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Understanding Gene Flow in Canadian Agricultural Crops

The natural process of gene flow is no different for biotech crops than conventional crops.

Natural process of gene flow in plants is a safeguard against biotech crop genes invading wild relatives

Gene flow describes the movement of genes from one plant to another, and is often used synonymously with the term “outcrossing”.  This process is not unique to crops that have been developed using recombinant DNA technology (rDNA), referred to as biotech crops. Rather, outcrossing is a natural process that occurs in the life cycle of most flowering plants.

Cross-pollination is natural

Flowering plants broadly fall into two categories with regards to outcrossing. They are either “cross-pollinators” or “self-pollinators.” Corn and canola are examples of cross-pollinators while wheat and barley are known as self-pollinators. In flowering plants, pollen dispersal is the main mode of gene flow. For gene flow to occur between plants they must be:

• sexually compatible
• within proximity such that pollen can physically move from plant to plant
• flowering at the same time

The physical distance between plants is an important factor influencing the level of outcrossing that may occur between crop plants and wild relatives.  The likelihood of gene flow diminishes as the distance between plants becomes greater, even if they are present within the same field or adjacent fields. Although the majority of canola pollen falls to the ground within a few metres of its origin, a small proportion can become airborne and must be carried by wind or insects to other plants. However, the amount of cross-pollination that actually occurs is low.  A recent large-scale study showed that cross-pollination only occurred between 0.03 per cent and 0.2 per cent between adjacent fields in canola.  

Concern expressed

Some fear that gene transfer between crops developed through rDNA technology and their wild relatives could produce a ‘superweed’ that may harm the environment.

In fact, after hundreds of years of manipulating plant genetics through plant breeding, referred to as domestication, crop plants have been selected for increased yield and quality while decreasing the ability of the crop to actually survive in the wild. To date, genes introduced into crop plants using rDNA technology have not caused the crop to become invasive of natural habitats or be harmful to the environment.

In reality, the majority of crop plants developed through rDNA technology, particularly those that are herbicide-tolerant, have improved environmental benefits through decreased herbicide use. Herbicide-tolerant crops are also more conducive to no-till farming, leading to reduced soil erosion and improved water quality.

Wild relatives

Outcrossing plants such as corn and canola originated in Mexico and Europe and thus have relatively few wild relatives in Canada. Wheat also originated in Europe and has limited wild relatives in Canada as well.

In the wild, gene selection pressure is contingent on their biological importance (such as ability to survive and reproduce) to the plant species and is carefully preserved over generations through reproduction.  Wild or weed relatives gain little advantage if they preserve a gene that confers herbicide tolerance, for example, if that herbicide is not present in their natural environment. 

“There are many natural barriers to interspecies crosses that make it an unlikely event,” says Keith Downey of Agriculture and Agri-Food Canada.  “What’s more, for a gene to become fixed in the new species, it must be transferred from the chromosome of the genetically-modified crop plant and inserted into the chromosome of the wild relative – an even rarer event.”

Regulatory restrictions and common field practices limit such gene flow even further for breeding purposes.  In Canada, canola Certified seed production fields are isolated by at least 100 metres from any other canola fields, sufficient to maintain genetic purity of at least 99.75 per cent.   However, when new novel traits developed through biotechnology are being evaluated, test fields must be separated by a 10 metre canola boarder strip surrounding the entire test area, or isolated by a distance of 200 to 400 metres from any other canola. 

Rigorous Government assessments are in place to evaluate potential for outcrossing

Before a novel agricultural plant or Plant with a Novel Trait (PNT) can be produced and marketed, it must undergo a series of mandatory safety assessments by the Canadian Food Inspection Agency (CFIA) to determine its environmental impact.  Among the criteria evaluated are the potential for gene flow to wild plant species and the potential impact of this gene flow on the environment.

All environmental safety assessments follow stringent government guidelines.  PNTs are evaluated for their novel gene product(s) regardless of the process used to develop them.  Evaluation is completed on a case-by-case basis with respect to the novel plants’ potential for gene flow to wild relatives, accounting for the particular plant species, its biology and ecology in Canada, and the genetic trait that has been introduced into the plant (such as herbicide tolerance).

An extensive review of more than 200 scientific publications concluded that PNTs are:

• no more likely than traditional crops to lead to super pests and diseases
• no more likely to become weeds outside farming situations than other cultivars
• no more invasive, persistent or likely to become weeds than conventional counterparts
• no more likely to transfer transgenes (developed through rDNA) or any other gene than other crop cultivars

Additional Resources:

• Assessment Criteria for Determining Environmental Safety of Plants with Novel Traits http://www.inspection.gc.ca/english/plaveg/bio/dir/dir9408e.shtml
• A companion document to Assessment Criteria for Determining Environmental Safety of Plants with Novel Traits – wheat http://www.inspection.gc.ca/english/plaveg/pbo/dir/dir9901e.shtml
• A companion document to Assessment Criteria for Determining Environmental Safety of Plants with Novel Traits – soybeans http://www.inspection.gc.ca/english/plaveg/pbo/t11096e.shtml
• Anonymous (1999). The Regulatory Directive DIR 1999-01: The biology of Triticum aestivum L. (Wheat).  Plant Biotechnology Office, Plant Health and Production Division, Canadian Food Inspection Agency, Nepean, Canada.
• Biotechnology Industry Organization, Fact Sheet on Cross Pollination in Bt and Herbicide-Tolerant Crops http://www.bio.org/foodag/action/fact8.asp
• Canadian Food Inspection Agency, Outcrossing to Wild Species http://www.inspection.gc.ca/english/sci/biotech/enviro/transfe.shtml  
• Canadian Food Inspection Agency, Plant Biosafety Office, The Regulation of Plants with Novel Traits in Canada http://www.inspection.gc.ca/english/plaveg/bio/pntchae.shtml
• Council for Biotechnology Information, In-depth Report: Gene Flow to Wild Plant Relatives
• Connor, AJ., Glare, TR., Nap, JP.  (2003).  The release of genetically modified crops into the environment.  Part II.  Overview of ecological risk assessment.  The Plant Journal, 33(1):19  
• Eastham K., Sweet, J. (2002).  Genetically Modified Organisms (GMOs): The significance of gene flow through pollen transfer. European Environmental Agency Environmental Issue Report No. 28. European Environmental Agency, Copenhagen, Denmark.
• Rieger, MA., Lamond, M., Preston, C., Powles, SB., Roush, RT. (2002).  Pollen-mediated movement of herbicide resistance between commercial canola fields.  Science, 296(5577):2386-8
• The Roundup Ready System for Wheat Update, Monsanto Canada, January 2002

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