 |
|
|
|
|
|
 |
Genetic engineering - an overview4. ENVIRONMENTAL ASPECTS OF GENETIC ENGINEERING As a result of our geographical isolation for millions of years, New Zealand has many plants and animals that are not found elsewhere. We enjoy a rich diversity of landscapes and many unique geographical features-a fact which is drawing growing numbers of tourists to these southern latitudes. This country is also unusual among western nations in its reliance on the primary production sector for export income. Both the protection of rare species and the ongoing use of land for agriculture rely on healthy functioning ecosystems that are well managed. Conservationists, environmentalists and scientists have all raised concerns about the possible impact of genetically modified organisms (GMOs) on such ecosystems. There is also the important matter of allowing time for public debate following recognition of these issues, since the development of molecular techniques enabling us to change the genetic makeup of species has raced ahead of ecological studies of the environmental impact of the newly created GMOs. (a) Molecular versus ecological understanding Genetically modified plants, animals and microorganisms all carry at least one new gene (or they can be missing a gene) designed to provide them with a new trait. While the majority of the first transgenic organisms so far have been herbicide- and insect-resistant plants, an enormous variety of new modifications are being explored. Research into the ecological implications of releasing modified organisms into the environment has also been taking place, but is hardly keeping up with the progress made in studying the molecular processes. Worldwide, plantings of genetically modified crops are increasing at a rapid pace. In 1996, there were less than 5 million acres planted worldwide but by 2002, this figure had jumped to 150 million acres. Most of this acreage is in the United States, Canada, Argentina and China. Just over two-thirds was used to grow herbicide resistant crop plants, while insect-resistant crops were planted on about a third of the land. Herbicide-resistant soybeans alone accounted for over half of the transgenic acres on a global basis. The increased use of GE crops in large-scale commercial plantings has sparked a debate about a number of environmental risks and benefits. The most frequently raised questions are:
(b) Transferring genes by drifting pollen Genes can be transferred from one organism to another by: Pollen or sperm. This process is known as 'vertical transmission'. or: (ii) By other means that do not involve pollen or sperm and known as 'horizontal transmission'. This process could theoretically happen even if the gene is not present in the pollen, through soil bacteria 'picking up' a fragment of rot or leaf DNA. Four factors are important in the consideration of the risk of gene transfer through pollen: how far the pollen moves; the viability or longevity of the pollen; the receptivity of the other plant to the GE pollen; and the concentration of competing viable pollen.
Planting transgenic trees for cropping, shelter or forestry near conservation areas would seem to present the greatest ecological risk, but for most GE crop plants, buffer zones are considered adequate measure to prevent unwanted plant drift. An important recommendation of the Royal Commission on Genetic Manipulation was that research be undertaken to determine the width of buffer zones required to ensure the protection of non-GE plants from GE pollen. Some soil bacteria can pick up pieces of DNA from other organisms and, incorporate it into plasmids. Such plasmids are present in most bacterial cells and the exchange of plasmids from one bacterial species to another is a natural process. Theoretically, bacteria could pick up DNA from a genetically modified plant and transport it along a chain of other microbial species until it eventually ends up in the human gut. Most genetic modifications use antibiotic-resistance genes to mark where a new gene has been spliced into a length of DNA. A widespread concern is that these genes could make their way into the human gut and spread resistance to antibiotics. While such gene transfers are possible between plants and soil bacteria, there is no evidence to suggest that such exchanges happen more in GE than in normal crops. To safeguard against the horizontal transfer of antibiotic resistance, the investigators use an antibiotic rarely used in human medicine. Alternative markers are being sought to do away with the use of such genes for this purpose. Traditionally, plant breeders have aimed to make the plants easier to grow, harvest and store. Most genetic modification has aimed to introduce resistance genes into crop plants. These genes enable plants to withstand herbicide treatment and attacks by insect pests. Plants equipped with such traits may be difficult to get rid of should they spread beyond the farm, but this is considered unlikely as crop plants tend to have few weed-like characteristics. A weed:
