Sir

In the Newsstory1 “Organic farmer takes gene battle to court”, the view is reported that “the probability of cross-pollination, and its effects on the environment and food safety, is too small to be studied effectively”. But the probability of cross-pollination is a different issue from the effect of transgenes on the environment and food, which is not one of probability, but one that is amenable to experimental study. What is a matter of probability is whether cross-pollination can occur at all.

Studies of cross-pollination in maize in the United Kingdom are unlikely to be as extensive or informative as they are for some other well studied indigenous crops such as sugar beet. The probability of genes escaping in beet can be more accurately estimated2,3, which may provide useful guidance for the maize problem.

Although quantification of rate of spread of transgenes through pollen and seed dispersal should be an essential part of risk assessment for genetically modified organisms, greater insight into the movement of genes can be gained by indirect methods. There are several documented cases of crossing having occurred between wild beet and sugar beet, evidence being derived from observations of morphological genetic markers (see, for example, 4–6).

These results accord with a comprehensive assessment of other UK crops7 in which the average isolation distance for outcrossing species is calculated. Figures of 180 m and 3,200 m are quoted for maize and beet, respectively, but the important point is that actual distances of gene flow are almost certainly not only crop-specific but also variety-, site- and season-specific. As a result, the so-called isolation distance quoted in the News story of “about 200 m” for maize could allow as much as 10% of alien pollen to effect fertilization if work on natural populations of radish is a reliable indicator8.

Bearing these sources of inaccuracies in mind, can further scientific studies help to assess the risk of alien gene flow? It is clear that transgenic sugar beet is just as effective as non-transgenic beet at crossing with the wild type. “Weed beets”, considered for many years to result from hybridization of sugar beet with wild beet, have been shown to be of such a hybrid origin using combinations of mitochondrial, chloroplast and nuclear DNA markers9. Microsatellite loci have been used to examine population structure among UK sea-beet populations10, pointing the way to their use for studying gene movement between the beet crop and wild forms. Other marker systems show that sugar beet and wild beet can be effectively discriminated11, and DNA microsatellite studies have demonstrated very high levels of polymorphism that could be used to discriminate between wild beets and sugar-beet varieties12, with a high likelihood of being able to detect hybrids even of an introgressed nature.

The application of such markers is not confined to beets. Hybridization rates between wild Brassica rapa and cultivated B. napus have been reported in Scientific Correspondence13. Experiments such as these are essential to indicate how we can estimate risks of transgenes being released into the environment. They also call into question the view that the probability of cross-pollination is too small to be studied effectively.