Topics: Gene flow, detection and impact

The science review centres on particular scientific topics being debated widely by the science community and others. The Forum closed for contributions on 15 October.

This section of the science review considers the status of our current knowledge on the movement of genes between organisms (known as gene flow), methods for gene detection, and means of controlling gene flow. In the context of genetically modified organisms, gene flow is important for risk assessment. For GM crops it is also relevant to the terms on which conventional, organic and GM agricultures might co-exist in future, because some farmers and consumers may want to avoid an incidental GM presence in their crops or food, or have a limit on the amount of GM presence.

The issues are wide-ranging. A few examples are listed which have been debated over recent years by scientists engaged in GM research, and these are mentioned here to help prime the review. We hope that scientists working in areas outside GM will make contributions that might offer insights.

The extent to which transgenes move from the genetically modified organisms to other organisms in the environment has important implications for determining the scope of risk assessments. For example, if transgene escape to wild relatives is possible, the impact of this is included in environmental risk assessments.

Co-existence is not purely a scientific issue, but the science on gene transfer between plants can help to clarify important questions - in what circumstances, by what mechanisms and to what extent will gene transfer occur? Science can also inform the consideration of possible measures to limit GM transfer.

An example of gene flow between crops is cross-pollination. If a field of GM maize is grown alongside a field of non-GM maize, pollen from the former is likely to fertilise the latter, resulting in a GM presence. The distance between compatible crops influences the degree of cross-pollination, and separation distances are a known mechanism for maintaining genetic purity and are used in the seed production industry.

Other pathways for gene flow are 'volunteers' - plants that grow from seed spilt by one crop into a following crop - and GM impurities in crop seed. GM and non-GM material may also become mixed after crops have been harvested - in storage or further down the production chain.

Gene flow from GM crops is therefore an obstacle to the co-existence of discrete agricultural systems (conventional farming, organic farming, and GM agriculture). It raises a supply chain problem because of a need for seed segregation, a spatial problem because hybridisation frequency is a function of separation distance, and a temporal problem because volunteers (remnants from previous crops) can become reservoirs for gene exchange after the original crop has been harvested.

Knowledge of the identity of the GMO is also necessary for the regulatory bodies that control the release of GMOs. On a global scale, detection and identification of non-approved GM products pose a number of technical challenges.

Key issues

  • Natural barriers to hybridisation are widespread amongst organisms, but how effective are these barriers? How good is our understanding of the mechanisms of cross-pollination?
  • In nature, how important and prevalent is horizontal gene transfer: the transfer of genes between unrelated organisms e.g. between plants and bacteria and do GM sequences make this more likely to occur?
  • If modified genes transfer to wild relatives, will they perpetuate or be lost?
  • What do we know about the spread of natural genes between conventional crops, feral populations and wild relatives on a regional scale and does this offer a rational basis for predicting transgenic gene flow?
  • Where cross-pollination is possible between crops, are separation distances effective in containing genes? Can we quantify this and with what level of confidence?
  • Is crop rotation a useful way of reducing gene cycling between crops, wild relatives and feral populations?
  • Could future developments in the design of GM crops prevent modified gene flow?
  • What are the technical limits for reliably detecting modified genes in supply chains and what approaches might be used to trace them cost effectively? Could GM seeds for example be designed so that they are visible to the untrained eye?
  • How do we detect imports of unapproved GM seeds?


Gene flow from genetically modified crops - A background paper that reviews gene flow from a risk assessment perspective. You may find it helpful in setting out some of the issues.

Gene flow through pollen transfer - Review commissioned by the European Science Foundation and the European Environment Agency on pollen-mediated gene flow, focusing on six major crop types oilseed rape, sugar beet, potatoes, maize, wheat and barley.

EU Co-existence report - This report describes scenarios for co-existence of genetically modified, conventional and organic crops in European agriculture.

Review of knowledge of the potential impacts of GMOs on organic agriculture

Horizontal Gene Transfer - Genetically modified crops and soil bacteria. This advice from the Advisory Committee on Releases to the Environment (ACRE), considers two scientific publications concerned with measuring this process in soil. You may find it helpful in setting out the issues and the way scientific advisory committees deal with them.

Containing Gene Flow? - Guidance prepared by the Advisory Committee on Releases to the Environment (ACRE) on principles of best practice in GM crops design which includes reference to the possibilities of minimizing or preventing gene flow in the environment.

Direct measurement of the transfer rate of chloroplast DNA into the nucleus -

Monitoring large scale releases of genetically modified crops (EPG 1/5/84) incorporating report on project EPG 1/5/30: monitoring releases of genetically modified crop plants - The report represents the combined final reports of two separate Defra monitoring contracts run between 1994-1997 and 1997-2000

Defra Research Report 17: The Risks and Consequences of Gene Transfer from Genetically-Manipulated micro-organisms in the Environment (Adobe Acrobat format, 100kb)