So, while the GE crop itself is not very likely to become a weed, could its pollen create new invasive weeds through spreading the foreign gene to other plants? Gene transfer through pollen is more likely between closely related species. In countries where modern crop plants originated, this kind of transfer is indeed possible and can create a problem (e.g. gene flow from GE rice to wilding rice). However, New Zealand's native flora is generally only very distantly related to the crop plants currently targeted for modification, so the risk of gene flow from agriculture to native plants is very low. A more pertinent debate may be the risk of gene flow from introduced domestic plants to introduced wild plants, such as brassicas and wild mustards. In Britain, scientists from the National Institute of Agricultural Botany discovered that wild turnip had acquired herbicide resistance from a neighbouring test site of genetically modified oilseed rape (canola). New Zealanders are fortunate that very few crop plants have weedy relatives growing in this country-weedy brassicas are a notable exception. Both New Zealand and overseas studies show that cross-pollination falls off rapidly within a short distance of transgenic plants. There is a very low risk of pollen travelling over several hundreds kilometres for most species, or even over a few kilometres for some wind-pollinated plants such as pine trees. A recent New Zealand study of the risks of genes flowing from GE oilseed rape to wild brassicas noted that the rape is unlikely to hybridise with any species apart from wild turnip and Brassica juncea. The Royal Commission on Genetic Modification concluded that if New Zealand is to release of GE plants into the environment, pollen drift should be investigated and appropriate distances separate GE and organically-grown crops. Chemical herbicides are now a standard part of conventional farming. They offer a cost-effective method of killing weeds and, as an alternative to mechanical cultivation, result in less topsoil lost and lower labour and energy costs. However, their negative effects include reduced soil fertility, water pollution, losses in earthworms and beneficial soil microbes, and a range of effects on human health. Most GE crops with a herbicide resistance have been modified to resist Monsanto's glyphosate-based Roundup (Roundup Ready soy accounted for more than half of the global plantings of soy in 1999). Roundup is the world's most frequently used herbicide and many, including Monsanto, would argue that it is one of the most environmentally friendly herbicides-it breaks down quickly in water and soil into harmless metabolites and is practically non-toxic to animals and humans. Monsanto's marketing strategy for the company's GE Roundup Ready crops is based on the promise that it will lead to fewer herbicide applications. However, statistics compiled by the United States Department of Agriculture (USDA) in 1997 show that expanded plantings of Roundup Ready soybeans resulted in a 72% increase in the use of glyphosate. One possible explanation is that, before the emergence of GE crops, the herbicide had to be applied early in the season, before the emergence of the main crop, so that it would not kill those plants. Now, given that the crop resists the herbicide, it can be applied at any time during the growing season. Perhaps in anticipation of increased use of the herbicide, Monsanto has successfully applied to have the minimum level for glyphosate in soybeans raised from six parts per million (ppm) to 20ppm in the United States and Europe. Apart from concerns that the use of Roundup itself could increase in the long-term, some scientists warn that Roundup resistance of GE crops could spread to other plants. The spread could lead to the need to use other, more toxic herbicide to control weeds. While Roundup is considered the most environmentally friendly herbicide, a Swedish study recently linked the compound to the increased incidence of non-Hodgkin's Lymphoma in developed countries. The commonly held belief that crossing different species leads to sterile offspring is true for most animals but is not always the case in the plant kingdom. Many species, even genera, can be crossed with good seed viability. However, this does not mean that a genetically modified plant can cross with non-modified plants easily, as there are three types of barrier-spatial, temporal and biological-that normally prevent plant species from hybridising.
Many of our common crop plants have arisen as the result of species hybridizing. Some of these crosses have been chance events, assisted by man as plants were moved around the globe, overcoming the spatial barrier. It is important to realise that crossing species barriers using standard breeding methods results in the thousands of multiple variant genes with unpredictable consequences.
