A review of pollen-mediated transgene flow has been published by a European Science Foundation (ESF) programme, 'Assessing the Impact of GM Plants' (AIGM). The AIGM programme includes researchers and other scientists from 10 European countries involved in assessing the environmental and agronomic impact of GM crops, including studies of gene flow and dispersal through pollen, hybridisation and gene introgression.

The newly published report considers the significance of pollen-mediated gene flow from six major crop types that have been genetically modified and are close to commercial release in the European Union. Oilseed rape, sugarbeet, potatoes, maize, wheat and barley are reviewed in detail using recent and current research findings to assess their potential environmental and agronomic impacts.

The AIGM rated oilseed rape as a high-risk crop for crop-to-crop gene flow and from crop to wild relatives. At the farm scale it is expected that low levels of gene flow will occur at long distances and thus it will be difficult to maintain complete genetic isolation. This may be a particular problem with varieties and lines containing male sterile components. These are liable to outcross with neighbouring fully fertile GM oilseed rape at higher frequencies and at greater distances than traditional varieties. Gene stacking in Brassica napus has already been observed in crops and the reports predicts that plants carrying multiple resistance genes will become common post-GM release. This will mean that GM volunteer plants may require different herbicide management. Since oilseed rape is cross-compatible with a number of wild relatives, the likelihood of gene flow to these species is high.

The report suggests that while pollen is important in the spatial dispersal of transgenes from oilseed rape, it has a short life-span and provides little temporal dispersal. Seed is viewed as a more significant contributor to the spatial dispersal of transgenes. The authors of the report conclude that our current status of knowledge of sexual barriers between B. napus and its related species makes predicting how hybrids will form or persist impossible. Until this research has been carried out it is suggested that the risk assessment of gene flow must take into account the specific trait introduced (e.g. herbicide resistance, oil quality), the biology of the plant (self-pollination or cross-pollination, seed dormancy) and the agricultural context (cropping systems, spatial organisation of the crops).

Sugarbeet is classified as medium to high risk for gene flow from crop to crop and from crop to wild relatives by the report. Pollen from sugarbeet has been recorded at distances of more than 1 km at relatively high frequencies. Cross-pollination in root crops is not usually considered an issue since the crop is harvested before flowering. However a small proportion of plants in a crop will bolt and transgene movement between crops may occur in this way. The proportion of bolting plants in sugarbeet root crops is typically less than 1 % and therefore the chances of widespread pollen mediated gene flow between root crops are minimal. It is recommended that GM varieties of sugarbeet should be screened carefully to check for vernalization susceptibility. Cultivation practices such as later sowing and removal of bolters from the crop by hoeing is also suggested as a means of preventing gene flow.

The report acknowledges that hybridisation and introgression between cultivated beet and wild sea beet can occur. Cultivated beet is highly interfertile with a number of agricultural variants also classified within Beta vulgaris ssp. vulgaris, and is highly interfertile with weed, feral and wild sea beet. It is recognised that the seed production areas of Europe present a high risk for gene flow from cultivated beet to wild beet and if the use of transgenic cultivars is inevitable in breeding districts, it is suggested that the use of transgenic male-sterile mother plants for seed production would minimize the gene flow to wild beet populations.

If virus-resistant transgenic sugarbeet varieties are grown, it is suggested that the spread of virus resistance genes into the wild population could have beneficial effects on the crop by removing susceptible plants that provide a source of future infection. However, it has also suggested that increasing the exposure of a virus to the resistance genes may enhance the possibility of the virus developing new strains that overcome the resistance. Management and cultivation practices for transgenic sugarbeet may also be significant in minimizing the possibility of gene flow.

The report regards potatoes as a low risk crop for gene flow from crop to crop and from crop to wild relatives. Cross-pollination between production crops is not usually considered an issue since the harvested tuber is not affected by incoming pollen. In true seed production areas, however, the likelihood of cross-pollination between adjacent crops leading to contamination is higher. The report suggests that the risk of gene flow exists if volunteers are allowed to persist in a field from one crop to the next. It is concluded that naturally occurring hybridisation and introgression between potato and its related wild species in Europe is unlikely.

The extent of pollen dispersal undoubtedly varies with cultivar, climatic conditions during flowering and presence and frequency of pollination vectors. The majority of field studies have detected pollen at a maximum distance of 20 m from the source with the exception of one study that re-corded outcrossing levels of 31 % at 1000 m.

