Aug 6, 2003

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Gene flow from crops to wild relatives

Spreading even where there is no advantage

Scientists in the US have used mathematical models to calculate the gene flow from crops to wild relatives. They have concluded that crop genes can become established in wild populations within just a few generations, even if the amount of pollen is not very high and the genes in question bring no advantage to the plant.

Genes from crops can enter the gene pool of their wild relatives via pollen and establish themselves there. Although this natural process is not new, in the context of the deliberate release of transgenic plants it is an explosive issue. After all, the transgenes could also become established in wild populations. The likelihood of the spread of genetically introduced genes and, in particular, the assessment of the potential consequences has often been the focus of intense debate.

Outcrossing happens - but what does it signify? Sunflowers: A team led by US ecologist Allison Snow discovered that a newly introduced Bt gene which protects sunflowers from insect damage gives the plant a survival advantage. Since feral sunflowers are regarded as a weed in some parts of the USA, problems could arise if the genes spread in wild populations.

Maize: It is particularly important to preserve the existing, highly diverse gene pool in areas crops originate from and where many wild relatives can still be found. Californian scientists caused quite a stir when they disclosed that they had found DNA from genetically modified maize in Mexican native varieties: Were the methods used by the scientists reliable? How could the transgenes have got there? What might the consequences be?

Researchers at the universities of Wisconsin and Minnesota have recently published their research findings in the “Proceedings of the Royal Society of London”, with the aim of making a contribution to this debate.

When developing their models, they focussed on two processes which play a decisive role in the evolution of plant communities: “genetic assimilation”, whereby genes in wild plants are replaced by crop genes, and “demographic swamping”, whereby the wild population declines because the hybrid offspring are less fit. Using their models, it should be possible to calculate how these two natural processes are influenced by rates of pollen flow and selection mechanisms.

The researchers initially started with a simplified basic model where a large, wild population of constant size receives pollen from a crop that differs from the wild relative by only a single gene. At distances ranging from ten to one hundred metres between the crop plants and the wild relatives, the calculations showed that there is a 90% probability that this gene will spread (and become established) in the wild population within 16 to 20 generations.

The natural processes were simulated in a more complex and realistic way in the expanded models by changing various parameters including the size of the wild population, the number of different or new genes and the rate of pollen flow.

With regard to genetic assimilation, all the models indicated that crop genes can become established in wild populations within a few generations. Sometimes just a very slight increase in the rate of pollen flow can make a big difference to the spread of the gene. Even genes which offer the plant no selective advantage can become established under certain conditions.

Just a small increase in pollen contamination can be enough to cause demographic swamping. In extreme cases it can lead to a drastic reduction in the wild population, known as “migrational meltdown”. Wild populations with low growth rates seem to be particularly sensitive to gene flow from crops.

The two processes are interlinked and can be mutually reinforcing.

The authors of the study stress that their models do not distinguish between crops produced through genetic engineering and traditionally bred crops. According to Ralph Haygood, one of the authors, how the genes get into the plants is not the issue, but what they do when they get there.