Transfer of herbicide resistance genes from Clearfield canola
by Bob Hartzler
Date September 4, 2002 - A primary concern with transgenic crops is the spread of the inserted gene from the crop to adjacent conventional fields or to compatible weeds in the area. Previous studies with ALS-resistant canola (Clearfield)1 have documented that pollen can transfer herbicide resistance to adjacent fields, but the earlier studies were conducted on small-scale research trials. Researchers from Australia published results of a comprehensive project investigating movement of the resistance gene from commercial fields during the first year of production of the Clearfield varieties (Rieger, M.A., M. Lamond, C. Preston, S. B. Powles and R. T. Roush. 2002. Pollen-mediated movement of herbicide resistance between commercial canola fields. Sci. 296:2386-2389.)
The researchers collected seed from 63 conventional canola fields growing near Clearfield canola fields in three Australian states. Seed from the conventional fields were grown out and treated with a lethal dose of chlorsulfuron (an ALS herbicide). Any plants developing from seed pollinated with pollen from a Clearfield field would carry the herbicide resistance trait, and therefore be able to tolerate the herbicide. Plants resulting from pollination with conventional pollen would be killed by the chlorsulfuron.
The herbicide resistant trait was found in 63% of the conventional fields, with the farthest cross-pollination occurring approximately 1.8 miles from the source field. Previous studies on small plots had shown a strong edge effect, with a relatively high frequency of resistance occurring immediately adjacent to the source field and then resistance dropped off very quickly as distance from the field increased. In this study the edge effect was not observed. The frequency of resistant plants in conventional fields located within 100 yards of a Clearfield canola field was similar at all sampling points, rather than being highest at the edge nearest the source of the Clearfield pollen. The researchers speculated that the random pollination events, rather than an exponential decline as distance from source increased, was due to the multiple pollination agents (wind and insect) and the large pollen source.
Although transfer of the herbicide resistance gene occurred at relatively long distances from the source field, the frequency of contamination was low, even in fields close to Clearfield canola fields. The highest contamination level in any field was 0.07% (700 seeds out of 1 million). The majority of fields had contamination levels less than 0.03%. It was also reported that transfer of the trait varied among different Clearfield varieties. This may have been due to difference in flowering timing of the canola varieties.
Several governments have adopted a 1% contaminant level as being acceptable for transgenes in conventional varieties, thus the reported contamination levels were at 'acceptable' levels. However, contamination levels might be expected to increase if the popularity of the the herbicide-resistant varieties increased, resulting in more source fields. Also, organic standards specify a zero tolerance for GMO genes. This study illustrates the difficulty/impossibility of maintaining a zero tolerance for these traits once they are grown commercially.
1The ALS resistance in Clearfield canola was introduced using 'conventional' breeding techniques (mutagenesis), rather than introduction of a gene from another organism. However, movement of the gene among canola populations should be similar as a transgenic trait, thus it serves as a good model to study this phenomenonc.
Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University
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