Unlike Bt-potato and Bt-cotton varieties, which were developed solely by Monsanto and distributed through seed companies wholly or partly controlled by Monsanto, Bt-corn varieties have been developed and are being marketed by a number of different companies. This article sets out some of the characteristics of the currently available commercial varieties of Bt corn.
All commercial transgenic insecticidal corn varieties contain toxin genes obtained from the soil bacterium Bacillus thuringiensis (Bt). In their natural form, Bt bacteria produce crystal protein toxins (abbreviated as Cry toxins) and spores, which make up the spray and powder formulations used by organic and other growers. Cry toxins rupture the insect's gut. The Bt-toxin genes spliced into corn allow the plant to produce its own Cry toxins. In 1997, an estimated 7 million acres out of about 80 million acres total of field corn were planted in Bt varieties in the United States. The primary insect pest targeted by the toxins is the European corn borer (Ostrinia nubilalis). However, other pests, such as the corn earworm (Helicoverpa zea), southwestern corn borer (Diatraea grandiosella), and lesser cornstalk borer (Elasmopalpus lignosellus), may also be controlled in some Bt corn varieties.
Only a few of the many known Bt toxins are active against the European corn borer. So far, companies have used only two in commercialized corn lines--one designated Cry IAb and another Cry IAc. Along with a Cry toxin gene, companies add different promoter genes--genes that determine how much, where, and when toxins is expressed. Bt corn also contains genes unrelated to the pest-control function, including marker genes, which are used to select plants that have been successfully engineered, and genes that may be carried passively into the corn on the gene-splicing vector.
The following list shows some of the characteristics of current commercially available Bt-corn variables:
1. Trade name is "KnockOut and NatureGard" and the Bt toxin gene is CrylAb. Tissue that produces the toxin is green pollen and the stalk. Toxin level is relatively high early season in green tissue, while late season has toxin levels declining rapidly. KnockOut was developed and is marketed by CIBA Seeds (now Novartis), and Mycogen. The genetic event name is 176.
2. Trade name is "Bt-Xtra" and the Bt-toxin gene is another CrylAb. Tissue that produces the toxin is leaf, kernel, stalk, silk. Toxin level is high in the leaves full season, much lower levels in kernel, stalk, and silk. Bt-Xtra was developed and marketed by DeKalb. The genetic event names are DeKalBt and DBT418.
3. Trade name is "YieldGard" and the Bt toxin gene is CrylAc. Tissue that produces the pollen is leaf, pollen, tassel, silk, and kernel. Toxin level is high in leaves, pollen, tassel, silk and kernel full season. YieldGard was developed by Monsanto and Northrup King (Novartis) and is marketed by Pioneer Hi-Bred, Cargil, DeKalb, and Golden Harvest. Genetic event name: Mon 810 and Bt 11.
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