Seeds may be a bigger danger than pollen in allowing GM crops to escape into the countryside.
By Helen Briggs
BBC News Online science reporter
Seeds can be carried long distances on farm machinery to cross with wild relatives, a French study has found.
Genes from commercial sugar beet turned up in wild plants growing more than 1.5 kilometres away, according to scientists at Lille University.
Seeds can be spread in soil on farm machinery
It suggests GM crops are likely to jump the confines of any buffer zone imposed.
"If GM sugar beets are established in regions where populations of the wild form also occur, then gene flow between wild and cultivated relatives is almost inevitable," says lead author Dr Jean-Francois Arnaud.
The French study looked at sugar beet, a crop that can easily cross breed with closely related wild plants such as sea beet.
The scientists used molecular biology techniques to see whether genes from the commercial crop could spread across the countryside.
They took DNA from plants in three areas: the field itself, wild sea beet growing some 1.5 km away and a linking "contact zone".
There must now be a major re-think on whether or not GM and non GM crops can co-exist
Pete Riley, Friends of the Earth
The scientists found there was gene flow from the commercial crop to the wild.
They believe the genes were spread accidentally by human means in seeds rather than by pollen.
"Accidental transport of seeds within soils carried on motor vehicles, or by other normal agricultural activities is the best explanation," says Dr Arnaud. "Our findings are consistent with the hypothesis of human-mediated long-distance dispersal."
The study, published in a journal of the Royal Society, comes as a nationwide debate on GM crops takes place in the UK.
The British government is due to decide later this year whether to license commercial GM crops.
The results of the farm-scale trials of four crops proposed for Britain, carried out over the last four years, will be revealed in September.
The French research shows that we are still only beginning to learn about the potential long term impacts of GM crops on the environment, says Pete Riley, GM campaigner for the green pressure group Friends of the Earth.
"There must now be a major re-think on whether or not GM and non GM crops can co-exist," he says. "The government must take note and refuse to allow the GM crops to be commercially grown in the UK."