Select Committee on Agriculture Minutes of Evidence



MEMORANDUM SUBMITTED BY PROFESSOR ALAN GRAY, INSTITUTE OF TERRESTRIAL ECOLOGY (R28)

SEGREGATION DURING GROWTH OF THE CROP—GENE FLOW

  At least in theory, the living products of GM and non-GM crops, whether seed, fruits, leaves or whole plants, can be separated on the farm by strict rules governing their sowing, harvest and handling. (In practice, of course, this may prove to be difficult where GM and conventional crops are grown on the same farm.) Of rather more concern, judging from media coverage, has been the possible mixing of GM and non-GM crops by cross-pollination and hybridisation—probably because this is determined by "natural" processes over which we appear to have little or no control, such as the wind or the vagaries of honeybee behaviour ("bees can carry pollen for many miles").

  This note briefly introduces the issues of segregation of the growing crop. The first, and arguably the most important, point is that the ability to segregate the crop by physical separation varies considerably from crop to crop. In the case of oilseed rape, for example, it is not possible to guarantee full segregation on anything but a regional scale of separation, although, as shown below, much can be done to minimise cross-pollination in this species. With wheat, and to some extent maize, segregation is feasible by growing crops at specified distances apart.

  The basic elements which determine the extent of gene flow at a given distance apart (defined in this note as the transfer of genes by cross-pollination) are the breeding system of the plant (whether it is self-pollinating or outcrossing— and its mode of pollination (by wind or by animal vectors, such as insects). In a sample of seed crops in the USA listed by Levin and Kerster (1974), the average isolation distance for self-fertilising species is around 300 metres ( 150 metres), and that in primarily or exclusively outcrossing species is 800 metres ( 240 metres). In practice, for any particular crop species there is known to be enormous variation in the levels of gene flow, depending not only on distance but on factors such as coincidence of flowering, the period of pollen viability, the variation in weather conditions, the size of the source, and recipient populations, the nature of the intervening vegetation, and so on. There is also considerable year-to-year variation.

  The difficulties of making accurate predictions (ie how much gene flow occurs at a specific distance) are partly illustrated in the figure [not printed]. This gives average dispersal curves for three sites in France in which herbicide-tolerant oilseed rape varieties were grown in adjacent fields; doubly-resistant genotypes were detected in plants grown from seed collected at various distances from the edge of the crop and in volunteers emerging after harvest (Champolivier et al, 1999). The results indicate average hybridisation rates of about 2 per cent at one metre, 0.2 per cent at 20 metres, and less than 0.01 per cent at 65 metres (oilseed rape isself-compatible and between 40-80 per cent of pollinations are self-pollinations). The best mathematical description of such a curve varies from species to species but has a very characteristic shape with a rapid fall from near-neighbour pollinations within a metre or two, according to some exponential power function, and a very long tail with gene flow occuring at extremely rare frequencies, sometimes over considerable distance.

  Dispersal curves such as this have been helpful in determining appropriate isolation distances for small-scale R&D releases of GM crops (so-called Part B releases). Working on a case-by-case basis, the Advisory Committee on Releases to the Environment (ACRE) has been able to agree appropriate isolation distances, sometimes combined with other risk management procedures such as a border or barrier of non-GM plants of the same species around the GM trial. Establishing such isolation has been made a requirement of the consent to conduct this trial.

  The separation distances which would provide acceptable segregation of GM and non-GM crops once the former were grown on a commercial scale presents more of a problem. In the applications to place on the market (Part C applications) considered by ACRE to date, the advice that a GM crop presents a low or effectively zero risk of harm to human health and the environment has included the explicit assumption that low but undefined levels of gene flow between crops (or to wild relatives of that crop) are possible and acceptable (ie do not constitute a hazard). This has been because the inserted gene (transgene) and its effect on the crop or wild relative was not considered sui generis to be a hazard.

  In making this assumption, ACRE has taken note of research such as that at the Scottish Crops Research Institute (eg Squire et al. 1999) on gene flow in oilseed rape at the landscape scale. One study of a patchwork of fields in Tayside used pollen traps, male-sterile bait plants and mathematical modelling to demonstrate a greater complexity and more gene flow than would be predicted from measures of dispersal from single-source fields. In addition, it should be noted that volunteers and feral populations (the latter being very common in oilseed rape growing regions) provide a means of transferring genes from GM to non-GM crops over time (so-called "green bridges"). Since seed from this crop may persist in the soil for 6-10 years, there is a considerable potential for transgenes to move around in space and time in regions where the crop is grown in high density year after year.

  The key question, therefore, becomes "What separation distances on average over a range of conditions will provide `acceptable' isolation of crops?".

  Fortunately for many crops, including those so far considered by ACRE for large-scale release, these distances have been calculated as part of the process of producing seed of known purity. Experience from around the world has led to internationally-accepted isolation distances for various levels of seed purity. In the UK these are governed by a range of legislation (eg for oilseed rape The Oil and Fibre Plant Seeds Regulations 1993 (as amended)), based on practical experience and extensive seed-testing over many years. Thus, to produce Pre-basic and Basic standard seed (with not less than 99.9 per cent purity), oilseed rape varieties must be separated by at least 400 metres. To produce Certified seed, the distance in this species is 200 metres (99.7 per cent purity), and at 50 metres a level of 99.5 per cent purity is achieved. By contrast, for wheat, barley and oats, a compulsory isolation gap of only 2 metres is required, although an isolation distance of 50 metres between different varieties is recommended. For maize, the highest standard of varietal purity requires 200 metres isolation.

  Thus, international seed certification standards provide a guide to the physical segregation of GM and non-GM crops during growth and flowering. For most crops they do not guarantee complete segregation, but when combined with agronomic practices designed to reduce gene flow, and perhaps regional agreements such as those for industrial and food oils in oilseed rape, can lead to extremely low levels of cross-pollination. It is very difficult, if not impossible for a crop such as oilseed rape, to guarantee that cross-pollination will never occur—a situation which emphasises both the need to focus on the risks posed by specific transgenes and the need to develop mechanisms for preventing hybridisation between GM and non-GM crops.

29 November 1999

REFERENCES

  Champolivier, J, Gasquez, J, Messeau, A & Richard-Molard, M (1999) Management of transgenic crops within the cropping system. In: Gene Flow & Agriculture—Relevance for Transgenic Crops, pp 233-240. BCPC Symposium No 72.
  Levin, D A & Kerster, H W (1974) Gene flow in seed plants. Evolutionary Biology 7, 139-220.
  Squire, G R, Crawford, J W, Ramsay, G & Thomson, C. (1999) Gene flow at the landscape level. In: Gene Flow & Agriculture—Relevance for Transgenic Crops, pp 57-64. BCPC Symposium No 72.


 
previous page contents next page

House of Commons home page Parliament home page House of Lords home page search page enquiries

© Parliamentary copyright 2000
Prepared 10 January 2000