The Environmental impact of Genetically Modified Organisms (GMOs)

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/228050297 Environmental Impact of Genetically Modified Organisms (GMOs) Chapter · September 2009 DOI: 10.1002/9780470015902.a0003255.pub2 CITATION READS 1 322 1 author: Rosemary S Hails Centre for Ecology & Hydrology 151 PUBLICATIONS 4,395 CITATIONS SEE PROFILE All content following this page was uploaded by Rosemary S Hails on 03 February 2015. The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the original document and are linked to publications on ResearchGate, letting you access and read them immediately. Impact of Genetically Modified Organisms Secondary article Article Contents . Introduction Maurizio G Paoletti, University of Padua, Padua, Italy . Benefits of Genetic Engineering in Pest Control . Disease Resistance in Crops Genetically engineered crops could prove environmentally and economically beneficial; however, current products – especially herbicide-resistant plants and Bacillus thuringiensisresistant crops – have potentially serious environmental and economic impacts, similar to the consequences of pesticide use. . Herbicide-resistant Crops (HRCs) . Bacillus thuringiensis for Insect Control . Single-gene Changes and Increased Pathogenicity . Threats from Modified Native Species . Discussion Introduction Despite the application of 2.5 million tonnes of pesticides worldwide, more than 40% of all potential food production is lost to insect, weed and plant pathogen pests prior to harvest. After harvest, an additional 20% of food is lost to another group of pests. The use of pesticides for pest control results in an estimated 26 million human poisonings annually worldwide. In the United States, the environmental and public health costs for the recommended use of pesticides total approximately $9 billion per year. Thus, there is a need for alternative nonchemical pest controls that genetic engineering might help provide. Disease and insect pest resistance to various pests has been slowly bred into crops for the past 12 000 years; current techniques in biotechnology now offer opportunities to further and more rapidly improve the nonchemical control of disease and insect pests of crops. However, relying on a single factor, such as the Bacillus thuringiensis toxin that has been inserted into corn, potato and a few other crops for insect control, leads to various environmental problems, including insect resistance and a serious threat to beneficial biological control insects and endangered species. A major environmental and economic cost associated with genetic engineering applications in agriculture relates to the use of herbicide-resistant crops (HRC). In general, HRC technology results in increased herbicide use, pollution of the environment, and weed control costs for farmers that may be 2-fold greater than standard weed control costs. Therefore, pest control with both pesticides and genetic engineering methods needs to be improved for effective, safe, economical pest control taking into account integrated and biological alternatives stemmed from agroecology. However, insufficient public and private funds are being spent to assess the effect of engineered plants on the environment compared with the amount devoted to developing transgenic crops. In addition, there needs to be greater focus on environmentally sustainable and socially acceptable research on transgenic organisms, with clear priorities toward reducing pesticides in the environment as well as soil erosion and depletion. Benefits of Genetic Engineering in Pest Control Since 1987, many crops have been genetically modified for features such as resistance to insects, resistance to pathogens (including viruses) and herbicides, and for improved features such as longer-lasting ripening, higher nutritional status, protein content, seedless fruit, and sweetness. Up to 34 new genetically engineered crops have been approved for entry into the market (Hammond and Fuchs, 1997). Since 1986, more than 2000 field trials have led to the release of transgenic plants around the world (Krattinger and Rosemarin, 1994; Paoletti and Pimentel, 1996). In 1998 and 1999, 27.8 and 39.9 million hectares of engineered crops were planted in countries such as the United States, Argentina, Canada and Australia. The USA alone contains 74% of the modified cropland planted. Worldwide, 19.8% of the acreage planted with genetically modified organisms (GMOs) has been planted with herbicidetolerant crops, 7.7% with insect-resistant crops, and 0.3% with insect- and herbicide-resistant crops. Five crops – soybean, corn, cotton, canola and potato – cover the largest acreage of engineered crops (James, 1999; Moff, 1998). Disease Resistance in Crops The crops currently on the market that have been engineered for resistance to