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Antibiotic Resistant Marker Genes in GM Plants!
The transfer of genes from one organism to another is a complex process, and one whose effectiveness is quite variable. When a new gene is inserted into an organism, therefore, researchers will often also insert a ‘marker’ gene to assist them in ascertaining whether the transferred genes have been successfully incorporated in the host organism.
Genes for resistance to a range to antibiotics have been used as selectable markers since the late 1980s. Plants containing a gene for resistance to an antibiotic such as kanamycin will grow on material which contains that antibiotic, whereas plants which have not incorporated the transferred gene will not do so. This makes it possible to screen out plants which have failed to take up the new genes.
The use of antibiotic-resistant genes in GM plants for human consumption has given rise to fears that these genes could be transferred to bacteria present in the human stomach, thereby making them resistant to commonly prescribed antibiotics. Fears have also been expressed that GM plants incorporating antibiotic resistance genes which are used for animal feed could lead to the spread of antibiotic resistance to humans through the transfer of resistant bacteria to those in contact with farm animals.
Though the possibility of transfers of these kinds cannot be ruled out, it should be noted that, despite extensive research, there has been no recorded case of antibiotic resistance being spread through the transfer of antibiotic resistant marker genes to animals or humans.
This kind of transfer would be an example of horizontal gene transfer which, even if it once occurred, is very rare in plants and animals. In 1997, the EU granted marketing approval for a GM maize incorporating a gene for resistance to the antibiotic ampicillin for use as animal feed and in the production of starch.
In the view of the European Commission’s scientific advisory committee, resistance to this antibiotic was already widespread on farms. It was considered improbable that the DNA would survive intact in the animals’ gut to be picked up by bacteria, while any processing of the maize into starch would degrade the modified DNA so that it was no longer functional. In 1998, an application for marketing approval for a potato modified to produce extra starch which contained an antibiotic resistance gene was withdrawn following objections by member states.
While stating that the risk of horizontal gene transfer is extremely low, the Food Safety Authority of Ireland has recommended that selectable markers based on antibiotic resistant genes should be avoided. Alternative marker systems which do not use antibiotic resistance have been developed, and it is unlikely that antibiotic resistant genes will be used as markers in new GM food products. It is also possible to remove marker genes from GMOs after organisms which have incorporated new genes have been identified.
In recent decades, resistance to antibiotics has grown to what many scientists consider an alarming extent due to factors such as the over-prescribing of antibiotics for minor illnesses and their widespread use in modern farming. At EU level, member states have been advised to clamp down on over-the-counter sales of antibiotics and to exert greater control of antibiotic use in human medicine, veterinary medicine, animal feedstuffs, and food.
In the future, enhanced knowledge of genomics and the increased sophistication of GM technology are likely to permit the rewriting of entire metabolic pathways in order to modify several plant traits in tandem. In the case of GM foods, it has been suggested that a possible increase in the use of genes without a history of food use may give rise to a greater risk of toxicity or allergenicity. These developments will necessitate more sophisticated analytical techniques to test the safety of GM crops and foods, and require that regulatory systems keep fully abreast of developments in food applications of genetic engineering.
GMOs and the Environment:
Another main area of concern raised by genetic modification is that of the possible effects on the environment. Sharp differences are apparent between, on the one hand, the biotechnology industry which sees genetic modification as an environmentally friendly technology that will lower the use of harmful pesticides and, on the other, environmental groups which believe that it will lead to ‘genetic pollution’ and inflict a range of direct and indirect damage on the natural world.
Each of the main applications of genetic modification in agricultural biotechnology to date – the insertion of genes for herbicide-tolerance and insect-resistance respectively in a range of plants – have given rise to concerns about their impact on the environment and biodiversity.
Herbicide-Tolerant Crops:
Weeds compete with crops for moisture, nutrients and light and can badly affect crop yields and quality .Though herbicides have been extensively used over the past half- century to combat weeds, the more effective they are in doing so the more likely they are to cause collateral damage to crops.
