 |
|
|
|
|
|
 |
Genetic engineering - an overview3. GENETIC ENGINEERING OFF THE FIELD The main focus of genetic engineering so far has been the study and improvement of commonly used crop plants, but an enormous variety of modifications are being explored in other primary production sectors, as well as in industries outside agriculture. From fertility control to garment production, there are many areas that stand to benefit from gene technology research. Farmers are constantly trying to improve the genetic makeup of their animals. Historically, this was achieved through selective breeding, but gene technology looks set to play an increasingly important role. Few early applications of gene technology added new traits to livestock. Rather they helped speed up the conventional process of selective breeding. DNA markers, which act as genetic 'signposts', can help identify animals or plants with a desired gene long before it would otherwise become obvious. Genetic engineering, which actually modifies the genetic makeup of organisms, is also being investigated, focussing on areas where the change would be hard to introduce by conventional means. The completion of the Human Genome Project will greatly extend our understanding of the genetics of a wide range of animal species. Several similar projects now under way will give us the complete genetic codes of a number of organisms, including some commonly used plantation trees. Pine (Pinus sp) and spruce (Picea sp) are already under close scientific scrutiny. The forestry industry relies on innovation to remain competitive and as the demand for both timber and paper rises worldwide it is moving towards more sustainable production methods, based on plantation rather than native forests. With better understanding of the genomes of important species, gene technology will become an even more important tool for forestry research institutions and companies. Fast identification of superior tree varieties is crucial and this is where gene technology is already playing its part. One of New Zealand's forestry companies, Carter Holt Harvey, currently grows several hundreds of pine trees that have had a marker gene inserted. The idea is that if desirable traits are linked with a marker gene, gene technology can provide a fast method of identifying superior trees. The alternative is to wait for thousands of seedlings to grow long enough for such traits to become obvious. The same GE tools are used to study the genetic diversity of tree populations, which is a component of biodiversity and is therefore important in ensuring the sustainability of the forest resource. Understanding how trees grow will also make it easier to identify potential areas for future improvement. One such improvement, developed in parallel to the techniques used in crop plants, is the insertion of herbicide and pest resistance. Tree pests and diseases cause extensive losses in productivity, and weeds challenge the establishment of tree plantations. Effective pest management strategies are thus important, and biotechnology may provide an alternative to chemical pesticides. Scientists at the Crown Research Institute, Forest Research, have produced herbicide-resistant seedlings of both pine and spruce trees in the laboratory. They are also studying the genetic regulation of flowering and seed production. Traditional tree improvement takes a very long time as each genetic cross takes many years to reach sexual maturity. The rationale behind the Forest Research project is that superior trees could be induced to flower earlier or produce more seed. On the other hand, Canadian researchers are already using gene technology to produce trees whose flowering-and therefore pollen production-is delayed or completely suppressed, without affecting the speed of their growth. Such an approach may allay the concerns of critics who worry that pollen from GE pine trees may contaminate distant plants. The Canadian team is also studying genes isolated from agricultural species and transferred to trees to get an insight into the evolution of higher plants. Overseas researchers are also investigating naturally derived products for managing forest insect pests. Examples include the Canadian neem tree (Azadirachta indica), which produces an insect repellent, and other species that produce their own weed-killing chemicals. The New Zealand Dairy Board is currently spending $30 million annually on biotechnology research to ensure that the industry stays abreast of international developments. But at the moment the commercial use of genetically modified organisms or GM products in the industry is limited, not least because the industry is acutely sensitive to consumer perceptions and aware of the importance of maintaining the confidence of its customers. Gene technology already plays a role is the production of chymosin, an enzyme to separate milk into curd and whey in cheese factories. Traditionally, the enzyme was obtained from the lining of calves' stomachs but these days most of the product, particularly for vegetarian cheeses, comes from GE microbes. The dairy industry says the product is purer and the cheese and whey products made with this type of chymosin can be declared kosher. Gene technology can be used to distinguish between cows with different genes that are desirable for meat or milk production. One such set of genes, responsible for only slightly different milk proteins, has already sparked interest in New Zealand. Most dairy cows in New Zealand have a mixture of the A1 and A2 genes, both of which produce a particular milk protein. The molecular structures of the two milk protein versions differ only slightly, but their effect on health is thought to be significant. Studies in countries where the entire dairy herd carries only the A2 gene suggest that A1 may be linked with the onset of juvenile diabetes and heart disease. Since New Zealand's incidence of diabetes is higher than in most other Western countries, scientists are interested in an efficient test that could quickly separate A2 from A1 cows. This would help with the breeding of an A2 herd and the production of A2 milk if it were proven to be beneficial. While the Dairy Board and a Dunedin biotechnology company are both pursuing this goal, there is a catch. More than one factor prompts the onset of both diabetes and heart disease. This raises concerns about the ethics of bringing a food product on the market for which the health benefits