When genes escape: does it matter to crops and weeds?
Judging by the questions they ask and the eyebrows they arch, the folks at the Botany 2003 meeting in Mobile, Ala., in late July hold a range of attitudes about genetically engineered crops. Yet just about everyone laughs with Rodemeyer.
The discussion of gene flow has changed in the past decade. The question is no longer, Can genes move? By now, scientists have tested some of the basic scenarios and reported their observations. The current consensus is that genes certainly can flow, says Allison Snow of Ohio State University in Columbus. Her tests and others' have shown that much. "The important question now is, What are the consequences?'" she says.
Researchers are starting to examine that question. The answers may strongly influence the future of genetic engineering in agriculture.
ROOTS When bioengineers first inserted foreign genes, or transgenes, into plants in the 1980s, the scientists generally expected crop-to-wild hybridizations to be only "rare and idiosyncratic," says Norman Ellstrand of the University of California, Riverside. However, interest in how cultivated plants consort with wildlings had started long before genetic engineering was even a glimmer in a test tube.
Ellstrand, a dedicated investigator of gene-flow questions, points out a 1886 treatise on domesticated plants that mentions their capacity for mating with wild relatives. Even the term superweed goes back at least to 1949, in a book on hybridizing plant species. The author raised the possibility that a traditional farm plant's wild side pairings might yield especially tough but undesirable offspring.
In a few cases, scientists have traced a trait moving from a conventional crop into the wild. For example, Ellstrand notes a 1959 report of a brainstorm that fizzled in India. Agriculturists encouraged farmers to plant a rice variety with red seedlings, easy to distinguish from a pale, weedy form that farmers had been removing from their paddies. The venture failed when the red color quickly migrated into the weed.
Scientists continue to examine conventional crops to gain insight into what genetic engineering might yield. For example, in 1998, Randal Linder of the University of Texas at Austin and his colleagues, including Snow, reported their study of wild sunflower patches that had grown near farmed sunflowers for up to 40 years. All of the 115 wild plants that the researchers tested carried at least one genetic marker characteristic of the commercial plants.
Tracking a rare genetic marker from a conventional alfalfa crop, Paul St. Amand of Kansas State University in Manhattan and his colleagues have documented the gene in stray plants outside farm fields. In some cases, the gene turned up as far as 230 meters away. "Data suggest that complete containment of transgenes within alfalfa-seed--or hay-production fields would be highly unlikely using current production practices," the researchers commented in their 2000 paper.
Ellstrand has built the case that opportunities abound for crossings of crops and weeds, in 1999, he reviewed the world's top 13 crops for human consumption (ranked by area harvested) and found reports that 12 crops hybridize with a wild relative somewhere in their range. Wheat, for example, has given rise to at least 21 natural hybrids, and certain crop-weed crosses of rice have yielded unusually fertile offspring. The exception was peanut plants, which typically self-fertilize.
Less-prominent crops, too, often mate with their wild relatives, Ellstrand says. He's added 31 plants, including grapes, avocados, lettuce, coffee, chocolate, and watermelons, to his list of crops that in some part of the world have hybridized with a wild mate.
LOOSE GENES The movement of genes from engineered plants has triggered more concern than gene flow from conventional crops ever did. Genetic engineering enables scientists to transplant a much wider range of genes than is available through traditional breeding.
Some experiments have observed neighboring barley picking up genes introduced into crops by genetic engineering. A marker from transgenic barley, for instance, traveled to up to 7 percent of conventional barley plants nearby that don't produce competing pollen. However, rogue pollination dwindled rapidly in frequency the farther researchers got from the source plants. Anneli Ritala of VTT Biotechnology in Espoo, Finland, reported in the January-February 2002 Crop Science.
Perhaps the most famous studies of transgene escapes aren't intentional experiments at all. For example, Mexicans are watching their traditional maize versions, or landraces, to see whether they'll pick up genes from the abundant U.S. crops of transgenic corn. Mexico itself has banned the growing of transgenic corn.
The ancestral home of corn lies in Mexico, where rich variety in the old landraces persists. Even today, the original lineage of crop corn survives in a lanky grass called teosinte, which has tiny stubs of seeds that only a botanist could love.
In 2001, California biologists reported traces of transgenes in landraces (SN: 12/1/01, p. 342). Other researchers challenged some of the findings as artifacts of the genetic techniques, and Nature eventually took the unusual step of saying there hadn't been enough evidence to justify its publishing the paper (SN: 4/13/02, p. 237).
Now, other labs have found signs of transgenes in maize landraces in Mexico. Sol Ortiz-Garcia of the Ministry of Environmental and Natural Resources in Mexico City described the findings of two research teams at the July botany meeting. Farmers who bought U.S. corn as animal feed may have tried growing some of it, or the feed corn may have sprouted spontaneously.
The teams are gathering further data to confirm the presence of the transgenes, but Snow says, "I believe it."
