By Alex Kirby
BBC News Online environment correspondent
One genetically modified crop proposed for possible use in the UK is certain to produce large numbers of hybrid plants, British researchers say.
The waterloving wild turnip
They believe oilseed rape will breed with a wild relative so that hybrids will be widespread and fairly frequent.
But they say the presence of hybrids poses no automatic risk in itself.
They call their estimate "a first step towards a larger risk assessment", but it does confirm the suspicions of some determined opponents of GM technology.
The researchers are from the University of Reading, the National Institute of Agricultural Botany in Cambridge, and the Centre for Ecology and Hydrology in Dorset.
Their study, Hybridization Between Brassica napus And Brassica rapa On A National Scale In The United Kingdom, is published in Sciencexpress, the online version of the journal Science.
Oilseed rape, known also as canola, is known to breed with B. rapa, whose popular names are bargeman's cabbage or the wild turnip. It grows abundantly along river banks in many parts of England.
Many wild trnips grow by crop fields
One of the worries about GM crops is how far gene flow may enable them to form hybrids and pass on characteristics that have been bred into them, like tolerance to particular herbicides.
There are several ways of suppressing hybridization, including the physical isolation of GM crops, ensuring male plants are sterile, molecular control systems, and chloroplast transformation (inserting the relevant gene into a different part of the plant).
Because the usefulness of these different methods will depend on how many hybrids there are, the team decided to assess likely numbers, using sources including population surveys, remote sensing, and profiles of the way in which pollen is dispersed.
They walked about 300 km (190 miles) of rivers in search of the waterside version of the wild turnip. A genetically slightly different "weedy" version grows away from water.
They estimate that 1.8 million waterside wild turnips are within 30 metres of rapeseed fields in the UK, and expect the probable emergence of 26,000 hybrid plants at these sites.
In calculating the likely rate of hybrid formation by long-range pollen dispersal, they think there will probably be 29 flowering hybrids in the average rape field where weedy wild turnips grow.
Cross-pollination of plants is certain
The weeds are mainly restricted to an area in northern England near the river Humber.
Across the UK, the authors conclude, there are likely to be about 17,000 hybrids annually as a result of long-range pollination of weedy B. rapa.
This is in addition to the 26,000 predicted in waterside populations through pollination from nearby GM crops, and a further 6,000 waterside hybrids caused by pollen transfer.
The researchers conclude: "We infer that widespread, relatively frequent hybrid formation is inevitable from male-fertile GM rapeseed in the UK... the substantial numbers of predicted long-range hybrids means physical isolation would tend only to suppress rather than prevent hybrid formation."
But they add: "The presence of hybrids is not a hazard in itself and does not imply inevitable ecological change... an estimate of hybrid abundance represents only the first step toward a more quantitative assessment of risk at the national level."
Oilseed rape has other wild relatives, but most are sterile and B. rapa is the one thought likeliest to cross-pollinate with the domesticated crop.
The researchers say they think their findings are probably applicable to almost all GM crops. The oilseed rape fields they studied were not a GM crop.