May 13, 2002

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Transgenes and pollen dispersal

Oilseed rape is good at outcrossing

With oilseed rape there is a high risk of gene transfer to other rape plants and to related wild species; this is the much-cited key statement of a current study produced by two British agro-ecologists for the European Environment Agency (EEA). Their report has caused an outcry, despite the fact that it simply summarizes previously published studies and some current projects. In early May 2002, Belgium refused approval for deliberate release trials of genetically modified oilseed rape. For GM opponents at Greenpeace, the study justifies the sensational headline "GM plants out of control". And yet although there is a very good chance that oilseed rape will outcross, whether or not this becomes a risk is quite another matter.

“Genetically modified organisms: The significance of gene transfer through pollen transfer” - in their study Katie Eastham and Jeremy Sweet look at six major crop types that have been genetically modified and could be grown commercially in Central Europe in the foreseeable future.

The crop types are characterised according to their outcrossing behaviour. Their ability to transfer genes both to neighbouring plants within the species and to wild relatives via pollen is considered.

Oilseed rape: biological containment virtually impossible

Of all the plants under investigation, oilseed rape has the greatest ability to outcross both crop to crop and crop to wild relatives.

• Pollen dispersal: Oilseed rape pollen is transported over large distances by wind and bees. Isolated outcrossings have been observed more than 4 kilometres away. As a rule, outcrossing rates fall dramatically as distance increases. With a distance of 100 metres separating two oilseed rape fields, genetic material originating from the variety planted in one field is found in 0.5% of the crop in the other field.
• Rape outcrossing to wild relatives: Rape pollen may encounter several wild relatives which could serve as potential crossing partners. The likelihood of this resulting in hybrids is rated as high particularly for turnip rape and, to a lesser extent, for wild cabbage and some types of mustard.
• Herbicide-tolerant weeds: The two ecologists believe that herbicide-tolerant weeds could emerge. However, whether the herbicide-resistance genes from the transgenic plants become established in the gene pool largely depends on whether the gene in question confers a survival advantage in a particular ecosystem.
• Rape outcrossing from crop to crop: If oilseed rape with a genetically engineered herbicide resistance were to be grown in Europe, Eastham and Sweet believe that rape plants with multiple resistance genes would emerge as a result of outcrossing processes.
• Colonisation outside the field: Oilseed rape plants can become established outside cultivated fields. This “volunteer rape” can be involved in outcrossings as a source or recipient.
• Conclusion: As far as Eastham and Sweet are concerned, they are currently unable to make safe, generally applicable assertions about where the crossing barriers between rape and its wild relatives lie. In most cases, evidence about the fitness of offspring and the persistence of genes in the ecosystem relates to individual cases and cannot be generalised. To obtain relevant experience, the EEA study recommends studying gene exchange between different (non-GM) species and populations. Research projects of this type are currently underway in several European countries, including as part of the BMBF’s SiFo biosafety research programme.

Sugar beet, maize, potatoes, wheat and barley

The other crop species included in the EEA study also showed great variation in their outcrossing behaviour.

• Sugar beet and maize readily outcross, although their pollen tends to be transported by wind rather than by insects. Although maize has no wild relatives in these latitudes, gene transfer from cultivated sugar beet to wild beets has already been observed: the genes from cultivated sugar beet were found in the gene pool of a coastal wild beet species in northeast Italy. This region is home to many sugar-beet-breeding facilities.
• Wheat and barley have only a very low outcrossing potential. Being self-pollinated, they tend to prefer their own pollen. Outcrossing to wild relatives appears to be very unlikely for both species. No viable hybrid offspring have yet been found.
• For potatoes too, gene transfer within the species or to wild relatives is extremely unlikely. Potatoes reproduce via tubers. The genes transferred to a potato plant via pollen do not enter its tubers and are therefore not passed on to the next generation. Although wild relatives of potatoes do exist in Europe, a large number of studies failed to identify any fertile offspring.

GM rape – under control after all?

Outcrossing (pollen-mediated gene transfer from one plant to another) is a natural process required for propagation. Many plants have developed the ability to produce large quantities of pollen and to disperse it over great distances. Plant breeders even make use of this process, but they control it by allowing only targeted gene transfer between selected parents. As far back as the beginning of the last century, seed production standards were developed to reduce to an acceptable level undesirable incrossings in the breeding stock via foreign pollen. These included minimum separation distances and other measures to prevent pollen dispersal.

As far as genetically modified crops are concerned, measures are currently being discussed in Europe with a view to minimising undesirable contamination of conventional seed and harvest products with GM pollen.

In their study, Eastham and Sweet recommend

• examining and possibly increasing the conventional separation distances used in seed production.
• In addition, research should be stepped up by developing concepts for biological containment. This involves integrating gene constructs for new traits into the plant genome in such a way that they cannot be transferred to other plants via the pollen.

Outcrossing is not automatically a risk

hether an outcrossing has harmful ecological and economic consequences depends not only on the crop type, but also on the trait of the transferred genes. So genes which confer a survival advantage must be assessed more critically than those which have a more neutral effect.

The EEA study, however, focuses on the likelihood of outcrossing and therefore on only one aspect of the risk assessment of genetically modified plants.