7 August 1998

Super Crops Could Create Super Weeds

The debate over transgenic crops is likely to intensify after the results of a study into weed hybridization were presented at the 1998 Ecological Society of America annual meeting in Baltimore. The study, focusing on herbicide resistant oilseed rape, from which canola oil is derived, showed that hybrid weeds may carry the beneficial transgene of the crop parent and still hang on to the aggressiveness of the weed parent.

It has been known for some time that that transgenic crops can pass their traits on to nearby weeds via hybridization. It was believed 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. This assumption may no longer be correct.

Allison Snow, from Ohio State University, and researchers from the Risoe National Laboratory in Denmark, 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.

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.

Because both oilseed rape and its weedy relatives were introduced to the US 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 US. 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.

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 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."