Canadian scientists have described transgene movement from a different crop. Farmers grow canola for the oil in its seeds, and controlling weeds in the fields had ranked high among canola-grower headaches. Starting in 1996, strains genetically engineered to withstand treatment by one of two herbicides have become popular in Canada. These strains could then be doused with pesticide powerful enough to wipe out troublesome weeds. About 70 percent of the country's crop carry a transgene to aid in weed control.
Those transgenic plants are hybridizing with Brassica rapa, one of the weedy parents of crop canola, according to Suzanne Warwick at Agriculture Canada in Ottawa. She and her colleagues documented the first crop-wild hybrid from a regular commercial field in the August 2003 Theoretical and Applied Genetics.
Transgenes also move from one type of crop canola to another. A canola field planted with one variety sprouted hybrid volunteers that combined the herbicide resistances of their parents, Linda Hall of Agriculture Canada in Edmonton, Alberta, and her colleagues reported in 2001.
The canola-transgene movement can complicate life for farmers, Hall says. Canola seeds that stay in the ground after the farmer has rotated crops can pop up as weeds in a wheat or barley field. If those volunteers have picked up unexpected herbicide resistance, the farmer's herbicide regimen may be insufficient.
BOTTOM LINE Is the canola-gene flow a lot or a little? It doesn't matter, says John Burke, now at Vanderbilt University in Nashville. In 2001, he and Loren Rieseberg of Indiana University in Bloomington published an analysis of what it takes for a new form of a gene to get established if it moves into a weed or other species. They reported that the rate at which a gene migrates makes little, difference, compared with whether it helps the plant survive and reproduce.
According to Burke and Rieseberg, if a transferred gene super-charges a plant into leaving more offspring, the gene will spread. "If it's disadvantageous or neutral, it won't do much, no matter how high the rate of gene flow," he says.
Some scientists looking for benefits to plants that receive stray transgenes have studied crops instead of weeds. They pitted the engineered version of a crop against its old-fashioned counterpart in a survival marathon. In the first test of survival advantages conferred by a transgene in a natural setting, Mick J. Crawley of Imperial College in Berkshire, England, and his colleagues chose 12 habitats. In each, they planted adjacent patches of transgenic and traditional versions of several crops: rape, maize, beets, and potatoes. The researchers then left the plants to fend for themselves.
After monitoring the experiment for 10 years, Crawley and his team reported in 2001 that none of the transgenic-plant populations had lasted significantly longer than the conventional ones did, and none of the patches had gained ground.
The experiment made the transgenics look pretty tame. Yet Crawley cautions that the crops his team examined had been engineered to resist herbicides, moth and butterfly caterpillars, and perhaps those qualities didn't matter much in the wild. Transgenes that confer different advantages, such as tolerance to drought or to other pests, might make more of difference.
Snow and Burke are approaching the problem by inserting transgenes into wild relatives of commercial plants. They both used wild sunflowers but studied different genes and got different results.
Snow and her colleagues began with wild sunflowers engineered to make the Bt pesticide, a toxin named for the Bacillus thuringiensis bacterium, in which the gene originates. The researchers used traditional breeding methods to move the gene into the wild sunflowers, which they planted in contained fields.
The souped-up wildlings set 50 percent more seeds than the regular wild ones did. "We were surprised," says Snow. Her team's results appeared in the April Ecological Applications.
In a series of studies, Burke and his colleagues are tracking a genetic construct called OxOx, which fortifies commercial sunflowers against white mold. The pathogen's abundant oxalic acid, or oxalate, breaks down plant tissue, and the transgene OxOx encodes the amino acid sequence for oxalate oxidase. The transgene is "making an antacid," Burke explains.
Earlier work showed that about two-thirds of all commercial U.S. sunflower fields lie near wild sunflowers that bloom at the same time. He calls gene flow between commercial and wild sunflowers "a virtual certainty." To mimic this potential spread, Burke and Rieseberg bred OxOx into a wild species and planted the enhanced offspring in cages in California, Indiana, and North Dakota.
The researchers exposed all the offspring plus unenhanced wild plants to white mold. Burke says that the gene gave different levels of protection from mold in the different states.
In none of the three states, however, did the genetically enhanced plants set significantly more seeds than the wild ones did, the researchers reported in the May 23 Science. If results from more years confirm these findings, the gene probably won't create aggressive weeds, they conclude.
Burke says his work "provides a nice counterpoint" to the study on the Bt gene. The disparity in outcomes, he says, emphasizes that for transgenes, "we need to be assessing the risks and benefits on a case-by-case basis."
SUN SPOTS--Sunflowers make an intriguing test case for studying gene flow, since North America has both thriving sunflower farms and abundant wild relatives.
GOLDEN SPREAD--A field of canola such as this one pollinates about 1 percent of like plants just outside its border. Transgenes from herbicide-resistant canolas in Canada have moved from one variety to another.