A combined method to study gene flow from cultivated sugar beet to ruderal beets in the glasshouse and open field

Gene flow from transgenic plants to compatible wild relatives is one of the major impediments to the development of the culture of genetically engineered crop plants. In this work the flow of two linked transgenes, EPSPs (conferring resistance to glyphosate) and gus genes of sugar beets, toward the ruderal beets in greenhouse conditions, in an open field (1 ha) and in normal seed multiplication conditions for sugar beet were studied. The data relate to only the 1999 summer season on one location of southwest of France. Care was taken as to the quality of the plant material and the efficiency of the analytical methods. Moreover, pollen dispersal and fertilization of the target plants were favorised and a detailed analysis of the hybrid offspring was performed. Greenhouse studies show that emasculated ruderal plants can be fertilized by the transgenic pollen of crop beets as easily as the male sterile isogenic line (near 100%). Only a part of the seedballs sown gave plantlets, the rest remains non-germinated in the soil. Both the hybrid progeny plantlets and the non-germinated embryos were analyzed. An efficient competition existed between the transgenic and the ruderal beet pollens, which hampers the production of a large transgenic hybrid progeny. In field, such a protection effect was also clearly operating when ruderal plants were implanted in groups (six plants) inside the transgenic donor source. The proportion of transgenic hybrids in receptor plots (containing 60 ruderal beets plus 20 male sterile beets) decreased rapidly with the distance from the transgenic source and was about 0.5% for ruderal beets and 1.5% for male sterile beets, at 200 m from the source. Moreover, the overall values were always higher in the east plots (under prevailing wind) than in the other directions, emphasizing the important role played by the wind in transgene dispersal. Male sterile receptor plots (80 × 2 plants) implanted one km from the transgenic donor source show a transgenic progeny. So even at this distance, pollen was still capable of fertilization. However, the percentage of the transgenic progeny of these male sterile plants was low (about 0.15%) and would be lower in the case of ruderal beets, owing to pollen competition. However, because non-germinated transgenic embryos have also been recovered indicating that care would have to be taken in rotation cultures involving genetically engineered beets.