SummaryWe present the first study on cross-fertilization between Bt and conventional maize in real situations of coexistence in two regions in which Bt and conventional maize were cultivated.A map was designed and the different crops were identified, as were the sowing and flowering dates, in Bt and conventional maize fields. These data were used to choose the non-transgenic fields for sampling and analysis by the real-time quantification systempolymerase chain reaction (RTQ-PCR) technique. In general, the rate of cross-fertilization was higher in the borders and, in most of the fields, decreased towards the centre of the field. Nine fields had values of genetically modified organism DNA to total DNA of much lower than 0.9%, whereas in three the rate was higher. Some differences were found when comparing our results with those of common field trials. In real conditions of coexistence and in cropping areas with smaller fields, the main factors that determined cross-pollination were the synchronicity of flowering and the distances between the donor and receptor fields. By establishing an index based on these two variables, the rate of the adventitious presence of genetically modified maize could be predicted, as well as the influence of other factors. By applying this index, and in the case of a fully synchronous flowering time, a security distance between transgenic and conventional fields of about 20 m should be sufficient to maintain the adventitious presence of genetically modified organisms as a result of pollen flow below the 0.9% threshold in the total yield of the field.

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Regulatory approvals for deliberate release of GM maize events into the environment have lead to real situations of coexistence between GM and non-GM, with some fields being cultivated with GM and conventional varieties in successive seasons. Given the common presence of volunteer plants in maize fields in temperate areas, we investigated the real impact of GM volunteers on the yield of 12 non-GM agricultural fields. Volunteer density varied from residual to around 10% of plants in the field and was largely reduced using certain cultural practices. Plant vigour was low, they rarely had cobs and produced pollen that cross-fertilized neighbour plants only at low--but variable--levels. In the worst-case scenario, the estimated content of GMO was 0.16%. The influence of GM volunteers was not enough to reach the 0.9% adventitious GM threshold but it could potentially contribute to adventitious GM levels, especially at high initial densities (i.e. above 1,000 volunteers/ha).

The introduction of genetically modified organisms (GMO) in many countries follows strict regulations to assure that only products that have been safety tested in relation to human health and the environment are marketed. Thus, GMOs must be authorized before use. By complementing more targeted approaches, profiling methods can assess possible unintended effects of transformation. We used microarrays to compare the transcriptome profiles of widely commercialized maize MON810 varieties and their non-GM near-isogenic counterparts. The expression profiles of MON810 seedlings are more similar to those of their corresponding near-isogenic varieties than are the profiles of other lines produced by conventional breeding. However, differential expression of approximately 1.7 and approximately 0.1% of transcripts was identified in two variety pairs (AristisBt/Aristis and PR33P67/PR33P66) that had similar cryIA(b) mRNA levels, demonstrating that commercial varieties of the same event have different similarity levels to their near-isogenic counterparts without the transgene (note that these two pairs also show phenotypic differences). In the tissues, developmental stage and varieties analyzed, we could not identify any gene differentially expressed in all variety-pairs. However, a small set of sequences were differentially expressed in various pairs. Their relation to the transgenesis was not proven, although this is likely to be modulated by the genetic background of each variety.