This has made the development of crops tolerant to herbicides an obvious goal for biotechnology firms and, to date, herbicide tolerance has been the characteristic most commonly engineered into transgenic crops. Though tolerance has now been engineered for most of the main herbicide groups, the most common are crops modified for tolerance to products containing glyphosphate – a broad-spectrum, non-selective, post-emergent herbicide which can be employed to control most of the major weed species in crops.
Starting with soybeans, tolerance genes for glyphosphate have been incorporated into a range of other crops-including maize, cotton, sugar beet, oilseed rape, and tobacco. In 1999, for example, it is estimated that 50 per cent or more of total soybean and cotton acreage in the United States was accounted for by varieties genetically engineered for glyphosphate tolerance.
Herbicide-Tolerant Crops and Herbicide Usage:
Some commentators have argued that the purpose of herbicide-tolerant crops is to enable farmers to spray as much herbicide as they want whenever they want without having to worry about damaging their crops. They maintain consequently that the development of these crops has simply been a device by agrochemical companies aimed at selling greater quantities of pesticide.
In response, the companies contend that the planting of herbicide- tolerant crops will require fewer applications of more effective, broad-spectrum herbicides. In support of this claim, they ask why farmers paying higher seed prices for GM seed, together with a technology fee, would want to incur further costs by using greater quantities of herbicide.
Many agronomists would also contend that there are environmental benefits from an increase in glyphospate use and a reduction in the use of other herbicide treatments. Glyphosphate herbicides bind tightly to soil particles and, over time, break down naturally in the soil into naturally occurring components, such as carbon dioxide. This contrasts with soil-acting herbicides which persist in the soil and are liable to leach into water systems with consequent adverse effects.
Herbicide-tolerant crops are also compatible with ‘no till’ methods which help preserve top soil. To the extent that less spraying is required, moreover, tractors have to pass over fields fewer times with less compaction and damage to soil. Concern has also been expressed about the indirect effects of herbicide resistant crops on the environment as a result, first, of gene escape and the possible transfer of GM crop traits to weeds and non-GM crops and, second, of their impact on wildlife habitats.
Gene Transfer from GM Plants:
Concerns about the escape or transfer of genes from herbicide-tolerant plants take several forms. First, the possibility that herbicide resistance could be spread from GM crops to weeds thereby creating herbicide resistant weeds or what are sometimes termed ‘super-weeds’. Second, the possibility that seeds shed during the harvesting of herbicide-tolerant crops could grow as weeds in future crops.
Third, the effects of the possible transfer of genes from GM crops to non-GM crops on organic farmers and farmers committed to growing conventional crops. These issues raise two main questions. First, how likely are gene escapes and transfers? Second, how serious are the resultant problems if such escapes and transfers occur?
The Likelihood of Gene Transfers:
Pollen from GM crops can be transported varying distances by wind, insects, or other means. Though cross- pollination of GM and non-GM crops and plants is possible, the barriers to its occurrence are greater than is sometimes appreciated. Gene transfer can only occur, first, where a GM crop plant has sexually compatible relatives in the area in which it is grown.
GM maize and potatoes, for example, have no wild or weed relatives in Europe, while oilseed rape and sugar beet do. Second, such transfers are highly unlikely among inbreeding crop species such as rice and soya in which crosses occur only between closely related parent plants, as they also are among crops such as cereals which are 99 per cent self- pollinating with only 1 per cent out-crossing.
The main risk of gene transfer from GM to non-GM crops arises where compatible relatives exist close to out breeding GM crop species such as oilseed rape. It is important to realize that, even where this risk is present, a number of steps must take occur before any transfer of modified genes can take place. In view particularly of the relative lack of research data on large-scale releases of GM crops such as oilseed rape under European and Irish conditions, a prudent approach should be taken to the assessment and management of such risks.
Gene Escape and Weeds:
There are also significant obstacles acting against plants modified for herbicide resistance or other traits subsequently becoming weeds themselves. Even if gene transfer from GM plants to wild relatives occurs and results in viable offspring, the effects are unlikely to be long-lived in most cases. With limited exceptions which arise to be addressed in the context of specific risk assessment and risk management strategies, crosses between cultivated crop species and wild relatives will generally not prove competitive in the wild.
