Philip Davies
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Philip Davies
The
cultivation of genetically engineered (GE) crops has attracted much
controversy. People are concerned about
effects of GE crops on human health and on the environment, following on from
earlier harmful agricultural technologies, including insecticides, herbicides,
nematicides, fungicides and mineral fertilisers. A key issue is the containment
of GE crops and modified genes after they are released into the environment.
Questions frequently raised include: Will GE crops become weeds difficult to
control? Can engineered genes escape from the crop into weed species and create
more aggressive weeds? Will engineered genes escape from the crop to pathogenic
bacteria or viruses? Can GE crop products be separated from non-GE products to
allow people the choice of eating non-GE products?
Can
the purity of non-GE crops be maintained to meet marketing requirements?
A responsible approach to regulating GE crops would ensure that such
questions are rigorously addressed before the release of the GE crop into the
environment. This, however, this has not been the case, and there is an
increasing number of examples of genetic
pollution - engineered genes contaminating crops, grain samples and foods.
In Canada, where GE canola has been grown for several
years, there are clear examples of cross-pollination between neighbouring GE
crops genetically modified for resistance to different herbicides, and non-GE
crops. Progeny grown from the cross-pollinated seed are resistant to multiple
herbicides. This has created problems
for chemical control of plants containing multiple resistance.1 It
has also created legal problems for a number of North American farmers who have
been found to have GE canola growing on their properties without the required
licensing agreements. In many cases, the GE canola is likely to have
contaminated the farmers’ paddocks through windborne seed or pollen. However
the courts have determined that it does not matter how the GE plants came to be
there, the fact that they were growing without a licence agreement is enough to
breach the genetic engineers’ intellectual property rights.2 These
cases have been very costly to the farmers concerned. Similar problems are
likely to follow in Australia with any commercial release of GE canola (see
also Chapters 3 and 7).
In the United States, a GE variety of maize known as
StarLink has been grown extensively. It was approved as an animal feed, but not
for human consumption because of its potential to trigger allergic reactions.
Despite attempts to segregate StarLink maize, there have been many reports of
food-grade maize contaminated with StarLink grain.3 This genetic pollution
has presented an unnecessary risk to human health. It has cost more than US $1
billion to test and recall contaminated products. Japan and the European Union have rejected many export shipments
containing traces of the StarLink grain.
Traces of StarLink maize have also been detected in
food products imported into the Australian food supply.4 Australia
imported GE maize from the USA in December 2002 with the condition imposed by
the Australian Gene Technology Regulator that it be used only as animal feed.
In February 2003, it was reported that some of this GE maize had contaminated a
silo of wheat awaiting export.5 Fortunately for the grain exporters,
the contamination was detected early enough to remove the GE maize before
loading the ship; detection by the importing countries may have led to
rejection of the shipment on arrival.
The development of experimental GE crops for the
production of pharmaceutical products also caused public concern when it was found
that a soya bean crop grown in the United States had been contaminated with
maize genetically engineered to produce a pig vaccine.6 The maize
weeds found in the soya bean field grew from seed spilt during the previous
season’s GE maize harvest. The contaminated crop was destroyed. This event also
highlights the ease with which crops containing pharmaceuticals may contaminate
food crops.
A longer-term consequence of genetic pollution is the
contamination of traditional land race
crop varieties, essential for maintaining the genetic diversity necessary for
sustainable crop production and development of new crop varieties.
Contamination of the traditional maize landraces in Mexico was first reported
in November 2001.7 Following initial controversy about the validity
of this report,8 further experiments have indicated that the
contamination does exist, although explanations of how it came about are
inconclusive.9 Although the consequences of contaminating the
centres of crop genetic diversity with artificial genes cannot reliably be
predicted, it is well recognised that these valuable gene pools should be
preserved in their natural state (as Chapter 14 also highlights with regard to
genetic erosion caused by the spread of monoculture crops).
Public concern about GE crops is now highly evident,
and concern among scientists is gaining increasing exposure in a media
environment not known for promoting open debate or diversity of opinion.10
It is essential that transparent and rigorous scientific and public evaluation
of GE crops be undertaken to protect our natural heritage. To contribute to that evaluation, this
chapter first discusses the role of plants in sustaining life, then gene
dispersal through pollen, and finally the potential hazards of genetically
engineered plants. With regard to the
latter, I present seven hypotheses related to the proposition that, because of
gene flow, GE crops will have significant effects on the environment and human
health.
