www.checkbiotech.org ; www.raupp.info ; www.czu.cz

With the inscription of 17 genetically modified (GM) maize (Zea mays L.)
varieties derived from the event MON810 in the common catalogue of varieties
of agricultural plant species of the European Union (EU) on 8 September
2004, the acreage of MON810 hybrids increased in Germany, France, and Spain,
and their commercial cultivation expanded to the Czech Republic and Portugal
in 2005 by Yann Devos, Dirk Reheul and Adinda De Schrijver.

On 14 December 2005, Germany accepted the listing of 3 GM MON810 hybrids
in the national catalogue, and on 30 December 2005, 14 additional Spanish GM
MON810 hybrids entered the common EU catalogue. These evolutions may further
boost the adoption of transgenic maize by European farmers and illustrate
the urgent need for legal and practical frames dealing with coexistence in
order to maintain conventional, organic, and genetically modified (GM) crop
production, and to guarantee a high degree of consumer choice.

In the EU, specific tolerance thresholds have been established or are
discussed for the adventitious and technically unavoidable presence of
approved GM material in non-GM produce: 0.9% for food and feed, 0.3-0.7% for
seeds (crop specific), and 0.1-0.9% for organic produce (country specific).
In addition to the mentioned thresholds, the product needs to be labeled as
consisting of, containing, or produced from a genetically modified organism
(GMO). In the case of maize seeds, a threshold of 0.3% is currently
proposed.

Member states will impose strict technical management measures to keep the
adventitious presence of GM material in non-GM produce below the labeling
thresholds. As maize is a cross-pollinated crop relying on wind for
dispersal of its pollen, on-farm measures may rely on spatial isolation.1
The task may be difficult, since various biological, physical, experimental,
and analytical parameters with varying levels of importance have been
identified to play a role in the study of cross-fertilization in maize. The
number of variables and their variability may hamper the comparison between
research results and make it difficult to define the appropriate length of
isolation distances and/or pollen barriers. How some of the parameters can
hamper the comparison between research results is addressed below.1

- Definition of isolation distance and pollen barrier: Although the terms
isolation distance and pollen barrier (or buffer zone) are clearly distinct,
they are regularly confused in the scientific literature. An isolation
distance separates fields by a zone of open ground or a zone with low
growing crops, while a pollen barrier consists of plants that are sown or
planted around the source or recipient field. If outer parts of fields
function as a barrier, the distance between inner parts increases. Barriers
may also produce competing pollen (if the barrier is of the same species as
the crop) and/or may serve as a physical barrier to air flow and
consequently pollen flow. A pollen barrier of maize has been proven to
reduce cross-fertilization levels more effectively than an isolation
distance of the same length.2 For the future, it might be advisable to match
the common vocabulary to similar definitions.

- Measuring cross-fertilization: Cross-fertilization is measured in
different ways. Out-crossing may be noted in the following ways: (1) in the
hybrid ears by phenotypic markers (e.g., xenia); (2) by detecting off-types
in hybrid progeny; (3) by exposing the seedlings to an appropriate selection
pressure (e.g., herbicide treatment in case of herbicide-resistant plants);
and (4) by the qualitative detection of transgenic DNA and/or proteins in
the seeds or seedlings. None of these methods quantifies the share of
transgenic DNA. A quantitative DNA analysis expresses the GMO content as a
percentage of haploid genomes. However, the latter results differ depending
on the genetic constitution of the analyzed tissue (zygotic or maternal),
the relative shares of these tissues in the sample, the ploidy levels of the
tissue (triploid endosperm vs. diploid maternal tissue), the moment of
sampling (early or late stage of kernel development), the copy number of
transgenic DNA, and the DNA extractability, which may differ between plant
tissues.3 As a consequence, results based on quantitative DNA analyses are
not smoothly convertible to results based on qualitative analyses.

- Hemizygosity: In the production of current GM hybrid varieties, the
transgene generally is present in either the seed parent or the pollinator:
as a result GM hybrids are hemizygous for the transgenic trait. Hence only
half of the pollen produced on the hybrid carries the transgene, and only
half of the cross-fertilization is measured compared to a pollen donor that
is homozygous for the screened trait.

