Columbus, OHIO (10/20/98)- Genetic engineering has already
produced plants with heightened resistance to herbicides and pesticides.
But if these transgenic plants hybridize with local weeds, could a new
superweed be the result?
Transgenic crops, i.e., those that are engineered with specialized traits
like herbicide resistance, could pass their special genes to nearby weeds
via a hybridization process called transgene escape. These transgenic superweeds
would be resistant to the herbicides that were designed to kill them. While
some food-producing plants, including corn, lack close wild relatives,
others like squash and sorghum do have close weed relatives.
"Gene flow from genetically modified crops can and probably will occur.
The risks of transgene escape are real, and they are quantifiable in most
cases. We need to discuss what levels of risk are acceptable for wide scale
release of crops in locations where they may have wild relatives. This
has yet to be the focus of the many discussions on the subject," says Dr.
Paul Arriola, Assistant Professor of Biology at Elmhurst College in Elmhurst,
Illinois, asserts: "
"When a crop grows near its weedy relative, it is inevitable that the
genetically-engineered trait will move into the weed," says Allison Snow,
associate professor of plant biology at Ohio State University.
Dr.
Snow evaluated the potential for gene transfer between a genetically engineered
form of oilseed rape and local weeds. Working in a an environmentally sealed
greenhouse in collaboration with researchers at the Risoe National Laboratory
in Denmark, she hybridizes transgenic oilseed rape, Brassica napus
and one of its wild relatives, Brassica rapa. The offspring reproduced
as prolifically as the original wild species. This suggests 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, she said.
Photos: Upper- Brassica
napus (the crop) Lower
Brassica rapa (the weed)
The gene for herbicide resistance persisted in about half of the new
hybrid weed population, as expected from Mendel's laws of inheritance.
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.
Tansgenic, herbicide-resistant oilseed rape has been grown in the commercial
U.S. market since 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.
Both oilseed rape and its wild 'weedy' relatives were introduced to
the U.S. by settlers from Europe. Indeed, the cultivated and wild versions
often grow side-by-side in the field. This is not 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 is
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," warned 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.
We're trying to project what could happen or will happen in the future.
That is 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.," she said.
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.
Dr. Arriolahas authored several papers on plant genetics and gene escape.
Dr. Snow spoke at the 1998 Ecological Society of America Annual Meeting
in Baltimore.
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