Expectations
It was originally assumed that genetically engineered plants would only very rarely spread in the environment, and persistence was improbable. According to the theory, the introduction of additional genes is effectively a disadvantage for the plants, as it will result in them being rapidly displaced in competition with other plants. Any spread of genetically engineered plants into respective centres of biodiversity is considered unlikely as long as there is no cultivation of genetically engineered crops in these regions.

Reality
Genetically engineered cotton has been spreading uncontrollably for several years in wild cotton populations in Mexico. Transgenes from herbicide-resistant and insecticidal cotton were evidently transferred into natural cotton populations (Gossypium hirsutum), even though these genetically engineered crops were not officially cultivated in this region at all.

Cotton plants produce a type of nectar whose production is increased by the infestation of pest insects feeding on the plants. This attracts predatory ant species, which then eat the insects feeding on the plants, and thus protect the plant in return. It has been observed that the production of plant nectar, as well as the number and composition of the associated ant populations, can differ in the offspring of transgenic cotton plants compared to the wild cotton plants.

Ants are not only important in controlling pests, but also in the dispersal of the cotton seeds. Therefore, any disturbance in the interactions between the transgenic plants and their environment can have significant long-term consequences. Higher nectar production, which attracts more ants, could result in the offspring of genetically engineered cotton acquiring invasive properties.

In fact, transgenic cotton plants are now spreading faster in wild populations than was originally expected. This may in part be due to the genetically engineered traits (insect toxicity and herbicide resistance), or a combination of these traits and associated side effects. New combinations of transgenes never tested in the laboratory have also been observed in the actively spreading plant populations.

Successive (hybrid) generations may have very different characteristics to the plants originally authorised for cultivation, e. g. greater invasiveness, which may enable them to spread even faster. Hence, there is a risk that natural cotton populations will be displaced by the spread of transgenic plants.

Consequences
The above case study illustrates how unintended genetic and metabolic interactions caused by genetic modification can promote the spread of transgenic plants. In this case, the damage is considerable as it poses a threat to one of the centres of biodiversity for wild cotton.

Against this backdrop, there is an obvious need to introduce criteria for the discontinuation of approval processes if uncontrolled spread in the environment cannot be definitively ruled out. There should, in addition, be an international consensus prohibiting the release of genetically engineered organisms if their spread in the environment cannot be controlled.

Further information:
TA report
Environmental risks new genetic engineering
Video on environmental risks