The possibility that genes introduced by genetic engineering may "escape" (be transferred via pollen) to wild or weedy related species growing nearby is often cited as one of the major risks of GMOs. Gene flow between crops and the wild species from which they were derived, however, is a well-documented natural phenomenon. Over the course of evolution, familiar crop species - wheat, potatoes, corn, canola, and numerous others - were modified from their original form because of hybridization with related species or weedy or culti-
vated strains growing nearby. Through this long-established mechanism for gene transfer, any gene in a cultivated crop or plant, irrespective of how it got there, can be transferred to its wild or semi-domesticated relatives.
The real concern is not that such outcrossing will occur—because we know that it does—but rather that negative consequences may result from it. In some cases, serious weeds are relatives to crops (Johnson grass to sorghum, wild mustards to canola, red rice to rice). If a wild plant's fitness is enhanced by a transgene that gave it protection from naturally occurring pests or diseases, would the plant become a worse pest (the "superweed" scenario), or would it shift the ecological balance in a natural plant community? Wild relatives of crops suffer from disease and insect attack, but few studies address whether resistance to pests in wild plants would result in significant ecological problems. Weeds often evolve resistance to disease by natural evolutionary processes. However, in some cases, gene transfer from crops could speed up this process considerably.
Wild races are especially important weeds in direct-seeded rice fields, which are becoming more common in Asia. It has been shown that genes often are naturally transferred between domestic rice and weedy wild races. In commercial fields planted to a genetically engineered herbicide-tolerant (HT) rice cultivar, weedy wild rice could be controlled by applying the herbicide, until the wild rice acquired the HT gene from the cultivar. At that point, the herbicide would become useless. In this case, the wild rice would not become a worse weed as a result of acquiring the HT gene. It would simply be more difficult to control and would nullify the benefit of the engineering effort. Weeds can evolve resistance to some herbicides without gene transfer, but the process takes much longer. For example, herbicides such as glyphosate (Round-
Up™) from Monsanto are difficult for plants to resist with their normally inherited genes. Nonetheless, in Australia decades of intensive use of glyphosate have led to the emergence of resistance in some weed populations.
Two other gene flow concerns deserve mention. First, nontransgenic crop plants may be pollinated by a GM variety growing in an adjacent field. If the GMO is engineered to produce a protein harmful to certain organisms, the protein may be present in the seed and progeny of the non-GMO plants. Conceivably, the gene transfer may escape the notice of those growing the non-GM variety and other organisms may unknowingly be exposed to the harmful protein. Second, gene transfer to diverse organisms (microbes, animals) is not impossible, but the probability of such an event is exceedingly low. It is not normally a major factor in biosafety reviews.
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