Genetically-Altered Crops Can Produce Tough, Hard-To-Kill Weeds

Scientists have known for years that transgenic crops -- those that are engineered with specialized traits like herbicide resistance -- can pass their traits on to nearby weeds via hybridization. These hybrid, transgenic weeds resist the herbicides that were designed to kill them.

“When a crop grows near its weedy relative, it’s inevitable that the genetically-engineered trait will move into the weed,” said Allison Snow, associate professor of plant biology at Ohio State.

Still, scientists hypothesized that hybridization might cause some negative characteristics to emerge in a weed that would limit its reproduction. For instance, a hybrid weed might produce fewer flowers or seeds than a pure weed.

Snow collaborated with Risoe National Laboratory in Denmark to find out whether this was the case for oilseed rape, the plant from which canola oil is derived. The study showed that the offspring of herbicide-resistant hybrids between transgenic oilseed rape and one of its weedy relatives reproduced as prolifically as unaltered weeds.

The study suggests that, at least in the case of oilseed rape, weeds that cross with commercial crops and acquire a specialized transgene will encounter few obstacles to prosperity in the field. The researchers presented their results August 6 in Baltimore at the 1998 Ecological Society of America Annual Meeting.

Snow and her colleagues crossed a transgenic, herbicide-resistant version of oilseed rape, or Brassica napus, with its weedy cousin, Brassica rapa, and cultivated their progeny in indoor growth chambers in Denmark. The researchers wanted to see how the transgenic weeds would fare compared to unaltered weeds that weren’t hampered by insects, disease, and herbicide as they would be in the field.

Snow said that even when the unaltered weeds were given the advantage of an ideal growing environment, they didn’t on average produce more fruits or seeds than the hybrid weeds.

The researchers crossed the hybrid plants with unaltered weeds, and saw that the gene for herbicide resistance persisted in about half of the weed population, as expected from Mendel’s laws of inheritance. What did change was the hybrid weeds’ appearance.

“By the third generation, the weeds that carried the gene for herbicide resistance looked exactly like normal weeds. The only way to tell them apart was to expose them to herbicide or test their DNA,” said Snow.

She added that the only way a farmer would be able to tell the difference would be to spray all the weeds with herbicide and see which ones survive -- but that would just give the herbicide-resistant weeds more room to grow.

Cultivated crop plants are generally less hearty than weeds -- they don’t produce as many branches or seeds, and they’re more vulnerable to bad weather, pests, and poor soil. That’s why scientists hoped that crop/weed hybrids wouldn’t be able to compete with their heartier pure-weed relatives. This latest study shows that hybrid weeds may carry the beneficial transgene of the crop parent and still hang on to the aggressiveness of the weed parent.

These studies picked up speed when transgenic, herbicide-resistant oilseed rape hit the commercial U.S. market in 1993. Since then, oilseed rape production has been on the rise. In 1997, American farmers produced six times more canola oil than they did just five years before, according to the United States Department of Agriculture. That year, the U.S. produced over $100 million worth of canola -- a popular cooking oil these days because it contains the lowest amount of saturated fat of any food oil.

Manufacturers use canola as an ingredient in soap, margarine, and lamp fuel. A refined version of the oil is used as a lubricant. The seed residue after oil extraction provides animal feed.

Because both oilseed rape and its weedy relatives were introduced to the U.S. by settlers from Europe, wild versions of the species often grow side-by-side with the cultivated plant. This isn’t a problem for corn, soybeans, potatoes, and tomatoes, none of which have weedy relatives in the U.S. Sunflower and squash plants, on the other hand, are both native born, so they naturally have genetically-compatible weeds growing nearby. That’s why genes from cultivated oilseed rape, sunflowers, and squash can escape from crop plants into the weed population.

In fact, previous studies have shown that oilseed rape pollen can reach weeds nearly one mile away.

“If farmers spray their crops with the same herbicide every year, the only weeds to survive will be the ones with the transgenes -- and then the transgenes will spread even faster,” said Snow.

As transgenes in the cultivated crop change, traits could accumulate in the weeds. For instance, a weed could develop a resistance to 3 or 4 herbicides as it acquired genes from consecutive generations of crops over many years.

“It’s hard to worry about a problem that may take 5 or 10 years to develop,” admitted Snow. “We’re trying to project what could happen or will happen in the future.”

“That’s why the area of crop transgenes is so controversial,” she continued. “Some people don’t even want to think about it, and other people think it’s a disaster. I think the truth lies somewhere in-between.”

Snow said that she and her colleagues would have to study oilseed rape and Brassica rapa in the field to fully gauge the benefits of herbicide resistance in the weed.

“I don’t like to paint this as an emergency, or as a disaster waiting to happen. I just think it’s important to understand what we’re doing when we create transgenic crops, and delay the possible negative consequences, if not prevent them in the first place,” said Snow.

One solution might be for scientists to insert genetically-engineered traits into the DNA of the cytoplasm of plant cells, instead of the nucleus. The DNA from the nucleus gets into every pollen grain of a plant, and will travel far and wide on the wind, or hitch a ride with pollinators such as bees. But a gene in the cytoplasm could only be inherited through the seeds of a particular plant.

In the future, Snow wants to study transgenes for insect-resistance. “We want to quantify how much of a benefit there is to a weed that acquires these transgenes,” she said. “It’s a very basic ecological question, but nobody knows the answer.”