THE ROLE OF PLANTS IN SUSTAINING LIFE
Plants
are the foundation for life on earth. Through photosynthesis, they harness the
sun’s energy to fuel the biochemical processes essential for the existence of
the diversity of life around us. Driving the growth of animals, the flight of
birds, the movement of fish through the oceans, is the energy that plants
capture from the sun.
Although plants provide the energy for biological
activity, they are dependent on other organisms for their long-term survival
and propagation. Plants could not exist in the absence of the diverse array of
organisms that form natural ecosystems. The flowering plants which cover the
forests, plains and agricultural lands rely on myriad bacteria, fungi and
insects to decompose dead organisms and mineralise the nutrients essential for
plant growth. Because most terrestrial flowering plants are also sedentary,
they must rely on insects, birds or animals to transport their seed or pollen
to different places.
Pollen is a specialised phase of the plant life cycle
which is essential for sexual reproduction. Also known as the male gametophyte, it is produced in
flowering plants in floral organs called anthers. Pollen is genetically
distinct from the plant it comes from because it has only half of the genetic
information encoded in the parent plant. The female gametophyte, or
egg cell, contained in ovaries at the base of the floral organ known as the
stigma, also possesses half of the genetic information encoded in the parent
plant. The primary function of pollen is to transport the plant’s genes to the
flower of another plant, where it may fertilise the female gametophyte,
creating a new seed containing the combined and complete genetic information
from both plants.
Plants
and most higher organisms contain duplicate sets of genetic information. These are similar, but not identical. One
set of genes derives from the male parent and the other from the female parent.
For example, for a single pollination event in wheat, the pollen parent may
contribute genes encoding red seed, short plant height and poor baking quality
and the female parent may contribute genes for white seed, tall height and good
baking quality. The resulting seed containing both sets of genes will express
characteristics, determined by the interaction of the genes. The gene for red seed may dominate the gene
for white, resulting in red seed; the gene for shortness may mask the gene for
tallness, producing a short plant; the genes for baking quality may interact
resulting in intermediate baking quality. When this seed grows into a mature
plant it will produce pollen (and eggs) with new combinations of the different
genes. In this example, there will be eight possible combinations, such as red seed, short stems, poor quality; white
seed, short stems, poor quality; red seed, long stems, poor quality, etc. In
reality, each parent contributes thousands of different genes. This results in millions of possible gene
combinations that create the vast array of genetic diversity observed within a
particular plant species.
Pollen is carried from one plant to another either by
animals, such as insects, birds and bats, or by the wind or water. Most cereal
crop species and many tree species are wind pollinated, whereas many
horticultural plants and pasture species are insect pollinated. Some insects,
such as the honey bee, are pollinators of thousands of different plant species,
whereas in other cases - for example, fig species pollinated by wasps - specific
relationships have evolved between particular plant and insect species. 11
For the honey bee and many other species, pollen is also an important food
source, providing protein and lipids not found in the nectar of the flowers
they visit.
ASSESSING POTENTIAL GE HAZARDS
The
complex relationship between plants and their insect pollinators has been
evolving for 100 million years. Over the relatively short period of the past
two decades, it has become possible to use recombinant DNA technology to
overcome the natural boundaries of gene flow and introduce artificially
constructed genes and genes from other organisms into plant species. Many of
these GE crops are grown over large areas of North America and China.
Although much controversy has arisen about the
potential impact of GE crops on natural ecosystems, agricultural ecosystems or
human health, limited scientific thought or analysis has resulted. Proponents
of GE crops frequently claim a scientific basis for their contention that GE
crops will have zero or minimal environmental impact. The bases of these claims
generally rely on the absence of empirical evidence for significant
environmental impact. In other words, unless a GE crop can be shown
unequivocally to be hazardous, it is assumed to be safe. This approach mirrors
the current scientific method by which knowledge is advanced through the
formulation and testing of hypotheses. A hypothesis – an explanation for a
particular natural phenomenon – remains valid until evidence that disproves the
hypothesis is found through observation or experiment.12 The formulation of hypotheses does not
require logic, it may involve some guessing or intuition, and a hypothesis will
remain valid until clearly disproved by empirical observation. In contrast, the
testing of hypotheses does require logic and reasoning to either disprove the
hypothesis or accept it as a reasonable explanation of the observed phenomenon.
A good hypothesis leads to the discovery of new scientific insights.