Outside farm areas, such transfers would not present problems in areas such as woodland, though they could cause difficulties in areas dependent on chemical control such as roadside verges, railway tracks and airport runways. In that event, they, like the GM crops from which they derived, would be resistant only to a restricted range of herbicides and could be attacked with alternative herbicide products.
Gene Transfer to Non-GM Crops:
The likelihood of gene transfer to non-GM crops depends, like that to wild plants, on the biology of the crop species and its location. The possibility of such transfer poses risks for organic farmers and farmers who wish to remain GM- free, and may present a threat to their status and viability. These concerns are not dissimilar to those which organic farmers have about the accidental spread of fertilisers, pesticides, or other chemicals onto their land from neighbouring farms.
As the commercial cultivation of GM crops compatible for breeding purposes with crops grown organically draws closer, effective procedures, such as prescribed isolation distances, will be needed to minimise the risk of gene transfers. It should be noted, in conclusion, that the genetic engineering of plant sterility might offer an effective, if controversial, solution to the spread of transgenes through pollen dispersal.
So-called ‘terminator technology’ being developed by US researchers enables the seeds of GM crops to be prevented from germinating. As the use of this technology would require farmers to buy new seed every year, its implications for poorer farmers in developing countries have been the subject of particular criticism. Though it is certainly open to objection on these grounds, its potential for environmentally benign applications in preventing unwanted gene flow should not be discounted.
GM Material in Seed of Conventional Varieties:
The risk of gene transfer from GM to non-GM crops was underlined by the disclosure in May 2000 of the presence of GM material in seed of two conventional oilseed rape varieties imported from Canada and sown commercially in 1999 and 2000 on farms in seven EU countries on an area in excess of 15,000 hectares. It is understood that this came about as a result of cross-pollination in Canada between the conventional oilseed rape and a variety genetically modified for herbicide tolerance.
The affected seed varieties were also included in small scale field trials undertaken by the Department of Agriculture, Food and Rural Development in 1997 and 1998. Such trials are carried out to test new crop varieties for potential value for cultivation and use to Irish agriculture. The affected varieties were submitted and accepted for testing on the basis that they were conventionally bred. The total area sown in each of the two years in which trials took place in this country was less than one-tenth of an acre.
Though the GM presence in the seed batches in question was at a relatively low level (between 1 and 2.4 per cent) and posed no threat to human health, the incident aroused understandable unease in this and other EU countries. The Minister for Agriculture, Food and Rural Development stated that the submission, accidentally or otherwise, of contaminated seed for variety trial evaluation in this country was a source of ‘great concern’ and indicated that the Irish Government was determined to prevent its recurrence.
The Department of Agriculture, Food and Rural Development, in consultation with the Environmental Protection Agency, is to establish an appropriate monitoring and control system to ensure that contaminated seed is not imported in the future.
Herbicide-Tolerant Crops and Biodiversity:
Herbicide-tolerant crops may prove to reduce total herbicide use, but their appeal to farmers is based squarely on their greater effectiveness in destroying weeds. It has been claimed as a result that their cultivation will inevitably have an adverse effect on the availability of habitats for insects and other invertebrates. It will also affect the organisms associated with the root systems of weed species.
This will in turn lessen the supply of food for the predators that feed on these insects and organisms. Others take a different view, arguing that, as a general rule, undisturbed ecosystems are more diverse and that there is no reason to suggest that a reduction in the biodiversity of hedgerows and other crop surroundings will necessarily result from the cultivation of herbicide- tolerant crops.
In Britain, it is estimated that, in the past twenty years, ten million breeding individuals of ten species of farmland birds have disappeared from the countryside. The intensification of agriculture is generally seen as the cause of these wildlife losses, in particular factors such as the increased use of agrochemical inputs, the introduction of new crop types, increased land drainage, reductions in traditional rotation practices, and hedgerow removals.
One U.S. scientist has estimated that pesticides alone may kill up to 67 million birds and between 6 and 14 million fish each year in the United States. Evaluations of the risk to biodiversity from GM crops should consequently take due account of the environmental damage caused by the corresponding conventional crops.