In a brief review article, published in the April, 2001 issue of Plant Physiology and entitled “When Transgenes Wander, Should We Worry? (125:1543-1545), Professor Norman Ellstrand <http://cnas.ucr.edu/~bps/ellstran.html> of UC, Riverside, California, reports that the flow of genes from crops to weeds is more widespread than previously suspected. In this article, the author discusses how the traditional view, that crop-to-wild hybridization occurs relatively rarely, began to be seriously challenged in the last two decades. The author then reviews his own work and that of others which support the view that widespread spontaneous hybridization between crop and wild relatives is the rule rather than the exception.
The author begins by noting that several biologists have independently recognized the problem of transmission of transgenes from genetically modified crops to their wild relatives. He quotes from a paper published in 1985 by two Calgene scientists, that “The sexual transfer of genes to weedy species to create a more persistent weed is probably the greatest environmental risk of planting a new variety of crop species.”
The general view prevailing around 1990 was that unidirectional exchange of genes from a cultivar to its wild relatives is an occasional event even in situations where the plants grow in close proximity. This view was based on the hypothesis that evolutionary processes reproductively isolate cultivated crop plants from their wild relatives.
In order to determine the extent of gene flow from cultivated plants to their wild relatives, Professor Ellstrand and his group carried out a number of experiments, allowing natural hybridization to occur by planting crops and their wild relatives in close proximity. In one of their earlier experiments, Dr. Norman Ellstrand and his group grew cultivated radish, Raphanus sativus in the center of a test plot surrounded by a wild radish population. They scored transfer of a marker gene Lap6, which encodes an allozyme protein, from the crop to the wild population. This gene is dominant in the cultivated radish and recessive in its wild counterpart. The presence and absence of Lap6 indicated to what extent Lap6 has been transmitted to the progeny of the weed population. Using this criterion, they found that every individual at 1-meter distance was a hybrid containing the allozyme allele. At 1-Km distance, the number of hybrid progeny was small, but not zero. When they compared the fitness of the hybrid population, they measured a 15% seed yield increase compared to that of the wild plants. They extended their studies to two sorghum species, S. bicolorrepresenting the cultivar and S. halepense, the pernicious Johnsongrass weed. With the exception of hybrid vigor exhibited by intraspecific hybrids in the radish system, the flow of genes from the cultivated sorghum to the wild species was of the same magnitude. These data – as well as those from other research groups reviewed by Ellstrand elsewhere — indicated clearly that the flow of genes from the crop cultivars to the wild population is a general feature of most cultivated crop plants from raspberries to mushrooms.
Dr. Ellstrand came across a number of instances where hybridization with wild relatives was implicated in the evolution of a number of aggressive weeds. One such instance is the evolution of a highly destructive sugar beet weed, resulting from natural hybridization between sea beet (Beta vulgaris ssp. maritima) and sugar beet (B. vulgaris ssp. vulgaris). The author also describes a well documented instance in which a wild species of rice became extinct, following natural hybridization between a cultivar rice and its rare relative. It is pointed out that the performance of transgenic crops is not likely to be different from those improved through traditional methods.
One of the reasons, is that a fitness boost occurs due to the expression of the newly acquired gene, e.g. such as infiltration of herbicide- or pest resistance genes into the weed. The risk of harmful gene flow at the crop-to-crop level is also noted. An example of wild rape is cited, in which instances have been reported where the weed has become resistant to three herbicides (Roundup, Liberty and Pursuit) in separate events.
The author concludes that the current practice of releasing genetically modified crops into the environment without containment of the introduced genes to just the target crop is inherently environmentally risky. To minimize this risk, he cautions molecular breeders to be very mindful of the possible negative environmental, and economic impacts of their products, taking into consideration the advanced knowledge from ecology and population genetics as well as social sciences and humanities. Certainly in the absence of failsafe mechanisms for genetic recall, the justification for creation and release of novel agricultural transgenics must continue to be weighed most carefully.