Assessing the risks of transgenic crops



		Related Why Files Field of genes Antibiotic resistance Environmental estrogens				22 OCT 1998. A bumper crop of genetically engineered seeds has scientists scurrying to predict their ecological effects. It's a matter of weighing costs and benefits: Corn and cotton carrying bacterial genes will poison insect pests, reducing farmers' costs and use of harmful pesticides. But could the new seeds spur the evolution of insects that resist Bacillus thuringiensis, organic farmers' favorite insect killer? There are no wild relatives in the United States for the major crops that have genetic resistance to Roundup, a low-toxicity herbicide, so there should be little danger of their spreading genes to wild relatives. But herbicide-resistant forms of two plants that do have wild relatives -- beets and rape seed -- are both on the market. If their pollen fertilizes wild plants, will we see a bloom of herbicide-resistant weeds? According to plant geneticist Paul Arriola, who studies the flow of genes from crops to wild plants, the movement of some genes is almost inevitable. He says it's too late to debate whether the transgenic crops (as genetically engineered seeds are known) should be used, since they are already a booming segment of the U.S. market. Arriola, an assistant professor of biology at Elmhurst College (Elmhurst, Ill.), who collaborates with Norman Ellstrand, a University of California - Riverside botanist in these studies, expects that farmers abroad will also want to benefit from the new transgenics, and thus the real question is no longer preventing the use of transgenics but rather minimizing their damage: "I represent the view that says we should proceed with caution." As Arriola explained in a debate at Nature's website, "The focus of assessment must now be on how and why [transgenic crops] are actually being put to use." .
		Pollen image courtesy of Minnesota Agricultural Experiment Station, Electron Microscopy Facility.				The concern about the effects on wild plants -- either weeds or crop relatives -- stems from the fact that transgenic crops, like conventional ones, release pollen, and pollen, like animal sperm, carries genes. Pollen fertilizes eggs in female flowers. Some flowers self-fertilize, while others require pollen from another plant. Pollen and eggs comprise botanical sex -- a method for sorting and resorting genes to increase genetic diversity and allow evolution to proceed. It's not the bugs... While much of the concern about genetically engineered crops has focused on the question of whether insects will evolve to resist the bacterial bug-killing proteins the plants make, many of the new transgenic cotton, potatoes and soybeans carry a gene allowing them to resist Roundup or another herbicide. To Arriola, Roundup resistance is "worrisome because it's conferred by one or a few genes, and it has the most likelihood of causing a problem if it does escape." (Hybrids would be less likely to actually display genetic traits that are carried on several genes.) .
		Domestic sorghum bicolor [left] and wild sorghum halepense aka johnson grass. Photography by Hugh D. Wilson, courtesy of the wonderful Vascular Plant Image Gallery at Texas A&M University Biology Department.				. We already know that conventional crops do pollinate -- or fertilize -- related species growing nearby. In research published in 1996, (see "Crop-to-Weed Gene Flow" in the bibliography), Arriola and Ellstrand found that crop sorghum would form hybrids -- would crossbreed -- with a second member of the Sorghum genus, the weed johnsongrass. The researchers looked at the two plants because they are closely related, and because johnsongrass is found near sorghum fields. At various distances from the crop, between zero and 12 percent of the weed seeds carried genes from the sorghum crop. While Arriola and Ellstrand looked no further than 100 meters from the crop, insects are known to carry some kinds of pollen at least a kilometer. While it's likely that genetically engineered crops will likewise hybridize with relatives, "there's no evidence that it's happened yet," Arriola says. And he reiterates that corn, cotton and soybeans and potatoes -- the major transgenic crops in United States, do not have wild relatives in this country. Survival of the fittest Once genes have transferred to another plant, "The rate of spread will always depend on the fitness effects of the genes and the level of gene flow from the crop to the weed," Arriola explains. Fitness is defined by whether the new genes help or hurt the recipient plant -- whether they will make them more "fit" in the battle for natural selection. Obviously, a gene giving resistance to a herbicide will only boost fitness if the herbicide is present. But it is likely to be present, since the whole point of using herbicide-resistant plants is to allow farmers to douse the field with that herbicide and snuff out competing plants. Since there's no proof that transgenic genes have yet transferred to related plants in the field, there's no way of knowing whether those hypothetical plants would survive. But researchers have found that hybrids of conventional crops and weeds can survive (see "Fitness of Interspecific Hybrids" in the bibliography). That lends legitimacy to computer predictions showing that herbicide- resistant weeds could become dominant in just two generations. Transgenic crops could also harm the primitive crop varieties or wild relatives that comprise a gene bank for plant breeders who may someday need, say, genetic resistance to a particular disease. The prospect that transgenic pollen could alter these priceless plant relatives is a major concern regarding corn in Mexico, where the crop is a food staple -- and where its ancestors grow wild. Solutions? Since genes are known to escape via pollen from several crops, what can be done to prevent problems as a growing number of acres of ever-more crop varieties are planted? One tack would be to eliminate close relatives from field margins, where hybridization is most likely. Growers could also mount search-and-destroy missions for hybrids that do escape -- a measure that would require expensive surveillance. Arriola, who does not oppose all transgenic seeds, says "We can identify problems that will surely arise, but there's no way to prevent all the problems. One hundred percent containment of transgenic pollen is impossible." . -- David Tenenbaum

						. Bibliography \| Credits \| Feedback \| Search ©1998, University of Wisconsin, Board of Regents.