- Analyzed plant tissue: The material to be analyzed for the adventitious
presence of GM material depends on the use of maize. In grain maize,
adventitious mixing is restricted to the grain fraction of the plant: the
cross-fertilization level is expressed per grain lot. In corn cob mix and in
fodder maize, transgene presence is diluted if expressed as a percentage of
genomes since vegetative plant parts (maternal tissue) are included in the
harvested material. In non-processed fresh sweet maize, cross-fertilization
is expressed per individual ear.

- Experimental design: The results of field trials will differ according to
the implemented design. In different studies, small recipient plots or even
individual plants have been planted at various distances from a source in
order to measure how far viable maize pollen can successfully fertilize a
maize ovule. Such designs do not reflect the real agricultural situation and
are not suited to quantify the adventitious GMO content of recipient fields
of commercial size. Individual plants or small recipient plots are much more
prone to cross-fertilization than large recipient fields, which may result
in an overestimation of the out-crossing level when making extrapolations.
Recent studies carried out in France5, Germany6, Spain7, and the UK8
mimicked worst-case commercial on-farm situations (e.g., pollen source next
to or completely surrounded by a recipient field) with a trend towards
out-crossing studies in real agricultural situations.9 As the probability of
cross-fertilization diminishes with increasing distances, sampling was
performed at different positions within the recipient fields in order to
calculate the average percentage of cross-fertilization over the whole
field. The recommendations previously made for isolation distances and/or
pollen barriers, based on discrete out-crossing levels, may therefore be too
conservative and thus larger than the ones actually needed.

Apart from the previously discussed parameters, out-crossing is also
affected by the distance between the pollen source and recipient; size,
shape, and orientation of the pollen source and recipient; wind
characteristics; rain; local environment; pollen viability; water status of
pollen; climatic conditions; male fertility; and flowering synchrony.1 When
research results are compared in order to define the appropriate isolation
distances and/or pollen barriers limiting out-crossing, the various
parameters at play should always be considered.

References

1. Devos Y, Reheul D & De Schrijver A (2005) The co-existence between
transgenic and non-transgenic maize in the European Union: a focus on pollen
flow and cross-fertilization. Environ. Biosafety Res. 4, 71-87

2. Melé E, Pe?as G, Serra J, Salvia J, Ballester J, Bas M, Palaudelm?s M &
Messeguer J (2005) Quantification of pollen gene flow in large maize fields
by using a kernel colour trait. In Messéan A, ed, Proceedings of the 2nd
International Conference on Co-existence between GM and non-GM based
agricultural supply chains, Agropolis Productions, pp. 289-291

3. Taverniers I (2005) Development and implementation of strategies for GMO
quantification in an evolving European context. Ph.D. thesis, University of
Ghent, Ghent, Belgium

4. Trifa Y & Zhang D (2004) DNA content in embryo and endosperm of maize
kernel (Zea mays L.): impact on GMO quantification. J. Agric. Food Chem. 52,
1044-1048

5. Bénétrix F, Foueillassar X & Poeydomenge C (2005) Coexistence OGM, non
OGM: des outils opérationnels pour gérer les productions. Perspectives
agricoles N° 317, 8-11

6. Weber WE, Bringezu T, Broer I, Holz F & Eder J (2005) Koexistenz von
gentechnisch verändertem und konventionellem Mais. Mais 1+2, 1-6

7. Melé E (2004) Spanish study shows that coexistence is possible. ABIC 3, 2

8. Henry C, Morgan D, Weekes R, Daniels R & Boffey C (2003) Farm scale
evaluations of GM crops: monitoring gene flow from GM crops to non-GM
equivalent crops in the vicinity: part I: forage maize. DEFRA report EPG
1/5/138

9. Messeguer J, Pe?as G, Ballester J, Serra J, Salvia J, Bas M & Melé E
(2005) Pollen mediated gene flow in maize in real situations of
co-existence. In Messéan A, ed, Proceedings of the 2nd International
Conference on Co-existence between GM and non-GM based agricultural supply
chains, Agropolis Productions, Montpellier, pp. 83-87

[www.isb.vt.edu]

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