TUCSON, Ariz. -- For the first time, researchers have shown that a gene artificially inserted into crop plants to fend off pests can migrate to weeds in a natural environment and make the weeds stronger.
Scientists studied genetically engineered sunflowers -- those modified with a gene that produces a chemical toxic to certain insects -- to see what happened when these foreign genes, called transgenes, were inadvertently passed along to weedy relatives.
"This is the first example of what might happen if a beneficial transgene accidentally spread to a wild population and then proliferated in subsequent generations," said Allison Snow, a study co-author and a professor of biology at Ohio State University.
"Many crops can exchange genes with nearby wild relatives," she said. "But few commercially grown crops in this country are genetically engineered. Of those that are, only canola and squash could cross with weeds."
In the current study, the resulting hybrid sunflowers that contained the transgene had 50 percent more seeds than control hybrids without the gene. These plants also had far less insect damage, suggesting that that the insecticidal gene was working by preventing insects from eating the plant.
"We were surprised that a single transgene could have such a big effect on seed production," Snow said. The researchers also found that the addition of this gene didn't harm the weeds' physical fitness, even when the sunflowers were deprived of water and nutrients.
"A plant with a transgene may have to divert more energy to handle this new compound it's making," Snow said. "Doing so could reduce the plant's ability to reproduce. But that certainly wasn't the case here."
Snow conducted the study with researchers from the University of Nebraska and Indiana University. The team presented their findings August 8 at the annual Ecological Society of America meeting in Tucson.
The researchers crossbred cultivated sunflowers that contained a Bt transgene -- a gene taken from the soil-dwelling bacterium Bacillus thuringiensis that produces chemicals toxic to certain insects -- with wild, non-Bt sunflowers. In this case, the Bt toxin repelled moths and butterflies, whose larvae are prime sunflower predators.
The scientists focused on the second generation of wild sunflowers that contained the transgene. The experimental populations were grown at two sites -- an open, pasture-like area in Nebraska and an intensely cultivated area in Colorado. The plants in Nebraska benefited more from the Bt transgene than those in Colorado, most likely due to differences in insect pressure, Snow said.
If a wild relative grows near a crop plant, chances are good that the two will crossbreed. Such unions happen in more than 20 species in the United States, including sunflowers, sorghum, carrots, radishes, rice and turf grasses. But these crops don't yet contain insect-fighting transgenes in their DNA.
It's the plants that do contain transgenes that concern researchers.
"Many genetically-modified cultivated crops could potentially crossbreed with weeds," Snow said. "Weeds are already hardy plants; the addition of transgenes could just make them tougher.
"While it's obvious that a single gene can have a huge impact on plant reproduction in a natural setting, there are still a lot of unknown effects, like whether or not the weed could spread at a faster rate."
Adding a transgene to a plant's DNA could potentially weaken that plant's ability to reproduce.
To test the effect of the transgene on the Bt sunflowers' physical fitness, the researchers used three separate populations of wild transgenic sunflowers in a greenhouse. One group was subject to drought; a second group was deprived of nutrients; and a third group, which received adequate water and nutrients, served as a control.
The transgene didn't pose any threat to any of the sunflower groups' fertility or growth. "There were no costs at all to inheriting the transgene," Snow said.
In order to keep the sunflowers from spreading outside of the area of the field experiments, the researchers removed all of the non-sterile plants that carried the Bt gene.
The researchers also collected seed heads from all of the remaining Bt sunflowers before the seeds had a chance to fall to the ground. During the two years following the study, the experimental plots and surrounding areas were sprayed with herbicides meant to kill wild sunflower seedlings.
Although the researchers let the sunflowers grow in natural conditions for this study, Bt sunflowers aren't currently sold in the United States. "Their commercialization depends on securing approval from the U.S. Department of Agriculture to grow Bt sunflowers, and if seed companies want to market the plants," Snow said.
She and her colleagues are continuing their research and trying to understand if wild sunflowers with beneficial transgenes could become troublesome weeds.
Snow conducted the study with Michael Reagon, a doctoral student at Ohio State; Diana Pilson, a professor of biology, and Matthew Paulsen and Nick Pleskac, all of the University of Nebraska; and Loren Rieseberg, a professor of biology, and Diana Wolfe, both of Indiana University, Bloomington.
The research was supported by the U.S. Department of Agriculture and seed manufacturers Pioneer Hi-Bred and Mycogen Seeds.
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