The exchange of genetic material between different organisms
Horizontal and vertical gene transfer are different phenomena and have different implications for the safety of genetically modified crops.
Horizontal gene transfer
This refers to the transmission and/or absorption of genetic material not involved with sexual reproduction and independent of acknowledged species boundaries. Depending on a variety of conditions, horizontal gene transfer (e.g. from a plant to a soil bacterium) may be possible, but very seldom occurs under natural conditions.
In the safety assessment of genetically modified plants, both the possibility and the probability of the foreign gene being transmitted by horizontal gene transfer has to be considered. For example, the antibiotic resistance genes used as marker genes might be transferred from a transgenic plant to soil or intestinal bacteria. Regulators need to ask if horizontal gene transfer is likely, and if it were to occur, what the consequences could be.
Vertical gene transfer
Two plants crossing sexually and passing their genes on to following generations is an example of vertical gene transfer (usually called “crossing”).
Gene transfer via pollen between plants of the same or related species takes place in the wild. Thus, the transfer of disease and pest resistance from cultivated plants to related wild species and vice versa has always taken place, regardless of how the resistance genes were acquired in the first place.
If crossing were to take place between a transgenic and a non-transgenic crop, the latter could acquire a novel gene. The safety assessment of transgenic plants includes assessing both the possibility of out-crossing to close relatives and evaluating the potential ecological consequences if it were to occur.
This includes a consideration of whether the transgenic cultivated plant is likely to find a related wild plant as an out-crossing partner. If the answer is “yes”, the consequences of a gene transfer must be assessed: could the foreign gene confer a trait that could enable the plant to gain an unfair advantage in the wild? What consequences might the dispersion of herbicide resistance genes have on wild species?