The hypothesis that GE crops will have no significant
environmental impact may be a good method for the advancement of scientific
knowledge. However, as a tool for making decisions about whether GE crops
should be released into the environment, it lacks the caution necessary for
responsible, long-term environmental stewardship. A more cautious and
responsible approach would be to adopt the hypothesis that GE crops will have a
significant impact on the environment, and approve them for release only if a
body of evidence accumulates to disprove this hypothesis (of relevance here is
the precautionary principle, which is discussed in detail in Chapter 3).
The remainder of this chapter will discuss hypotheses
related to the proposition that, because of gene flow, GE crops will have a
significant effect on the environment and human health. The hypotheses are
presented together with supporting evidence and discussion of the type of
observations and experimental data required to disprove the hypotheses.
Pollen
is the primary vector through which engineered genes may be transferred to
non-transgenic crops. The distance travelled by pollen depends on the plant
species and method of pollination. For short, self pollinated cereals such as
wheat, barley, rice and oats, which are generally wind pollinated, the
distances recorded for pollen dispersal are in the order of tens of metres or
less.13 These distances are greater for taller outcrossing cereals
such as maize and sorghum. The possible distance for cross-pollination is
determined both by the physical distance that pollen can travel and the time for
which pollen may remain viable. Cereal pollen is generally intolerant of
desiccation, and therefore survives only a matter of hours after release from
the anther into the atmosphere. However, if a GE cereal crop is growing in a
paddock adjacent to a non-GE crop, it is highly likely that some GE pollen will
pollinate the non-GE crop, resulting in GE seed contamination.
For crops that are insect pollinated, such as canola,
sunflower, and many legume species, the opportunities for cross pollination
between GE and non-GE crops are greater than for wind-pollinated species. Bees
can carry pollen over distances in the order of five kilometres, so any non-GE
crops within a five-kilometre radius of a GE insect pollinated crop such as
canola risk being pollinated by GE pollen (see Chapter 7 for the concerns of
organic farmers about this risk). There is evidence to support the hypothesis
that GE crops will pollinate and contaminate non-GE crops.14 In
North America, there have been several cases in which GE pollen has cross
fertilised non-GE crops, resulting in GE seeds being harvested from a
supposedly non-GE crop.15 In some cases, the farmer being prosecuted
by the company that developed the GE crop for growing the crop without a
license.
Because numerous examples exist where GE crops have
cross pollinated with non-GE crops this hypothesis cannot be disproved and we
must therefore accept the proposition that GE crops will pollinate and pollute
non-GE crops. This does not mean that all GE crops will cause contamination in
all cases, but it is a clear signal that a cautious approach is necessary.
GE crops will pollinate weed
species to produce GE weeds: Hypothesis 2
In evolutionary terms, most
crop species evolved recently. They developed larger and more palatable seeds
and fruits than their wild relatives 5000 to 10 000 years ago with the
development of agriculture and the repetitive selection of the largest and
tastiest crops by the first farmers. Because crop species have only recently
evolved, cross pollination is possible between many crops and their wild
relatives, although the evolutionary distance between them often means that
interspecific cross-pollination is less successful than cross pollination
within the crop.
There is good evidence that
GE crops will pollinate weed species to produce GE weeds. Experiments have
shown that engineered genes from canola are transferred easily to the weedy
relative Brassica campestris.16
Canola is also capable of cross pollinating with several other weed species
including wild raddish (Raphanus
raphanistrum) and buchan weed (Hirschfeldia
incana) and can pollinate related crop species, including broccoli,
cauliflower, cabbage and mustard.17 Other reports indicate that GE
sunflower18 and GE sugar beet19 are capable of cross-pollinating
with related weed species.
The hypothesis that GE crops
will pollinate weed species to produce GE weeds is well supported with
experimental data for many crop species. For some crops, the hypothesis may be
disproved if it is demonstrated that related weed species do not exist in areas
where the crop is grown.
GE
crops will carry genes which adversely affect human health: Hypothesis 3
Non-GE pollen is known to cause hay fever and
allergies due to specific proteins expressed in the pollen.20 When
new proteins are introduced into plants through genetic engineering, it is
essential to test whether these proteins occur in pollen and to assess their
allergenicity (see also Chapter 5). A protein naturally found in soil bacteria
- for example some of the enzymes used to provide herbicide resistance to GE
crops - may not normally affect humans, but if the same protein is found in
pollen it may cause an allergic response.