plant pathogens are listed in Table 1. Disease-resistant engineered crops have some potential advantages because few current pesticides can control bacterial and viral diseases of crops. In addition, these engineered plants help reduce problems from pesticides. The large-scale cultivation of plants expressing viral and bacterial genes might lead to adverse ecological consequences. The most significant risk is the potential for gene transfer of disease resistance from cultivated crops to weed ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net 1 Impact of Genetically Modified Organisms Table 1 Plants genetically engineered for virus resistance that have been approved for field tests in the United States from 1987 to July 1995 (Krimsky and Wrubel, 1996) Crop Disease(s) Research organization Alfalfa Alfalfa mosaic virus, Tobacco mosaic virus (TMV), Cucumber mosaic virus (CMV) Barley yellow dwarf virus (BYDV) Beet necrotic yellow vein virus CMV, Papaya ringspot virus (PRV) Zucchini yellow mosaic virus (ZYMV), Watermelon mosaic virus II (WMVII) CMV ZYMV ZYMV Soybean mosaic virus (SMV) SMV, CMV Maize dwarf mosaic virus (MDMV) Maize chlorotic mottle virus (MCMV), Maize chlorotic dwarf virus (MCDV) MDMV MDMV MDMV CMV Tomato spotted wilt virus (TSWV) PRV TSWV PRV Plum pox virus Potato leaf roll virus (PLRV), Potato virus X (PVX), Potato virus Y (PVY) PLRV, PVY, late blight of potatoes PLRV PLRV, PRY PLRV, PVY PYV SMV Tobacco etch virus (TEV), Tobacco vein mottling virus TEV, PVY TEV, PVY TMV TEV TMV, Tomato mosaic virus (ToMV) CMV, Tomato yellow leafcurl virus TMV, ToMV ToMV CMV CMV CMV CMV CMV Pioneer Hi-Bred Barley Beets Cantaloupe and/ or squash Corn Cucumbers Lettuce Papayas Peanuts Plum trees Potatoes Soybeans Tobacco Tomatoes 2 USDA Betaseed Upjohn Harris Moran Seed Michigan State University Rogers NK Seed Cornell University New York State Experiment Station Pioneer Hi-Bred Northup King DeKalb Rogers NK Seed New York State Experiment Station Upjohn University of Hawaii Agracetus USDA Monsanto Frito-Lay Calgene University of Idaho USDA Oregon State University Pioneer Hi-Bred University of Florida North Carolina State University Oklahoma State University USDA Monsanto Upjohn Rogers NK Seed PetoSeed Asgrow Harris Moran Seeds New York State Experiment Station USDA ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net Impact of Genetically Modified Organisms relatives. For example, it has been postulated that a virusresistant squash could transfer its newly acquired virusresistance genes to wild squash (Cucurbita pepo), which is native to the southern USA. If the virus-resistance genes spread, newly disease-resistant weed squash could become a hardier, more abundant weed. Moreover, because the USA is the origin for squash, changes in the genetic makeup of wild squash could conceivably lessen its value to squash breeders. The US Department of Agriculture argues that viruses do not appear to infect wild squash, basing this conclusion largely on a survey of only 14 wild squash plants in which no viral infection was detected. Sampling 14 wild squash plants is insufficient evidence. Another area of concern is the production of virusresistant sugar beets, where a similar exchange of genes between cultivated and weed populations of beet (Beta vulgaris L.) is likely, since production areas containing wild and/or weed beet populations are separated by only a few kilometres. Genetic exchange could take place by wind pollination, biotic pollination, or the common gynoniecy of wild beets (Boutin et al., 1987; Cuguen et al., 1994). A genetic introgression from seed beet to weed beet populations has already been observed in Europe (Santoni and Berville, 1992). Some plant pathologists have also suggested that development of virus-resistant crops could allow viruses to infect new hosts through transencapsidation. This may be especially important for certain viruses, e.g. luteoviruses, where possible heterologous encapsidation of other viral RNAs with the expressed coat protein is known to occur naturally. With other viruses, such as the Papaya ringspot virus (PRV) that infects papaya, the risk of heteroencapsidation is thought to be minimal because the papaya crop itself is infected by very few viruses (Gonsalves et al., 1994). Virus-resistant crops may also lead to the creation of new viruses through an exchange of genetic material or recombination between RNA virus genomes. Recombination between RNA virus genomes requires infection of the same host cell with two or more viruses. Several authors have pointed out that recombination could also occur in genetically engineered plants expressing viral sequences of infection with a single virus, and that large-scale cultivation of such crops could lead to increased possibilities of combinations (Hull, 1990; Palukaitis, 1991; de Zoeten, 1991; Tepfer, 