Modified Crops Could Lead to "Superweeds," Study Suggests

Ben Harder
for National Geographic News
August 16, 2001
Genetic engineering holds great potential payoffs for farmers and
consumers by making crops resistant to pests, diseases, and even
chemicals used to kill surrounding weeds. But new research raises
concerns that altering crops to withstand such threats may pose new
risks—from none other than the weeds themselves.

The threat comes from the weeds' ability to acquire genes from the neighboring agricultural crops.

Researchers found that when a weed cross-breeds with a farm-cultivated relative and thus acquires new genetic traits—possibly including artificial genes engineered to make the crop hardier—the hybrid weed can pass along those traits to future generations.

"The result may be very hardy, hard-to-kill weeds," said Allison Snow, a plant ecologist at Ohio State University in Columbus who conducted the experiments over the past six years along with two colleagues. They presented their results last week at the annual meeting of the Ecological Society of America in Madison, Wisconsin.

The findings suggest that genetic engineering done with the aim of improving crops—giving them new genetic traits such as resistance to herbicides or pests—could ultimately have unintended and harmful consequences for the crops if weeds acquire the same trait and use it to out-compete the crops.

"Gene movement from crops to their wild relatives is an ongoing process that can be ultimately harmful to crops," said Snow.

Long-Term Legacy

The scientists conducted the experiments at the University of Michigan Biological Station in Pellston. They used two species of radish: one a common edible radish, the other a wild relative. The wild version is a tenacious weed that reproduces more readily than the crop and can take over agricultural fields if not controlled.

The scientists began by cross-breeding the cultivated radishes and the weedy radishes to produce hybrid, weed-like radishes. Then they designed studies to measure and compare the reproductive success and other traits of the hybrid and non-hybrid radishes.

The original crop radishes used to produce the hybrids had not been genetically engineered. But the scientists wanted to monitor the effects of the "borrowed" genes in weed populations over subsequent generations. They did this by selecting several genetic traits in the cultivated radishes, including flower color and fertility, to serve as "markers" indicating the spread of crop genes into the hybrid population.

When the two groups of plants were grown in pots under the same conditions, the non-hybrid radishes had more seeds and reproduced more than the hybrid plants, especially in the first generation of the experiment.

Nonetheless, the scientists found that traits acquired from the crop radishes, such as their flower color, showed up in subsequent generations of hybrids.

"Even though the effects of delayed flowering and reduced fertility inhibited the movement of certain crop traits to later generations, we did find evidence of crop genes in every generation," Snow said.

A second experiment, conducted in field plots, supported these findings and will be published soon.

Need for Caution

The results of the experiments challenge a common belief that hybrids gradually die out over several generations, Snow explained. "There has been an assumption that [crop] genes wouldn't persist in crop-weed hybrids" because hybrids are thought to be less successful at reproducing, she said.

Not so, Snow concluded after reviewing her team's data: Hybrid wild radishes survived in all six generations that were grown since the study began.

Although the genetic traits the scientists monitored were natural and not genetically engineered, the findings nonetheless suggest that artificial improvements introduced into crops through genetic engineering could spread to weeds and become permanent traits of the weed population.

So strengthened, the weeds may pose a serious risk to the long-term health of agricultural crops. The danger exists in a number of crop plants—including canola, rice, sunflower, sorghum, squash, and carrots—that are closely related to weeds with which they compete.

Snow is concerned that the transfer of genes from crops to related weeds could rapidly render many herbicides ineffectual. That situation, she said, would be much like bacterial diseases acquiring resistance to antibiotics.

Because plant hybrids arise in a single generation, however, it could happen much more quickly.

"Modern agriculture is heavily dependent on herbicides," she said, "so people will notice when those don't work anymore."

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