Issues Related to Release of GM Wheat: Gene Flow and Selection

Release of genetically modified (GM) wheat will require segregation of GM and non-GM wheat to satisfy international markets. Before GM wheat is released, it will be important to understand the fate of a GM trait within the agronomic production system. The objective of this study was to evaluate the effect of gene flow and selection pressure on the frequency of GM traits in non-GM wheat and wheat volunteers. Gene flow of GM traits to non-GM wheat will occur through pollen or seed movement. Gene flow is inevitable. When a GM trait does not confer a selective advantage in the production system, the frequency of the GM trait within non-GM wheat will be a function of the rate of gene flow. Low rates of gene flow will lead to low levels of GM contamination in the nonGM crop. With repeated gene flow events, the frequency of the GM trait will slowly increase in the non-GM crop. When the GM trait has a selective advantage, the frequency of the GM trait will increase rapidly in volunteer wheat populations. Herbicide tolerance is an example of a GM trait that provides a high selective advantage when the herbicide is applied in the production system. Predictive models show that even with very low rates of gene flow, frequent application of a highly effective herbicide will quickly increase the frequency of the herbicide tolerant GM trait in volunteer populations. This has negative implications for control of volunteers and the ability to maintain tolerance levels of GM traits in non-GM wheat crops. Introduction Development of genetically modified (GM) wheat through recombinant DNA technologies is a reality in a number of wheat breeding programs. The Canadian Food Inspection Agency (CFIA) has established an extensive protocol for testing and evaluation of GM crops through regulation of Plants with Novel Traits (PNT’s). Environmental safety assessments focus primarily on the impact of the new trait on weediness characteristics of the crop, the effect of gene-flow to wild relatives, the potential for the crop to become a pest, the potential impact of the trait on non-target organisms and biodiversity. Comparisons are made relative to a known non-GM counterpart. In most cases the environmental safety assessments focus on impacts outside of the agronomic production system. However, some GM traits can have significant impacts on crop production practices, pedigreed seed purity, ability to manage volunteers, and the ability to produce non-GM wheat crops. Surveys conducted by the Canadian Wheat Board show that there is significant customer resistance to GM wheat (Canadian Wheat Board, 2001). As a result, initial release of GM wheat will require segregation of GM and non-GM wheat to satisfy different customer demands. Under these circumstances, it will be important to understand the fate of GM traits in wheat within the production system. The objectives of this paper were to: 1) briefly review the pollination biology and out-crossing rates of wheat and 2) assess the potential effect of gene flow and selection on the frequency of GM traits in non-GM wheat and wheat volunteers using basic population genetic principles. Gene flow in wheat In plants, genes move between populations either through pollen or seed movement. Pollen movement in wheat is facilitated by wind and gravity. Anthers normally dehisce within the floret, followed by filament elongation and extrusion of the anthers outside of the floret. A small amount of pollen is shed on the stigma within the floret, while 80% of the pollen is shed outside of the floret. Florets that have not been successfully self-pollinated will remain open and be receptive to pollen from other sources for up to 13 days after anthesis (de Vries, 1971). Estimates of out-crossing rates in wheat are dependent on synchrony of flowering between males and females, the presence of receptive females and the availability of single dominant nuclear genes to facilitate detection of outcrossing. Seed movement may occur in a number of ways such as movement with farm or transport equipment, animals, or wind and water. Seed that has previously been contaminated with a GM trait through prior pollen or seed movement can contribute to the introduction of GM traits into fields that were not previously planted to a GM crop or adjacent to a GM crop. Frequency of seed movement is expected to by highly variable and difficult to predict. However, extrapolations from weed seed studies and crop mixtures may be helpful in establishing a range of values. Methods Potential gene-flow rates were estimated using published estimates of out-crossing rates in wheat. Hucl and Matus-Cdiz (2001) compared out-crossing rates among four wheat cultivars (Katepwa, Roblin, Oslo and Biggar) using a dominant blue aleurone trait in the pollen source to quantify outcrossing rates. Out-crossing rates varied considerably among the different cultivars (Figure 1). Katepwa showed the lowest level of out-crossing and Oslo showed the highest level of out-crossing. Out-crossing was reported up to 27 m from the pollen source. The pollen source plot size in this study was small (5m2) and sample sizes evaluated were low (less than 700 seeds/sample). Others studies have shown that out-crossing rates in wheat range fall within the range 0.1% to 10.1% (Griffin, 1987; Martin, 1990; Hucl, 1996 and Enjalbert et al., 1998).