(h) Butterflies, corpses and superbugs Apart from herbicide resistance, another major research focus is the development of crop plants that resist insect pests. The most frequently used genes are taken from the bacterium Bacillus thuringiensis (Bt), which is also used to produce insecticide sprays to combat insect pests in conventional and organic agriculture worldwide. The Bt endotoxins are mainly toxic to moths, butterflies and beetles. Some strains can affect flies and mosquitoes. Bt is popular because it kills a narrow range of target pests, degrades very quickly under UV light, and is very effective. It is not poisonous to mammals so has become the spray of choice against pests where people may become exposed (such as the aerial spraying of Auckland suburbs for tussock moth). Bt genes have been incorporated into many plants, which raised concern that its continuous presence will make it more likely that pest insects will become resistant. The best-known example of resistance in a pest insect is that of the diamond-backed moth, which developed resistance in the field after being heavily treated with commercial formulations of Bt in the mid-1980s in Hawaii. Studies carried out in New Zealand suggest that pests that have a wide host range are likely to develop resistance more quickly if several of their host plants are modified. The pollen may not be expressing Bt, but if it is from a GE plant, it will still be carrying a silent, non-expressed gene. However, several studies using Bt corn have generated evidence of possible effects on non-target beneficial species. The monarch study nevertheless illustrates the importance of ongoing risk assessments regarding the impact of GMOs on other species and their ecosystems. Such findings have also prompted further studies to determine how any negative impacts on non-target species could be averted, for example by avoiding the expression of Bt or other toxins in pollen. New Zealand has a high percentage of endemic species. The introduction of plants and animals from around the world has had major impacts on the original ecosystem. Competition with and displacement of native plants in the remaining areas of native bush is a continuing problem. Paradoxically, genetic modification has the potential to contribute both to the loss of more species and to the better protection of those species that are already endangered. Some scientists believe that gene technology could even bring back some of the species that conventional methods could not save from extinction. In New Zealand, the cloning of the huia has been suggested, and across the Tasman, scientists are considering cloning the Tasmanian tiger. Others question whether this is really feasible, given that animal cloning requires intact nuclei rather than naked DNA and the low chance of obtaining suitable nuclear material from long-preserved specimens. On the negative side, the Royal Society in Britain has echoed some scientists' concerns that the herbicide-resistant crops may reduce plant biodiversity-with knock-on effects in insects and birds. The Society says more effective destruction of weeds will lead to a reduction of micro-habitats for insects and other invertebrates, including many natural control agents such as insect predators. Genetically modified animals may lead to further extinctions. Researchers at Purdue University, USA, found that salmon modified with a human growth hormone grow much larger than the rest, and so are more attractive to hen salmon. However, large numbers of their offspring die before reaching sexual maturity. Using the Japanese medaka, the research team found that these GE fish attracted four times as many females as their smaller unmodified rivals, but that only two thirds of the GE medaka survived to reproductive age. The researchers tested what became known as the 'Trojan Gene' hypothesis in a computer model, and showed that if 60 GE fish got into a wild population of 60,000, the population would become extinct in just 40 generations. (i) GE to protect the environment On the positive side, GE has the potential to improve pest control (e.g., by making insect-killing viruses faster and more efficient). Further environmental gains will be achieved through GE plants designed to extract and accumulate heavy metals and other toxic pollutants from contaminated soils. Other teams are studying ruminant physiology to test whether sheep and cattle really need the bacteria that make them produce methane. (New Zealand is unique among other western countries in that its contribution to the greenhouse effect is mostly due to methane emissions, rather than carbon dioxide. Already, methane-free sheep are being bred through conventional methods, but gene technology will boost our understanding of ruminant digestion and perhaps bring a solution to the problem in time.) GE also offers some hope for managing New Zealand's most menacing pest, the possum. To date, these animals have been controlled only by inefficient and labour-intensive methods such as poisoning or trapping. Several teams are currently working on ways of making possums infertile by setting off an auto-immune reaction against their own reproductive organs. The researchers are taking a two-pronged approach to achieve such an effect. They are developing a new bait, most likely to be made of GE carrots, which feeds possums one of their own proteins. They are also searching for a possum-specific pathogen, such as a virus, bacterium or parasitic worm, which would also carry a possum protein and thus set off the auto-immune reaction. If a contagious pathogen can be found, this 'vectored' option could render the entire possum population infertile. Gene technology tends to focus on single genes, and assumes they will perform the same function wherever they are located. To some degree this ignores the fact that a gene may act differently, depending where it is spliced into its host's DNA. Furthermore, it is now clear that some genes have more than one function and not all of these functions may be known, or their interrelationships understood. As in conventional plant breeding, results of gene transfers are likely to be variable and will include some unwanted outcomes. The key lies in properly designed experiments. Laboratory tests may show how a GMO will behave but field trials are essential to study how the organism behaves in nature. Both approaches must be pursued before contemplating the release of GE organisms. |
|
