One big family

Oilseed rape hybrids are seldom: There are numerous studies and experiments on the outcrossing behaviour of oilseed rape. Pollination trials conducted in semi- realistic conditions indicate that oilseed rape can outcross with turnip, wild radish and several species of mustard and produce viable "hybrid seed".

Whether outcrossing actually occurs in natural conditions depends on various different factors.

• Wild species capable of crossing must be present in the region where the GM oilseed rape is grown or released.

• The transgenic rape and potential crossing partners must flower at the same time. Winter rape sown in autumn flowers earlier than many of its potential crossing partners such as wild radish and field mustard.

• The outcrossing must produce viable (fertile) offspring, but this is hampered by the fact that related varieties have different sets of chromosomes.

This is all to do with plant biology and whether conventional or genetically modified rape is involved in these processes is irrelevant.

If a viable rape hybrid were to arise, it would be theoretically possible for it to spread within an ecosystem and to suppress other related species. The only circumstance in which such a scenario could occur, however, is if the trait transferred from the cultivated rape conferred a survival advantage compared with "normal" plants of the same species. This applies to a few traits only.

• If, for example, genetically modified rape with herbicide resistance were grown, the relevant gene could transfer not only to other rape plants in the area, but also to related wild species. The wild plants would subsequently become resistant to the complementary herbicide . However, they would have a survival advantage only if this herbicide were also present in their environment, i.e. if it were sprayed on fields and adjacent areas, for example. If this occurred over several years, the herbicide-resistant plants could gradually begin to dominate. They would then become farmland weeds which could no longer be controlled with the herbicide.

• If a gene which modifies the composition of fatty acids is inserted into cultivated rape, a transfer to wild related species would probably have little effect. The extra fatty acid gene would hardly enhance the plant's fitness . However, indirect effects cannot be completely ruled out. For example, the plants' modified fatty acid composition could alter their susceptibility to disease or chewing pests.

The likelihood of an outcrossing to rape-related weed species may well be low, but it is a natural process in rape. In field conditions wild types of turnip in particular make potential crossing partners, as do wild radish and field mustard to a lesser extent. In terms of outcrossing behaviour, there are no fundamental differences between genetically modified and conventional oilseed rape.