1993). It has recently been shown that RNA transcribed from a transgene can recombine with an infecting virus to produce highly virulent new viruses (Greene and Allison, 1994). A strategy for reduced risk would include: (1) identification of potential hazards; (2) determination of frequency of recombination between homologous, but nonidentical sequences in crops and weeds; and (3) determination of whether or not such recombinants can have selective advantage (Tepfer et al., 1994). Fernandez-Cuartero et al. (1994) have demonstrated that even though a particular pseudorecombinant strain was at a competitive disadvantage relative to a parent Cucumber mosaic virus (CMV) strain, a spontaneous recombinant that arose from the pseudorecombinant (resulting from pseudorecombination, or the situation in which gene components of one virus are exchanged with the proteins of another coat) showed enhanced fitness relative to either of the other original strains. Assessment of transgenic virus-resistant potatoes in Mexico An in-depth assessment of potential socioeconomic implications related to the introduction of some genetically modified varieties of virus-resistant potatoes (Potato Virus Y (PVY), Potato Virus X (PVX), Potato Leaf Roll Virus (PLRV)) in Mexico underscores the importance of this technology. This type of genetic modification could prove especially beneficial to large-scale farmers, but only marginally beneficial to small-scale farmers, because most small farmers use red potato varieties that are not considered suitable for transformation. In addition, 77% of the seeds that small farmers use come from informal sources, not from the seed providers that could sell the new resistant varieties (Quaim, 1998). The mycoplasma and virus diseases in Mexico are not currently controlled with pesticides, and rank second and third in economic damage, respectively. The major pest, the fungus Phytophthora infestans, ranks first in economic damage and requires, in some cases, up to 30 fungicide applications for control (Parga and Flores, 1995). Thus, the interesting new genetically altered varieties of potatoes are of little benefit to small farmers. Herbicide-resistant Crops (HRCs) Several engineered crops that include herbicide resistance are commercially available, and 13 other key crops in the world are ready for field trials (Table 2). In addition, some crops (e.g. corn) are being engineered to contain both herbicide (glyphosate) and biotic insecticide resistance (BT d-endotoxin) (Gene Exchange, 1997). Some specialists suggest that herbicides adopted for herbicide-resistant crops employ lower doses compared with atrazine, 2,4-D and alachlor. However, the resistance of the crop to the target herbicide would, in practice, suggest to the farmer to apply dosages higher than recommended (Paoletti and Pimentel, 1996). In addition, the costs for this new HRC technology are about two times higher in corn than the recommended herbicide use and cultivation weed control programme (Pimentel and Ali, 1998). (Charles Bonbrook has reported that HRCs in soybeans cost two times more and use two to four times more herbicide.) ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net 3 Impact of Genetically Modified Organisms Table 2 Herbicide-resistant crops (HRCs) approved for field tests in the United States from 1987 to July 1995 (Adapted from: Krimsky and Wrubel, 1996, and Gene Exchange, 1997) Crop Herbicide Research organization Alfalfa Barley Canola (oilseed rape) Glyphosate Glufosinate/bialaphos Glufosinate/bialaphos Northrup King USDA University of Idaho Hoechst-Roussel/AgrEvo InterMountain Canola Monsanto Hoechst-Roussel/AgrEvo ICI UpJohn Cargill DeKalb Holdens Pioneer Hi-Bred Asgrow Great Lakes Hybrids Ciba-Geigy Genetic Enterprises Monsanto DeKalb Pioneer Hi-Bred Du Pont American Cyanamid Monsanto Dairyland Seeds Northrup King Calgene Monsanto Rhone Poulenc Du Pont Delta and Pine Land Phytogen University of Florida University of Idaho USDA USDA Monsanto American Cyanamid Louisiana State University Monsanto UpJohn Pioneer Hi-Bred Northrup King Agri-Pro UpJohn Hoechst/AgrEvo Du Pont Hoechst-Roussel American Crystal Sugar American Cyanmid Monsanto Canners Seed AgrEvo Glyphosate Corn Glufosinate/bialaphos Glyphosate Sulfonylurea Cotton Imidazolinone Glyphosate Bromoxynil Sulfonylurea Peanuts Potatoes Rice Soybeans Imidazolinone Glufosinate/bialaphos Bromoxynil 2,4-D Glyphosate Imidazolinone Glufosinate/bialaphos Glyphosate Glyphosate Glufosinate/bialaphos Sugar beets Tobacco Tomatoes Wheat 4 Sulfonylurea Glufosinate/bialaphos Glyphosate Sulfonylurea Glyphosate Glufosinate/bialaphos Glufosinate/bialaphos ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net Impact of Genetically Modified Organisms Integrated pest management (IPM) could benefit from some HRCs, if alternative nonchemical methods