A number of studies performed in the last ten or so years have found that commercial plants exchange genetic material with wild relatives in their vicinity with considerable regularity. But now one of the first studies of what actually happens to a transgene when it moves from a genetically modified cultivar into a wild population indicates that such transfers need not have a major environmental impact.
Results of the field study, which was conducted by plant scientists at Vanderbilt University and Indiana University, are reported in the May 23 issue of the journal Science. The subject of the study was a transgene that can provide commercial sunflowers with additional protection against a disease called white mold, which is caused by a pathogen named Sclerotinia sclerotiorum.
The experiment found that wild sunflowers already possess a degree of resistance to white mold that the commercial variety lacks. As a result, wild sunflowers that pick up the transgene do not gain a reproductive advantage that would cause them to spread widely. Although subject to some significant caveats, the finding suggests that this particular transgene is unlikely to spread throughout the wild sunflower population, making wild varieties hardier and more aggressive.
White mold is one of the worst diseases afflicting commercial sunflowers. Sunflower is one of the world's four most important oilseed crops, with a value of $40 billion per year. White mold infection, which causes the rapid wilting and death of cultivated sunflower plants, is the source of economic losses that range from $50 million to $80 million annually.
Efforts of sunflower breeders to improve the resistance of their plants to this disease using traditional techniques have been unsuccessful and the application of chemical fungicides is costly and often ineffective. The situation led Pioneer Hi-Bred International – a subsidiary of E.I. du Pont de Nemours and Company – to genetically modify one of their varieties of sunflower by inserting a gene isolated from wheat. Much of the damage that white mold inflicts on sunflowers is caused by its production of an organic acid called oxalic acid. The wheat gene, called OxOx, allows the sunflowers to produce a compound called oxalate oxidase that breaks down oxalic acid.
Pioneer Hi-Bred then funded a study to help determine the likelihood that the wheat gene would spread into the surrounding wild sunflower community, which was carried out by John Burke, assistant professor of biological sciences at Vanderbilt, and Loren Rieseberg, professor of biology at Indiana University.
In the first stage in the study, the researchers surveyed the areas in the United States where commercial sunflowers are grown looking both for the incidence of white mold infections among wild sunflowers and for the points of contact between wild and commercial varieties. It took two years and involved driving more than 8,000 miles throughout the Midwest and Southwest.
Despite its prevalence among cultivated sunflowers, the researchers found surprisingly few wild sunflowers suffering from white mold infection.
The survey also found wild sunflowers growing next to fields of cultivated sunflowers almost everywhere that they looked. They also confirmed the conclusion of previous studies that cultivated sunflowers hybridize (exchange genetic material) with their wild relatives when they come in contact.
"We showed that contact between cultivated and wild sunflowers occurs throughout the entire range of cultivation, making gene escape inevitable," says Burke.
The close relationship between cultivated and wild sunflowers means that the risk that transgenes will jump from commercial plants to their wild cousins is particularly high. But there is another factor that governs whether these genes will spread widely throughout the wild population. If the transgene confers a reproductive advantage to the wild plants that obtain it, then it will spread. If it does not provide such an advantage, however, then it won't spread and so shouldn't become an environmental problem.
"It's time to move beyond all the hand-wringing about whether and how often transgenes are going to escape and start attacking the real root of the problem, which is what impact will specific transgenes have if they get out," Burke says.
In order to address the question of the likely impact of OxOx transgene escape, Burke and Rieseberg simulated the early stages of gene escape by "backcrossing" the gene into wild sunflowers. They grew the resulting plants – roughly half with the transgene – in containment cages at three sites, one in Indiana, one in North Dakota and one in California. Just before the plants flowered, they inoculated half of them at each location with Sclerotinia sclerotiorum.
"Faced with such a severe pathogen challenge, we expected the plants with the OxOx gene to have quite an advantage," says Burke. "Somewhat surprisingly, that was not what we found."
The researchers discovered that although the transgene did provide some protection against becoming infected, the transgenic plants did not produce any more seeds than those without the OxOx gene. As a result, the gene does not appear to confer any reproductive benefit even under the unusually severe exposure to the pathogen that the researchers imposed.
When taken with the survey results that found little evidence of white mold infection among wild sunflower populations, Burke and Rieseberg conclude that it appears that wild sunflowers already possess some level of resistance to white mold that their commercial cousins lack. "It looks like we are giving the wild sunflower a degree of resistance to white mold that it already has, so it isn't a big advantage," says Burke.
Because wild sunflowers with the transgene don't look as if they will reproduce at a faster rate than those without it, the scientists have concluded that the OxOx gene appears unlikely to spread widely through wild populations and so does not represent a significant environmental threat.
There are some important caveats to their conclusion, however. The study was performed in a single season. There could be multi-year cycles of white mold infection that would give the transgenic sunflowers a selective advantage that is not reflected in the study. Also, the experiment did not examine the effects of environmental stress, like drought. Stress could work either way: It could enhance the advantage of the transgenic plants or penalize them in ways that would restrict their spread. Finally, the study only looked at three locations. There was enough variation among the locales to suggest that there could be some circumstances under which wild, transgenic sunflowers might do better than indicated. The researchers recommend that these questions be addressed in future studies.
"I believe there is a middle ground between dismissing genetic modification of crops entirely or introducing them without appropriate scientific study," says Burke. "That is doing the research necessary to provide an informed judgment of the relative risks and benefits of genetic modifications on a case by case basis."
For more news about Vanderbilt research, visit Exploration, Vanderbilt's online research magazine at www.exploration.vanderbilt.edu.