For maize, the report suggests that this crop should be regarded as a medium to high-risk crop for gene flow from crop to crop. Evidence suggests that GM maize plants would cross-pollinate non-GM maize plants up to and beyond their recommended isolation distance of 200 m. Contamination of a conventional maize crop with GM maize may affect the market acceptability of the harvested crop due to reduced quality. It is suggested that further problems may be encountered in maintaining the genetic purity in seed crops. Maize pollen has been shown, by the action of wind, to cross with other cultivars of maize at up to 800 m away and small quantities of pollen are likely to travel much further under suitable atmospheric conditions. The report suggests that the extent of gene flow between GM and non-GM maize crops is mainly dependent on the scale of pollen release and dispersal and on the distances between source and recipient populations. The use of separation distances within fields of 2 ha or more of 200 m to maintain 99 % grain purity and 300 m to maintain 99.5 % grain purity are recommended. The reports concludes that there appear to be no known wild relatives in Europe with which maize can hybridise and so no risk of contamination of wild plants.

The report classifies wheat as a low-risk crop for gene flow from crop to crop and from crop to wild relatives. It concludes that cross-pollination under field conditions normally involves less than 2 % of all florets so any outcrossing usually occurs with adjacent plants. Hybrids formed between wheat and several wild barley and grass species generally appear to be restricted to the first generation with little evidence for subsequent introgression due to sterility. Similarly, barley is also rated as a low risk crop for gene flow from crop to crop and from crop to wild relatives. Barley reproduces almost entirely by self-fertilisation, producing small amounts of pollen so that most outcrossing occurs between closely adjacent plants. There are no records of naturally occurring hybrids between barley and any wild relatives in Europe.

The status of some fruit crops, such as strawberry, apple, grapevine and plum, is less clear. Some of the crops have outcrossing and hybridisation tendencies that suggest that gene flow from GM crops to other crops and to wild relatives is likely to occur. The report suggests that for raspberry, blackberry and blackcurrant the likelihood of gene flow is less easy to predict, partly due to lack of available information. At present none of these crops has pollen which can be completely contained. This means that the movement of seed and pollen will have to be measured and managed much more in the future.

The report concludes with some recommendations for techniques and management systems that could be used to minimise direct gene flow between crops, and to minimise seed bank and volunteer populations. The use of isolation zones, crop barrier rows and other vegetation barriers between pollen source and recipient crops are recommended as a means of reducing pollen dispersal. However, it is acknowledged that changing weather and environmental conditions will mean that some long distance pollen dispersal will occur.

The authors of the report suggest that the possible implications of hybridisation and introgression between crops and wild plant species are so far unclear because it is difficult to predict how the genetically engineered genes will be expressed in a related wild species. The fitness of wild plant species containing introgressed genes from a GM crop will depend on many factors involving both the genes introgressed and the recipient ecosystem. The report states that while it is important to determine frequencies of hybridisation between crops and wild relatives, it is more important to determine whether genes will be introgressed into wild populations and establish at levels which will have a significant ecological impact.

More research into biological and physical barriers that can minimize gene transfer through cross pollination is recommended by the report. Seed persistence and dispersal can lead to gene flow via temporal and spatial means and it is suggested that more information is needed on the role of seed banks and dispersed seed of GM crops on contamination of subsequent crops. Further research is also needed in order to provide good scientific information on both seed and pollen mediated gene flow if better management systems and steward-ship schemes are to be devised to minimise GM contamination.

As regards future monitoring of experimental and commercial releases of GM crops, the report recommends that this should be based on good scientific knowledge of the behaviour and ecology of the GM crop and its wild relatives. Further studies on gene flow and introgression are viewed as a key part of this requirement.

The full text of the report, Genetically Modified Organisms (GMOs): The Significance Of Gene Flow Through Pollen Transfer, is available at the European Environment Agency website. The report’s authors were Katie Eastham and Jeremy Sweet with contributions from other participants in the AIGM programme.

Contact: Dr J. B. Sweet, NIAB, Huntingdon Road, Cambridge, CB3 0LE, UK.
Email: jeremy.sweet@niab.com
URL: http://www.eea.eu.int/.