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Sci Tech
Tracing hybridisation through gene flow
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One of the purported advantages of growing GM crops is the need to use lesser amount of herbicides. Yet, super weeds have resulted from accidental crosses between GM and neighbouring crops. These super weeds call for older and stronger herbicides to be used to remove them. R. Prasad looks into the issue.
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Bt cotton may not be the best option for small farmers as it calls for setting aside 20 per cent of acreage for non-Bt cotton. GEAC has not recommended it for India's small farmers.
THE COMMERCIAL cultivation of genetically modified (GM) Bacillus thuringiensis (Bt) cotton has become a reality in India notwithstanding the protests by environmentalists.
The Bacillus bacterium is commonly found in soils and has efficient pest controlling capability. When introduced into the transgenic plant, the bacteria help the plant ward off pests. The pay off from such a transgenic plant would be reduced need or total elimination of costly pesticides. The government's decision comes after a protracted delay, as environmentalists debated on a host of issues.
GM cotton is cultivated in several countries. According to latest figures nearly 5.5 million farmers in the U.S., Argentina, Canada and China grow GM crops covering a total area of more than 50 million hectares.
Bt cotton cultivation in China is a big success story. Their GM cott
on has been on sale since 1997. Around 2 million Chinese cotton farmers now grow Bt cotton, in fields covering 7000 square kilometres. As envisaged by the technologists the farmers' production costs have dropped by 28 per cent and the average income has gone up by more than $150 per year. More importantly the use of toxic pesticides has plummeted by 80 per cent and pesticide poisonings have gone down.
China has not stopped with GM cotton alone. Other GM crops on sale in China include pest and disease-resistant tomatoes and sweet peppers. It has released rice varieties resistant to three major pests and done field trials on GM wheat. In the pipeline are GM potatoes, rape, peanuts, cabbage, melons, maize, chillies, papaya and tobacco.
Acceptability to transgenic crops has never been so easy. But one of the most feared and less understood issues is the possibility of creating `super weeds' due to cross-pollination from GM plant to non-GM plant. The apprehension is not misplaced as several articles published in New Scientist and other journals have pointed out the possibility of such a super weed creation.
The Canadian episode has only confirmed the worst fears. Oilseed rape plants resistant to three or more herbicides have been found in Canada. These super weeds have resulted from accidental crosses between neighbouring crops that have been genetically modified to resist different herbicides.
Contrary to the benefits assured by growing GM crops, the super weeds call for older, stronger herbicides to be used to remove them. What is alarming is the speed of the process. It has happened in just three or four years' time. Multiple resistant oil seed rape appeared as weeds in the following year's crop, especially around field margins where seeds spilled during harvest.
The problem of contamination either through pollens or seeds has reached a different proportion in Canada. It is now difficult to grow conventional or organic strains without their being contaminated.
Canada's experience provides valuable lessons for countries like India which is venturing into GM crops. But not all crops behave the same way.
For instance, wheat and soybeans are relatively safe from contamination because they usually pollinate themselves. But oilseed rape accepts pollen from neighbours. Experiments have shown that pollen from GM oilseed rape travels at least 800 m, much further than expected.
This is nearly eight times the official Canadian `safe' separation of 100 metres for rape grown to supply pedigree seed and four times the country's 175-metre separation for rape grown to supply oil or food.
Deborah Letourneau, a professor of environmental studies at the University of California, Santa Cruz and a member of the U.S. National Research Council Sub-Committee on Environmental Consequences of Genetically Modified Plants feels how potentially misleading it can be to use average distances to estimate hybridisation rates between transgenic crops and their wild relatives.
According to Letourneau, the possible risk is underestimated by concentrating on the average hybridisation distances. New evidence according to him, suggests that GM plants might hybridise at higher rates than `equivalent' non-transgenic plants that were conventionally bred for herbicide resistance.
Another challenge to the technology is the presence of GM plants surviving in fields from year to year as a result of acquiring resistance to more than one weed killer. This happens by crossing with other GM strains. Such `stacked' resistance can be managed by using other herbicides.
The paramount concern therefore is to prevent or minimize the chances of creating super weeds. This calls for different challenges depending on whether genes travel by pollen or seed. Different methods of spreading genetic changes require different strategies to ensure that super weeds are not created.
The most important and practical method of sanitising GM crop is to grow GM and non-GM crops very widely separated. Indian government's stipulation which requires Bt cotton crop to be surrounded by a five-row deep sanitising band of non-Bt cotton is intended to avoid such super weed creation.
According to Dr. Cindy Sagers of Biological Science, Fulbright College, U.S., in some part of its range, almost every crop species can hybridise with a native, Dr.Sagers studies the risk of modified genes from rice plants escaping into naturalised red rice populations. Red rice is already a leading weed pest of rice in the United States, and the concern is that introduced genes will make them a more serious threat to crops.
Scientists have introduced about 40 novel rice genes to the commercial rice genome, with characteristics ranging from roundup resistance to specific beetle or pathogen resistance. However, a little to nothing is known about what might happen to weeds capable of cross breeding with rice if these genes are introduced into rice crops.
Current examination of cross-contamination of genetically modified organisms involves looking at the characteristics of weed-crop hybrids in greenhouse laboratory studies. However, Sagers also plans to reconstruct the history of hybridisation by examining genetic relationships among cultivated and red rice populations. The process of fertilization can also be tracked by tracing the gene flow in nuclear and chloroplast markers as these markers differ depending upon how plants are fertilized. The two markers help in identifying the fertilization process. For instance, if the two markers contain differences in their genetic code, then one can deduce that the gene movement from the GM crop to the weed has been primarily through pollens.
Similarly if the two markers tell the same story one may conclude that the gene movement is primarily through seeds. Identifying the gene flow is just one of the challenges. The real challenge arises in tackling the gene flow as the two different routes pose different problems in managing the flow.
According to Dr. Sagers, rice seeds can lie dormant for up to 12 years in the soil posing problems in eradicating them. This makes managing the modified gene flow in seeds quite difficult.
To manage pollen, strips of buffer plants can be used to trap the pollen and keep modified genes from escaping into the weed population.
The major concern in the developed countries where GM crops like canola, sorghum, rice and radishes have been introduced is the presence of close relatives, and in some cases the same species growing nearby. But the status is vague as regards genetically modified corn, cotton and soybean crops.
Probably examining the gene flow may be one of the ways of getting better clues to understand the strategies to be adopted in future to manage GM crops.
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