can be applied first to control weeds and the target herbicide could be used later, only when and where the economic threshold of weeds is surpassed (Krimsky and Wrubel, 1996). Generally, though, the use of HRCs will lead to increased use of herbicides and environmental and economic problems (Pimentel and Ali, 1998). Most HRCs were developed for Western agriculture (Krimsky and Wrubel, 1996). One innovation that would help developing countries is the control of parasitic weeds – such as Orobanche and Stringa, both of which severely reduce grain yields. Trials on broomrape have demonstrated that HRCs can produce at least double the yields of control crops. However, the authors observed that this technology could only be used with crops that are not potentially interbreeding with wild weed relatives (Joel et al., 1995). For example, in northern African countries, most crops, such as sorghum, wheat and canola (oilseed rape), have wild weed relatives, thereby increasing the risk that genes from the herbicide-resistant crop varieties could be transferred to wild weed relatives (Mikkelsen et al., 1996; BSTID, 1996). The risk of herbicide-resistant genes from a transgenic crop variety being transferred to weed relatives has been demonstrated for canola (oilseed rape) and sugar beet. Mikkelsen et al. (1996) have shown that herbicide-resistant genes from transgenic canola move quickly into weed relatives. Boudry et al. (1994) also revealed gene flow between cultivated sugar beets and wild/weed beet populations. Repeated use of herbicides in the same area creates problems of weed herbicide resistance (Wrubel and Gressel, 1994). For instance, if glyphosate is used with HRCs on about 70 million hectares, this might accelerate pressure on weeds to evolve herbicide-resistant biotypes (Krimsky and Wrubel, 1996). Sulfonylureas and imidazolinones in HRCs are particularly prone to rapid evolution of resistant weeds (LeBaron and McFarland, 1990; Wrubel and Gressel, 1994). Extensive adoption of HRCs will increase the hectarage and surface treated, thereby exacerbating the resistance problems and environmental pollution problems (Krimsky and Wrubel, 1996). Bromoxynil has been targeted in herbicide-resistant cotton by Calgene and Monsanto (Table 2). This herbicide has been used on winter cereals, cotton, corn, sugarbeets and onions to control broad leaf weeds. Drift of bromoxynil has been observed to damage nearby grapes, cherries, alfalfa and roses. In addition, leguminous plants can be sensitive to this herbicide (Abd Alla and Omar, 1993), and potatoes can be damaged by it. Herbicide residues above the accepted standards have been detected in soil and groundwater, and as drift fallout. Rodents demonstrate some mutagenic responses to bromoxynil (Rogers and Parkes, 1995). Beneficial Stafilinid beetles show reduced survival and egg production, even at recommended dosages of bromoxynil. Crustaceans (Daphnia magna) have also been severely affected by this herbicide. Many crops have been engineered for resistance to PAT (phosphinothricin acetyl transferase) (Bommineni et al., 1997). The crops with resistance to this herbicide include alfalfa, corn, barley, wheat, rice, canola, peanuts, soybeans, sorghum, tomato and sugar beet (Table 2). Finally, turfgrass (Agrostisis stolonifera) and other components have been engineered for resistance to glufosinate/bialaphos. Toxicity of herbicides and herbicide-resistant crops Toxic effects of herbicides to humans and animals have also been reported (Cox, 1996). For example, the Basta surfactant (sodium polyoxyethylene alklether sulfate) has been shown to have strong vasodilative effects in humans and cardiostimulative effects in rats (Koyama et al., 1997). Treated mice embryos exhibited specific morphological defects (Watanabe and Iwase, 1996). Most HRCs have been engineered for glyphosate resistance (James, 1999). Although adverse effects of herbicide-resistant soybeans have not been observed when fed to animals such as cows, chickens and catfish, genotoxic effects have been demonstrated on other nontarget organisms (Cox, 1995a,b). Earthworms have been shown to be severely injured by the glyphosate herbicide at 2.5–10 L/ha (Reanova et al., 1996). For example, Allolobophora caliginosa, the most common earthworm in European, North American and New Zealand fields, is killed by this herbicide. In addition, aquatic organisms, including fish, can be severely injured or killed when exposed to glyphosate (WHO, 1994). The beneficial nematode Steinerema feltiae, a useful biological control organism, is reduced by 19–30% by the use of glyphosate. There are also unknown health risks associated with the use of low doses of herbicides (Wilkinson, 1990). Due to the common research focus on cancer risk, little research has been focused on neurological, immunological, developmental