GE proteins may also appear in foods which themselves are not
genetically engineered. For example, pollen is an important component of honey,
and if bee hives are located within 5 kilometres of a transgenic canola crop,
or a weed which has inherited a GE protein through cross-pollination, the honey
is likely to be contaminated with the GE protein. Keeping account of the spread
of GE proteins is almost impossible.
One of the greatest concerns for human health is the development of molecular farming technology whereby
crops are genetically engineered to produce pharmaceuticals, vaccines, enzymes,
hormones or industrial oils or polymers. The products of many of these GE crops
would have obvious detrimental consequences for human health if mistakenly
consumed as food. For example, a banana genetically engineered to produce a
vaccine may not be visibly distinguishable from a normal banana and, without
extraordinary care, may end up in food markets; or a pharmaceutical produced in
a canola crop may end up in a non-GE crop through cross pollination. It is very
difficult to segregate agricultural commodities for food and non-food use, as
the StarLink maize example discussed earlier demonstrates.
The hypothesis that GE crops will carry genes which adversely affect
human health cannot be disproved if crops are genetically engineered to produce
non-food products such as pharmaceuticals, which may have serious consequences
if consumed at unspecified dosage. Although obtaining evidence to reject this
hypothesis would not be simple, some GE crops have already been allowed to
enter the human food chain without any publicly accessible or credible,
scientific evaluation (see also Chapter 5).
GE crops will adversely
affect insect populations: Hypothesis 4
A gene that has been frequently incorporated in
genetically engineered crops - known as the Bt toxin - is derived from the
bacterial species Bacillus thuringiensis.
Bt toxins kill certain species of butterfly, moth, fly and beetle and they are
put into crop species to protect them from insect attack. For example, Bt
cotton is resistant to the cotton bollworm and Bt corn is resistant to the corn
borer.
There is increasing evidence, however, that insects
other than the crop pest are affected by the GE crop. It has been observed, for
example, that lacewing larvae that feed on insects which have been raised on Bt
maize have significantly greater mortality than when raised on conventional
maize.21 It has also been observed that if monarch butterflies
consume Bt maize pollen present on milkweed leaves, their mortality increases
significantly.22 In different transgenic crops containing Bt genes,
it is likely that other predatory and parasitic insects will be similarly
affected.23
The hypothesis that GE crops will adversely affect
insect populations has ample supporting evidence, particularly for the use of Bt
genes. For GE crops in which Bt is not used, there has been little analysis of
the effect of the crop on insect species.
GE crops will adversely
affect soil organisms: Hypothesis 5
The impact of the Bt toxin on insects, earthworms and
other soil-inhabiting organisms is also of concern. Although the Bt toxin
exists naturally in the environment in Bacillus
thuringiensis, it was not present in such large quantities until the
development of GE crops containing it. The long-term effect on soil
microorganisms is unknown. Another concern is the impact of chemicals applied
to crops engineered to be resistant to various herbicides. GE herbicide-resistant
varieties sold as a package with proprietary herbicides encourage increased use
of specific herbicides, whose effects on soil flora and fauna are unknown.
The hypothesis that GE crops will adversely affect
soil organisms is very difficult to disprove, especially considering that many
of the organisms living in the soil are so little studied that they have not
yet been allocated scientific names.
Engineered genes released into the environment cannot be contained:
Hypothesis 6
Many examples exist of the detrimental impact of
technology on the environment. In Australian agriculture, the use of many
chemicals - including the chlorinated hydrocarbon pesticides - have been
banned. When a chemical pollutant is no longer allowed to be used, its existing
residues slowly disintegrate through physical and biological processes until
eventually it exists in negligible quantities. On the other hand, when a
genetically engineered organism is released into the environment, it has the
capacity to reproduce and spread and may be impossible to recall. Many weed
species are plants that have been introduced into a new environment where they
have attained some selective advantage over the species that already exist
there. If a GE plant has some selective advantage, it may become established as
a weed. This is already being observed in agricultural systems, where GE crops
with resistance to particular herbicides are becoming weeds in subsequent crops
because they can no longer be controlled with particular herbicides In Canada, for example, Brassica napus (oil seed rape) crops have been found to contain
volunteers that are resistant to three difference herbicides, namely
glyphosate, imidazoline and glufosinate.24
Finding evidence to disprove the hypothesis that
engineered genes released into the environment cannot be contained may involve
experiments examining the prospects for cross-pollination to weedy species.
