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Superweeds

The fight over transgenic farming is anything but academic. In 1996, the first year transgenic seed was widely available, farmers planted 1.74 million hectares (4.3 million acres) of the new varieties. This year, according to Clive James, head of the nonprofit International Service for the Acquisition of Agribiotech Applications, as many as 50 million hectares worldwide-an area bigger than Germany-are planted with genetically modified crops. “It’s one of the fastest adoptions of technology I’ve ever seen,” James says.

About three-quarters of that land is in the United States, most of it planted in bioengineered corn and soybeans. But the technology is growing even faster in Argentina-the area the country devoted to transgenic soybeans tripled between 1997 and 1998. Although exact figures are not available, China, the world’s biggest producer of cotton and tobacco, is, according to James, “aggressively increasing” the land planted with genetically altered versions of both crops.

By far the most important bioengineered trait today is herbicide tolerance, which accounts for two-thirds of all transgenic crops. A technology dominated by Monsanto, it lets plants withstand the use of selected weed-killing chemicals, so that farmers can apply them without fear of destroying their crops. Monsanto’s “Roundup Ready” soybeans, which resist the company’s Roundup herbicide, were introduced in 1996; last year, they covered an estimated 10 million hectares-a third of the U.S. farmland devoted to that crop. Next in importance is insect-resistant corn, including DekalBt corn, modified by Monsanto’s recently acquired Dekalb subsidiary to produce a bacterial insecticide, and StarLink corn, produced by AgrEvo, a joint venture of German chemical giants Hoechst and Schering. Principally aimed at fighting off the European corn borer, transgenic corn last year occupied 6.5 million hectares in the United States-a fifth of the nation’s total corn crop.

More-much more-is on the way. As sales of bioengineered seeds rose from $75 million in 1995 to more than $1.5 billion last year, half a dozen huge companies in Europe and the United States positioned themselves to exploit a market that is widely believed to be on the verge of exploding. According to U.S. Department of Agriculture records, some 4,500 genetically altered plant varieties have been tested in this country, more than 1,000 in the last year alone. About 50 have already been approved for unlimited release, including 13 varieties of corn, 11 tomatoes, four soybeans, two squashes, and even a type of radicchio. Hundreds more are in the pipeline, among them plants that will produce industrial and pharmaceutical chemicals (see past issue “The Next Biotech Harvest”).

This rush to market alarms some biologists, who believe transgenic crops are being released before the environmental implications are understood. The most immediate worry is whether genetically engineered crops will spontaneously breed with their wild relatives, creating hybrid “superweeds.” Just as a single Brazilian bee researcher created a continent-wide nuisance by accidentally letting aggressive African bees hybridize with gentle domestic bees, the release of alien genes could, in theory, produce noxious “killer-bee” plants.

Surprisingly little is known about such natural hybridization, explains plant geneticist Norman C. Ellstrand of the University of California at Riverside. Until recently, agricultural scientists focused on protecting farmers; the small amount of hybridization research done in the past primarily concerned the introgression of genes from the wild into cultivated species, rather than the other way around. “People had the idea that [crop-weed hybridization] wasn’t a very common or interesting phenomenon,” Ellstrand says. “But when they finally got around to looking at it, they basically spent a lot of time being surprised at what could happen.”

Initially, scientists thought genes were unlikely to flow from transgenic crops to weeds, because known crop-weed hybrids are often sterile. But last September, Bergelson and two Chicago colleagues startled researchers with a study of Arabidopsis thaliana, a mustard species often used as a test organism by plant geneticists. Usually, the plant pollinates itself, implying to scientists that foreign genes in transgenic A. thaliana would not escape by hybridization. But after the researchers planted ordinary A. thaliana, transgenic herbicide-resistant A. thaliana, and a naturally occurring, herbicide-resistant mutant variety, they learned that the transgenic plants were 20 times more likely to outcross than the mutants-they were “promiscuous,” as a headline in the journal Nature put it. “Nobody knows why,” Bergelson says. “We’re still trying to find the mechanism that drives the pattern we saw. There’s a lot we don’t understand, including how common it is.”