and reproductive effects of herbicide exposures (Krimsky and Wrubel, 1996). Much of this problem is due to the fact that scientists may lack the methodologies and/ or the diagnostic tests necessary to evaluate properly the risks caused by exposure to many toxic chemicals, including herbicides. While industry often stresses the desirable characteristics of their HRCs, environmental and agricultural groups, and other scientists, have indicated the risks (Pimentel et al., 1998). For example, research has shown that the application of glyphosate can increase the level of plant oestrogens in the bean, Vicia faba. Feeding experiments have shown that cows fed transgenic glyphosate- ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net 5 Impact of Genetically Modified Organisms resistant soybeans had a statistically significant difference in daily milk-fat production as compared to control groups (Hammond et al., 1996). Some scientists are concerned that the increased milk-fat production by cows fed these transgenic soybeans may be a direct consequence of higher oestrogen levels in these transgenic soybeans (Hammond et al., 1996). Economic impacts of herbicide-resistant crops The herbicides for which HRCs are being designed are more expensive than many of the herbicides they are intended to replace (Figure 1). While some analysts project that switching to bromoxynil for broadleaf weed control in cotton could result in savings of $37 million each year from reductions in herbicide purchases, few other economic product evaluations demonstrate cost savings with the use of HRCs (Krimsky and Wrubel, 1996). Furthermore, recent problems with use of glyphosate-resistant cotton in the Mississippi Delta region – crop losses resulting in up to $500 000 for the cotton crop of 1998 – suggest that this technology needs to be further developed before some farmers will reap economic benefits (Fox, 1997). In addition, a recent study of herbicide-resistant corn suggests that the costs of weed control might be about two times greater than normal herbicide and cultivation weed control in corn (Pimentel and Ali, 1998). While some scientists suggest that use of HRCs will cause a shift to fewer broad-spectrum herbicides (Hayenga et al., 1992), most scientists conclude that the use of HRCs will actually increase herbicide use (Rissler and Mellon, 1993; Paoletti and Pimentel, 1995, 1996; Pimentel and Ali, 1998). Millions of pounds (active ingredient) 600 Herbicides 400 300 200 Insecticides 100 Fungicides and other* 1980 1984 1988 1992 Annual expenditures (millions of $) Year Herbicides 4000 3000 2000 Insecticides 1000 Fungicides and other 1986 1988 1990 1992 Year Figure 1 The choice of herbicide resistance as the main target for engineered crops seems related to the promising trend of herbicide sales and returns rather than to other environmental strategies such as potential reduction of pesticides in the environment (from McCullum et al., 1998). * Other 5 rodenticides, fumigants and molluscicides. 6 Bacillus thuringiensis for Insect Control 500 More than 40 Bacillus thuringiensis (BT) crystal protein genes have been sequenced, and 14 distinct genes have been identified and classified into six major groups based on amino acids and insecticidal activity (Krimsky and Wrubel, 1996). Many crop plants have been engineered with the BT d-endotoxin, including alfalfa, corn, cotton, potatoes, rice, tomatoes and tobacco (Table 3; Figure 2). The amount of toxic protein expressed in the modified plant is 0.01–0.02% of the total soluble proteins (Strizhov et al., 1996). Some trials with corn demonstrate a high level of efficacy in controlling corn borers (Steffey, 1995). However, corn borers are a minor pest in the USA and major insecticides are applied to control corn rootworms in corn monoculture. Corn engineered with BT-endotoxin has the potential to reduce corn borer damage by 5–15% over 28 million hectares in the US, with a potential economic benefit of $50 million annually (Steffey, 1995). Some suggest that corn engineered with BT toxin will increase yields by 7% over similar varieties (Moff, 1998; Rice and Pilcher, 1998; James, 1999). Trials in Italy demonstrated that engineered corn increased yields by 2–28%, but has a 1.8% higher grain moisture level at harvest. However, it is too early to tell if all these benefits will be realized consistently. Potential negative environmental effects also exist because the pollen of engineered plants contains BT, which is toxic to bees, beneficial predators and endangered butterflies like the karnala blue and monarch butterflies (in the US). Cotton was the first crop plant engineered with the BT dendotoxin. Caterpillar pests, including the cotton bollworm and budworm, cost US farmers about $171 million/ year as measured in yield losses and insecticide costs. The widespread