However, there remains the potential for engineered genes to escape from the GE
crop by other means, such as horizontal gene transfer.
Engineered genes may be
transferred to non-plant species: Hypothesis 7
The
movement of genes between species outside of the normal reproductive process is
referred to as horizontal gene transfer.
One example of horizontal gene transfer in nature is the transfer of DNA from
the HIV retrovirus into human DNA to cause disease. Another example is crown
gall disease of stone fruits where the bacterium Agrobacterium tumefaciens transfers some of its DNA into the
plant’s DNA, causing disease. Genetic engineering is an artificial form of
horizontal gene transfer, by which genes may be transferred between species. There
have been several examples of the transfer of genes from plants to fungi or
bacteria. In one case, engineered genes introduced into Datura, Brassica, and Vicia species were shown to be
transferred to the fungus Aspergillus
niger25
No adequate means exist for testing the environmental
impact of an engineered gene that finds its way into a species in which, in
nature, its presence was not intended. For example, a Bt toxin gene may escape
from a GE crop plant into a fungus that is the food source of a particular
insect. What will be the effect on the insect and the ecosystem? With the
escape of engineered genes, it will become virtually impossible to preserve
forests and natural ecosystems in their natural state. This is because there will be no way of
ensuring that the engineered genes keep out of these ecosystems.
Another potential hazard is the development of new
strains of virus. Most GE crops contain a piece of DNA from the Cauliflower
Mosaic Virus to control the functioning of the engineered gene. There is a risk
that viruses that attack GE plants may combine their genes with the engineered
gene to create viruses with new characteristics, such as increased virulence on the host or the ability to infect
different species. 26
There is ample evidence to support the hypothesis that
engineered genes can be transferred to non-plant species. Because there is
uncertainty about gene escape to other organisms and further uncertainty about
the behaviour of the modified gene in that organism, release of the GE crop to the
environment cannot be considered prudent.
Conclusions
The
seven hypotheses presented above assert that genetically engineered crops may
be a hazard to the environment and to human health. GE crops may cross-pollinate
with non-GE crops and cross-pollinate with related weed species. GE crops may
carry genes that adversely affect human health either through allergenicity or through
toxicity. GE crops may also adversely affect insect species or soil organisms,
and the engineered genes may find their way into non-plant species, in which
their behaviour and environmental consequences are unknown. Unlike chemical
environmental pollutants, GE organisms are living, replicating entities and
cannot be readily recalled once released to the environment.
Evidence to refute these hypotheses is either difficult
to obtain, or does not exist. In the absence of clear scientific evidence,
supporters of genetic engineering often argue that decisions concerning the
release of GE crops should be based on risk-benefit analysis, where GE crops
are approved for release if the perceived benefits outweigh the estimated
risks. This approach is imprecise, as quantification of the risks involves a
large degree of conjecture. In addition, this approach is inequitable, as the
benefits are generally restricted to the organisations who have developed the
technology, whereas the risks are borne by the entire human population and the
environment.
A preferable approach to assessing the release of GE
crops would incorporate the Precautionary Principle which states:
When an activity raises threats of harm to human health or the environment, precautionary measures should be taken even if some cause and effect relationships are not fully established scientifically. In this context, the proponent of an activity, rather than the public should bear the burden of proof. The process of applying the precautionary principle must be open, informed and democratic and must include potentially affected parties. It must also involve an examination of the full range of alternatives including no action. 27
In
conclusion, unless the hypotheses presented above can be clearly disproved, any
release of genetically engineered crops should not be permitted. To proceed
otherwise would be to jeopardise human health and wellbeing, and endanger the
ecosystems which sustain life on earth.
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2 MacKay, W. & Andrew, J. 2001. Federal Court of Canada. Monsanto Canada Inc. and Monsanto Company vs Percy Schmeiser and Schmeiser Enterprises Ltd., 29 March 2001, Docket T-1593-98, Neutral Citation 2001 FCT 256.
3 Smyth, S., Khachatourians, G. & Phillips, P. 2002. Liabilities and economics of transgenic crops, Nature Biotechnology, 20: 537-41.
