阅读理解
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. This is due to 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(除莠剂)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.
The results of the experiments challenge a common belief that hybrids (杂种)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. However, Snow''s research contradicted this assumption: 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 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 (chemicals which kill weeds) 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."
单选题
The word "this" in Line 4, Para. I refers to ______.