use of BT-cotton could reduce insecticide use ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net Impact of Genetically Modified Organisms Table 3 Transgenic insect-resistant crops containing BT dendotoxins. Approved field tests in the USA from 1987 to July 1995 (Krimsky and Wrubel, 1996; Gene Exchange, 1998) Crop Research organization Alfalfa Apples Mycogen Dry Creek University of California Asgrow Cargill Ciba-Geigy Dow Genetic Enterprises Holdens Hunt-Wesson Monsanto Mycogen NC+Hybrids Northrup King Pioneer Hi-Bred Rogers NK Seed Calgene Delta and Pineland Jacob Hartz Monsanto Mycogen Northrup King University of Wisconsin Rutgers University University of Wisconsin USDA Calgene Frito-Lay Michigan State University Monsanto Montana State University New Mexico State University University of Idaho Louisiana State University University of Wisconsin Auburn University Calgene Ciba-Geigy EPA Mycogen North Carolina State University Roham & Haas Campbell EPA Monsanto Ohio State University PetoSeeds Rogers NK Seeds University of California, Davis USDA Corn Cotton Cranberry Eggplant (aubergine) Poplar Potatoes Rice Spruce Tobacco Tomatoes Walnuts Figure 2 The development of Bacillus thuringiensis (BT) crops and their entrance in the market. BT has been known as a target insecticide for caterpillars (Lepidoptera) since the beginning of the twentieth century. ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net 7 Impact of Genetically Modified Organisms and thereby reduce costs by as much as 50–90%, saving farmers $86–186 million/year in the US. The development of insect resistance to transgenic crop varieties is one highly possible risk associated with the use of BT d-endotoxin in genetically engineered crop varieties. Resistance to BT has already been demonstrated in the cotton budworm and bollworm (Bartlett, 1995). If BTengineered plants become resistant, a key insecticide that has been utilized successfully in integrated pest management (IPM) programmes could be lost (Paoletti and Pimentel, 1995). Therefore, proper resistance management strategies with use of this new technology are imperative. Another potential risk is that the BT d-endotoxin could be harmful to nontarget organisms (Losey et al., 1999). For example, it is not clear what potential effect the BT dendotoxin residues that are incorporated into soils will have against an array of nontarget useful invertebrates living in the rural landscape (Paoletti and Pimentel, 1995). Effects of BT d-endotoxin on nontarget organisms It has also been demonstrated that predators, such as the lacewing larvae (Crysoperla carnea) that feed on corn borers (Ostrinia nubilalis), grown on engineered BT-corn have consistently higher mortality rates when compared to specimens fed with non-engineered corn borers. In addition, the treated larvae need three more days to reach adulthood than lacewings fed on prey from non-BT corn. Monarch butterfly caterpillars have been seriously affected by eating milk weeds having some pollen of BTcorn deposited on it (Losey et al., 1999). Single-gene Changes and Increased Pathogenicity Most single-gene changes are probably not likely to affect adversely the pathogenicity and virulence of an organism in nature. However, some single-gene changes can have detrimental consequences. Certain genetic alterations in animal and plant pathogens, for example, have led to enhanced virulence and increased resistance to pesticides and antibiotics. For instance, some oat rust microbes, initially nonpest genotypes for a particular oat variety, became serious pests after a single gene change allowed the rust to overcome resistance in the oat genotype (Wellings and McIntosh, 1990). An important fungal disease of rice, rice blast, has some genotypes with single-gene changes that cause the fungal organism to be potentially pathogenic to rice cultivars (Smith and Leong, 1994). A similar phenomenon of singlegene changes resulting in pathogenicity has been documented with a related fungal pathogen that infects weeping 8 lovegrass. This phenomenon has led plant pathologists to develop the ‘gene-for-gene’ principle of parasite–host relationships in which a single mutation in a parasite overcomes single-gene resistance in the host. Furthermore, numerous instances have been documented in which insects, through a single-gene change, have overcome resistance in plant hosts or have evolved resistance to insecticides. More than 500 species of arthropods have developed resistance to pesticides (Georghiou, 1990). Threats from Modified Native Species Lindow (1983) has reported that there is little or no danger from the ice-minus strain (the organism that has been suggested as a potential remedy to orchard freezing in early spring) of Pseudomonas syringae (Ps) because Ps is a native US species that produces related phenotypes in nature. Other