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.
|