are difficult to prove. The humble grass itself has also attracted some GE interest, especially the ryegrass varieties used on pastures. Gene technology could not only improve the nutritional composition of pasture grasses, but also create varieties that would need less fertiliser. Ryegrass is often grown in combination with clover, which can obtain its nitrogen through a symbiosis with soil bacteria. These bacteria live in root nodules and produce as much nitrogen as the plant needs, while they, in turn, live off other nutrients the plant produces. Scientists are investigating whether this symbiosis could be transferred to ryegrass and other pasture grasses, as this could lead to significant drop in the use of nitrogen fertilisers. Most pasture grasses are also currently having their genes mapped. This process can only lead to better understanding and improvement of growth patterns and nutritional content. New Zealand scientists have already used selective breeding to produce a range of sheep breeds of different meat quality. The same conventional methods have also produced the Belgian Blue cattle breed, which has double the amount of muscle. The difference is caused by a simple mutation - a deletion in the gene for myostatin. Sheep are now the target of an experiment to find out whether GE can achieve the same muscle-doubling effect. To knock out the myostatin gene, scientists insert a short string of DNA from a bacterium that disrupts the gene code sufficiently to make it unreadable for the cell's DNA translation machinery. But increased meat production is not the primary focus of the research. The first goal is to understand how the myostatin gene contributes to the process of muscle development and growth. Another goal is to elucidate how the gene might be used to prevent elderly people's muscles weakening or the weakening of muscles that accompanies conditions such as muscular dystrophy or some bone fractures. Australian researchers use GE to improve the quantity and quality of meat produced by sheep. They have designed a gene that alters the amount of growth hormone circulating in the blood of sheep. Early results from this work show that the transgenic animals grow larger and have a much lower carcass fat content than normal sheep. Currently, 20 sheep with this gene are undergoing field trials at an Australian research centre to provide more information on the effects of GE on the utilisation of food, the composition of the carcass and the growth of wool.
Many sheep farmers face hard problems with blowfly strike because it happens unpredictably and randomly throughout a flock. Australian researchers are trying to overcome this problem by providing sheep with a gene that will give them genetically-inherited natural resistance to blowfly strike. The concept involves the transfer to sheep of a gene obtained from plants. This gene carries the information for a protein called chitinase, which is a natural insecticide against blowfly larvae and yet is harmless to sheep and other mammals. The plant gene is being engineered to produce the chitinase protein in the sweat glands of sheep, so that sweat secreted onto the sheep's skin will contain quantities of the active protein. Blowfly larvae imbibing this material will then become affected by the insecticidal action of the chitinase protein. Tests have already been completed with chitinase protein obtained from tobacco plants. Currently, the research is concentrating on modifications to the plant gene to make the chitinase protein more effective against the blowfly larvae so that less will need to be produced in the sweat glands. Growing wool needs large quantities of the sulphur amino acid, cysteine, which in sheep must be supplied by the diet. When food is short, the lack of cysteine can limit the growth or quality of wool. To overcome this limitation, scientists overseas are using genetic engineering to provide sheep with the missing biochemical pathway that currently prevents the animals from making their own cysteine. To achieve this, two genes have been isolated from bacteria and modified to operate in sheep. These genes contain the information needed for the sheep to make two enzymes that, together, can synthesise cysteine from the sulphide present in their gut. Currently, the concept has been shown to operate effectively in transgenic mice. When given a cysteine-deficient diet, the transgenic mice continue to grow normally. Their non-transgenic littermates lose weight and much of their hair in the absence of this essential amino acid. The gene has now been transferred to transgenic sheep and these animals will be used to breed progeny to test the gene's effectiveness in preventing wool defects associated with cysteine deficiency. In 2000, scientists at AgResearch discovered two genes that influence a ewe's ability to produce twins. Both, called Inverdale and Woodlands, occur naturally in sheep but gene technology will help farmers to detect the animals with the desired gene faster, without having to wait for them to start breeding. The scientists involved in the studies predict that sheep will become an important model for fertility studies in the coming years and that New Zealand will pioneer this field, based on the sheer amount of effort that has already gone into conventional sheep breeding. These genes are also of interest to human fertility researchers, because some causes of infertility are likely to be genetic. In some cases the genes required for fertility may well be present, but silenced by cellular regulatory mechanisms. Both sheep genes follow complex patterns of inheritance. Whether or not they are expressed depends on whether they are passed on along the paternal or maternal line. Since the creation of Dolly the sheep, cloning techniques have advanced to the point that it is now feasible to have flocks of identical sheep or cattle for study purposes. Cloning by nuclear transfer is still in experimental stages, but the process allows scientists to transfer the nucleus from one cell into a recently ovulated egg. The ultimate goal of this cloning research is to produce animals from cells that have been genetically modified in the laboratory. The advantage of this approach is that cultured cells can be modified and selected in the laboratory and then allowed to develop into a transgenic animal, instead of relying on the small number of transgenic offspring previously produced by injecting DNA into embryos. |