The implications are ominous. A decade ago, for instance, European sugar beets spontaneously mixed with a wild relative, creating a hybrid species that is now a continent-wide problem. Whereas the sugar beet is biannual-the root is harvested at the end of the second year -the new weed is an annual. At the end of the year, Ellstrand says, “the root turns into a chunk of wood that damages farm equipment or gets into the sugar-beet processing plant and screws up the machinery. You can’t kill it with an herbicide because any herbicide that gets the weed hits its relative. It’s not until the thing blooms and flowers that you see it, and by that time it has set seed that gets into the beet field forever.”

Transgenic crops have already shown the potential to create similar problems. The prospect of herbicide- or insect-resistant superweeds is particularly dismaying. In 1995, Monsanto and AgrEvo introduced herbicide-tolerant oilseed rape (Brassica napus), the plant that is the source of canola oil. One year later, an 11-member team from the Scottish Crop Research Institute reported, to scientists’ surprise, that pollen from oilseed rape fields can travel as much as two kilometers. At almost the same time, three Danish geneticists discovered that transgenic Brassica napus readily breeds with a weedy relative, Brassica campestris. The resulting plants look much like B. campestris-but are unaffected by herbicides. Taken together, says Dean Chamberlain of the University of North Carolina at Greensboro, the two reports “showed that hybridization is a real concern and that you need a very large buffer area around your plot to control it.”

When Ellstrand reviewed the literature on the 30 most agriculturally important plant species, most scientists he consulted believed few hybridize easily. In fact, he found evidence that more than 25 of the crops can break the species barrier, sometimes with unrelated species. Included in that list is wheat, which Robert S. Zemetra and his colleagues at the University of Idaho reported in April can outcross with bearded goatgrass, a problem weed in the western United States.

“What really shocks me as a biologist is that you have two species with different numbers of chromosomes hybridizing,” says Allison Snow, a botanist at Ohio State. “Goatgrass has 28 chromosomes and wheat has 42, but they can cross.” Biologists have regarded viable offspring from such mismatches as almost impossible. As a result, they thought the range of species that could hybridize was limited. The goatgrass-wheat hybridization suggests that the range is bigger than had been thought.

“You get very low rates of reproduction,” Snow says. “But when you’re talking about acres and acres of wheat with goatgrass all around them, even a very low probability event can occur.” If hybridization created insect-resistant goatgrass in areas where the weed’s spread is naturally controlled by insects, she says, “that could end up being the only kind of goatgrass you have, and then you might end up with even larger infestations of it than we already have.” Such fears are one reason that insect-resistant Bt crops-which contain genes from the bacterium Bacillus thuringiensis-have been targeted by activists.

In the United States, transgenic corn is unlikely to pose much risk of hybridization because it has no close relatives. But Mexico has teocinte, the wild plant that may be the ancestor of modern corn. What would happen if Mexican farmers planted bioengineered corn? Could the new genes affect the fitness of teocinte, which some agricultural ecologists view as a potential storehouse of valuable genes for future corn breeders? “With the information we have now,” Snow says, “it’s hard to tell when the long-term risks are serious enough to ban certain crops.”

Looming behind the ecologists’ fears is the belief that molecular biologists who work with DNA on the laboratory bench don’t understand fully how it behaves in the field. According to Rosemary S. Hails of the British National Environmental Research Council’s Institute of Virology and Environmental Microbiology, “The risk assessment of transgenic organisms is a multidisciplinary subject, which should include ecologists, molecular biologists, agronomists and sociologists.” Instead, companies tend to delegate decisions about the release of transgenic crops to molecular biologists-who are not trained to appreciate the full complexity of how the genetic code interacts with environmental factors.

“How fast would a new weed get around?” Snow asks. “Nobody really knows. I’m sort of assuming that most of these crops will be approved eventually and people like me will study what the consequences are. Then, after the cat is out of the bag, we may figure out how to regulate this technology.”

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