investigators have demonstrated that there are different genotypes of Ps and some of these genotypes have genes for pathogenicity (Lindgren et al., 1988). Because some native species have the ability to alter their interactions within an ecosystem, the genetic modification and release of native species into the natural ecosystem may not always be safe. For example, from 60–80% of the major insect pests of US and European crops were once harmless native species (Pimentel, 1993). Many of the insects moved from benign feeding on natural vegetation to destructive feeding on introduced crops. For instance, the Colorado potato beetle moved from feeding on wild sandbar to feeding on the potato that was introduced from Peru and Bolivia. This insect has become a serious pest of the potato in the USA and Europe. Discussion Both pesticides and biotechnology have definite advantages in reducing crop losses to pests. At present, pesticides are used more widely than biotechnology, and thus are playing a greater role in protecting world food supplies. In terms of environmental and public health impacts, pesticides probably have a greater negative impact at present because of this more widespread use. Genetically engineering crops for resistance to insect pests and plant pathogens could, in most cases, be environmentally beneficial, because these more resistant crops could allow a reduction in the use of hazardous insecticides and fungicides in crop production. In time, there may also be economic benefits to farmers who use genetically engineered crops; this will depend, though, on the prices charged by the biotechnology firms for these modified, transgenic crops. There are, however, some environmental problems associated with the use of genetically engineered crops in ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net Impact of Genetically Modified Organisms agriculture. For example, adding Bacillus thuringiensis (BT) to crops like corn for insect control can result in any of the following negative environmental consequences: (1) development of resistance to BT by pest species in corn and other crops; (2) health risks from exposure to the BT toxin, to humans in their food and to livestock in feed; (3) the toxicity of pollen from the BT-treated corn to honey bees, beneficial natural enemies, and endangered species of insects that feed on the modified corn plants or come into contact with the drifting pollen; and (4) potential hazard to above ground and soil organisms affected by crop residues containing BT. A major environmental and economic concern associated with genetically engineered crops is the development of herbicide-resistant crops (HRCs). Although in rare instances HRCs may result in a beneficial reduction of toxic herbicide use, it is more likely that the use of HRCs will increase herbicide use and environmental pollution. In HRC soybeans, two to four times more herbicides are required compared with conventional soybeans. In addition, farmers will suffer because of the high costs of employing HRCs – in some instances, weed control with HRCs may increase weed control costs for the farmer 2fold; (Paoletti and Pimentel, 1996Pimentel and Ali, 1998). More than 40% of the research by biotechnology firms is focused on the development of HRCs. This is not surprising, because most of the biotechnology firms are also chemical companies who stand to profit if herbicide resistance in crops results in greater pesticide sales (Krimsky and Wrubel, 1996). Theoretically, the acceptance and use of engineered plants in sustainable and integrated agriculture should consistently reduce use of pesticides, but this is not the current trend. In addition, most products and new technologies are designed for Western agriculture systems, not for poor or developing countries (Moff, 1998). For instance, if terminator genes enter into the seed market, there will be no possibility of traditional and small farmers using their plants to produce their seeds (Berlan and Lewontin, 1998). In assessing socioeconomic implications of North–South biotechnology transfer, it has been shown that small, subsistence farmers are only marginally interested in the introduction of virus resistance engineered potatoes (Quaim, 1998). Thus, genetic engineering could promote improvements for the environment; however, the current products – especially the herbicide-resistant plants and the BTresistant crops – have serious environmental impacts, similar to the consequences of pesticide use. References Abd Alla MH and Omar SA (1993) Herbicides effects on nodulation, growth, and nitrogen yield of faba bean introduced by indigenous Rhizobium leguminosarum. Zentralblatt fur Mikrobiologie 148(8): 593–597. Bartlett AC (1995) Resistance of the pink bollworm to BT transgenic cotton. 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