4 Young, P. 2002. Australian Biotechnology News, cited at: <http://communitycouldron.com/bulletinpages/main.html>.
5 Cuming, M. 2003. Contamination raises fears about quarantine protocols, Stock and Land, 13 February, p. 3.
6 Goldenberg, S. 2002. Pig vaccine contaminates US crops, The Guardian (London), 24 December; Fox J. 2003. Puzzling industry response to ProdiGene fiasco, Nature Biotechnology, 21: 3-4..
7 Quist, D. & Chapela, I. 2001. Transgenic DNA introgressed into traditional maize landraces in Oaxaca, Mexico, Nature, 414: 541-43.
8 Metz, M. & Fütterer, J. 2002. Suspect evidence of transgenic contamination, Nature, 416: 600-01; Kaplinsky, N., Braun, D., Lisch, D., Hay, A., Hake, S. & Freeling, M. 2002. Maize transgene results in Mexico are artefacts, Nature, 416: 601.
9 Quist, & Chapela, Transgenic.
10 Suzuki D. and Knudtson P. 1988. Genethics: The Ethics of Engineering Life, Allen and Unwin, Australia; Ho, M-W. 1999. Genetic Engineering: Dream or Nightmare? The Brave New World of Bad Science and Big Business, Third World Network, Malaysia; Murray, D. 2003. Seeds of Concern: The Genetic Manipulation of Plants, UNSW Press, Sydney.
11 Meeuse, B. & Morris, S. 1984. The Sex Life of Flowers, Rainbird Publishing Group, London.
12 Popper, K. 1963. Conjectures and Refutations: The Growth of Scientific Knowledge, Routledge & Kegan Paul, London.
13 Ritala, A., Nuutila, A., Aikasalo, R., Kauppinen, V. & Tammisola, J. 2002. Measuring gene flow in the cultivation of transgenic barley, Crop Science, 42: 278-85.
14 Rieger, M., Lamond, M., Preston, C., Powles, S. & Roush, R. 2002. Pollen-mediated movement of herbicide resistance between commercial canola fields, Science, 296: 2386-88.
15 Hall et al., Pollen.
16 Mikkelsen, T., Anderson, B. & Jørgensen, R. 1996. The risk of crop transgene spread, Nature, 380:31.
17 Salisbury, P. 2002. Genetically Modified Canola in Australia; Agronomic and Environmental Considerations. Australian Oilseeds Federation.
18 Faure, N., Serieys, H. & Berville, A. 2002. Potential gene flow from cultivated sunflower to volunteer, wild Helianthus species in Europe, Agriculture Ecosystems and Environment, 89: 183-90.
19 Lavigne, C., Klein, E. & Couvet, D. 2002. Using seed purity data to estimate an average pollen mediated gene flow from crops to wild relatives, Theoretical and Applied Genetics, 104: 139-45.
20 Petersdorf, R., Adams, R., Braunwald, E. Isselbacher, K., Martin, J. & Wilson, J. (eds.). 1983. Harrison’s Principles of Internal Medicine, Tenth edition, McGraw-Hill International, p. 376.
21 Hilbeck, A. 2001. Implications of transgenic, insecticidal plants for insect and plant biodiversity, Perspectives in Plant Ecology Evolution and Systematics, 4: 43-61.
22 Losey, J., Rayor, L. & Carter, M. 1999. Transgenic pollen harms monarch larvae, Nature, 399: 214; Hansen Jesse L. & ObryckI, J. 2000. Field deposition of Bt transgenic corn pollen: Lethal effects on the Monarch butterfly, Oecologia, 125: 241-48.
23 Schuler, T., Poppy, G., Kerry, B., & Denholm, I. 1999. Potential side effects of insect-resistant transgenic plants on arthropod natural enemies, TIBTECH, 17: 210-16.
24 Hall et al., Pollen.
25 Hoffmann, T., Golz, C. & Schieder, O. 1994. Foreign DNA sequences are received by a wild-type strain of Aspergillus niger after co-culture with transgenic higher plants, Current Genetics, 27: 70-76.
26 Schoelz, J. & Wintermantel, M. 1993. Expansion of viral host range through complementation and recombination in transgenic plants, The Plant Cell, 5: 1669-79; Rubio, T., Borja, M., Scholtof, H. & Jackson, A. 1999. Recombination with host transgenes and effects on virus evolution: An overview and opinion, Molecular Plant-Microbe Interactions, 12: 87-92.
27 Anonymous. 1998. The precautionary principle, Rachel’s Environment and Health Weekly, 19 February, 586; see also Chapter 3 of this volume for commentary concerning the precautionary principle.