voluntarily withdrawn from the list of approved varieties by the United States Environmental Protection Agency when it was found to have little or no Bt expression in the ears and was not found to be effective against second generation corn borers. (Current status of Bt Corn Hybrids, 2005)
Effects of Bt corn on nontarget insects
In May 1999, a laboratory at Cornell University published the results from a laboratory trial that appeared to indicate that the pollen of genetically modified Bt corn presented a threat to monarch caterpillars. Critics claimed that the popular media was wrong to report that monarch butterflies were threatened because this experiment did not duplicate natural conditions under which monarch caterpillars may come in contact with corn pollen. (Cornell News, 1999)
In 2001 the scientific journal the Proceedings of the National Academy of Sciences published six comprehensive studies that showed that Bt corn pollen does not pose a risk to monarch populations for the following reasons:
(Sears, et al., 2001)
Monarch populations in the USA during 1999 increased by 30%, despite Bt corn accounting for 30% of all corn grown in the USA that year. The beneficial affects of Bt corn on Monarch populations have been attributed to reduced pesticide use. (Trewavas and Leaver, 2001).
Numerous scientific studies continue to investigate the potential effects of Bt corn on a variety of nontarget invertebrates. A synthesis of data from many such field studies(Marvier et al. 2007) found that the measured effect depends on the standard of comparison. The overall abundance of nontarget invertebrates in Cry1Ab variety Bt corn fields is significantly higher compared to non-GM corn fields treated with insecticides, but significantly lower compared to insecticide-free non-GM corn fields. Abundance in fields of another variety, Cry3Bb corn, is not significantly different compared to non-GM corn fields either with or without insecticides.
Preventing Bt resistance in pests
By law, farmers in the United States who plant Bt corn must plant non-Bt corn nearby. These non-modified fields are to provide a location to harbor pests. The theory behind these refuges is to slow the evolution of the pests to the Bt pesticide. Doing so enables an area of the landscape where wild type pests will not be immediately killed.
It is anticipated that resistance to Bt will evolve in the form of a recessive allele in the pest. Because of this, a pest that gains resistance will have an incredibly higher fitness than the wild type pest in the Bt corn fields. If the resistant pest is feeding in the non-Bt corn nearby, the resistance is neutral and offers no advantage to the pest over any non resistant pest. By ensuring that there are at least some breeding pests nearby that are not resistant, it increases the chance that the resistant pest will mate with one. Since the gene is recessive, all offspring will be heterozygous. All offspring from that generation will no longer be resistant to Bt and therefore no longer a threat. By doing this, scientists and farmers hope to keep the resistant gene in very low numbers and utilize genetic drift to further insure that any resistance that emerges does not spread to the rest of the pest population.
The non-Bt pesticide status of the refuges is being compromised by wind-born pollen drifting into the non-Bt corn fields. Corn harvested from the supposed Bt-free zones has shown traces of Bt toxin. The levels found in the non-Bt corn decreases with distance from the Bt-corn fields indicating that the pollen is wind-borne rather than another method of transfer. The concentrations in the refuge fields were found to be low-to-moderate.
Possible solutions to the cross-pollination problem are to plant a wider refuge field or plant varieties of corn that bloom at different times than the Bt fields do. (Chilcutt & Tabashnik, 2004)
The StarLink corn controversy
StarLink was a variety of Bt corn patented by Aventis Crop Sciences (a subdivision of Aventis, acquired by Bayer AG in 2002), intended for use in animal feed.
U.S. regulatory authorities permitted the commercial sale of StarLink seed, with the stipulation that crops produced must not be used for human consumption. This restriction was based on the possibility that a small number of people might develop an allergic reaction because the version of the Bt protein used in StarLink is less rapidly digested than other Bt varieties.
StarLink corn was subsequently found in food destined for consumption by humans, with an episode involving Taco Bell taco shells being particularly well publicized. This led to a public relations disaster for Aventis and the biotechnology industry as a whole. Sales of StarLink seed were discontinued.
Corn sent by the UN and the US as help to Central African nations was also found to contain some StarLink corn. The nations involved refused to accept the aid.
The southern portion of the U.S. corn belt planted the greatest amount of StarLink corn. It is this portion of the U.S. where corn borer damage creates the greatest economic loss to farmers.
Greenpeace, which opposes genetic engineering in general, responded with a movement to ban the production and distribution of StarLink corn.
Notably, no adverse reactions were reported due to the inadvertent human consumption of StarLink corn.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Transgenic_maize". A list of authors is available in Wikipedia.|