Gene flow from transgenic crops to wild relatives

Abstract

Gene flow from transgenic crops to wild relatives resulting in transgenic hybrid populations is a possible outcome of the utilization of genetically modified crops. In order to develop a transgene flow model, canola (Brassica napus) was transformed with two GFP constructs, mGFP5er (GFP) and pSAM 12 (GFP linked to a synthetic Bacillus thuringiensis (Bt) cry1Ac gene). GFP fluorescence was analyzed during leaf development, and was variable at each leaf position over time and among different leaves on the same plant. Nine GFP/Bt and three GFP transgenic events were hybridized and backcrossed two generations with a wild relative, Brassica rapa. The resultant hybrids were insecticidal to corn earworm larvae to the same degree as the transgenic canola parents, and homozygous canola lines and hybrid populations that contained individuals homozygous for GFP demonstrated significantly higher fluorescence than hemizygous hybrid generations.

Two experiments were performed to evaluate the consequences of introgression of transgenes into hybrid generations on crop production. First, vegetative growth potential and nitrogen use efficiency was analyzed for progressing transgenic hybrid generations along with transgenic Brassica napus and non-transgenic B. rapa. Under optimal conditions, B. rapa exhibited higher rates of growth when compared to canola and transgenic hybrid generations. Second, the competitive ability of transgenic hybrids, non-transgenic hybrids, canola, and B. rapa when grown with a model crop, wheat (Triticum aestivum), was analyzed, and transgenic hybrids were the least competitive with wheat when compared to the other Brassica competitors (25.5% versus 47.1% crop reduction, respectively).

Gene flow of GFP and Bt transgenes was quantified under field experiments. Under a high crop to wild relative ratio (600:1), hybridization frequency with B. rapa differed among transgenic canola events (ranging from ca. 4% to ca. 22%). Under a lower crop to wild relative ratio (180:1), hybridization frequency with B. rapa was consistent among the canola lines at ca. 2%. Backcrossing frequency between interspecific hybrids and B. rapa was determined in two field experiments. Seeds produced from maternal hybrid plants were ca. 47.5% transgenic backcrosses, and when the wild relative served as the maternal parent, backcrossing frequencies were low at ca. 0.074%.