GM SCIENCE REVIEW
An open review of the science relevant to GM crops and food
based on interests and concerns of the public
PREPARED BY THE GM SCIENCE REVIEW PANEL (JULY 2003)
Printed in the UK on recycled paper with a minimum HMSO score of 75.
First published in July 2003. Department of Trade and Industry.
© Crown copyright. DTI/Pub 6704/0. 1K/07/03/NP. URN 03/949.
Foreword by Profesor Sir David King
Members of the Panel
Chapter 1: General introduction
1.1 Why have a Science Review?
1.2 What has the Science Review involved?
1.3 How is scientific knowledge acquired?
1.4 Who has been involved in the Review?
1.5 What is the structure of the Report?
1.6 What is the relationship between this Review and the work of the UK
statutory advisory committees on GM?
1.7 How will the Report be used?
Chapter 2: Methodology
2.2 The website
2.3 The Science Review Panel
2.4 Open meetings
2.5 Strand co-ordination
2.6 The Framework of the Review
2.7 The review of public concerns (the Corr Willbourn report)
Chapter 3: The role of science in the regulatory process
3.1 Substantial equivalence
3.2 The precautionary principle
Chapter 4: How reliable is GM plant breeding?
Does GM work? Is GM technology too imprecise? Are GM genes more unstable than
resident genes? Is it necessary to produce many transgenic plants to obtain an acceptable
Chapter 5: The safety of food and animal feed derived from GM crops
5.2 Possible nutritional and toxicological differences in GM food
Could GM derived food be more toxic, more carcinogenic, or nutritionally less adequate
when compared to other foods? And what is the potential for GM technology to produce
foods with enhanced nutritional content or reduced toxicity compared with their non-GM
5.3 Food allergies from GM crops
Is the risk of suffering food allergies greater in GM food?
5.4 The fate of transgenic DNA
Could transgenes (or parts of their DNA sequences) in food survive digestion and behave
differently in comparison to traditional foodstuffs in their ability to relocate, recombine or
modify the consumer’s genome or that of associated gut microflora? If so, would this pose an
increased risk to health compared to the consumption of non-GM derived food?
5.5 The effect of GM derived feed in the food chain
Could the consumption of GM derived feed and crops by farm animals prove more of a
health hazard to consumers of the resulting food products, or to the animals, than the use of
Chapter 6: Environmental impacts of GM crops
6.2 Invasiveness/persistence of GM plants
Could GM plants be invasive or persistent, and what might be the impacts?
6.3 Toxicity to wildlife
Could GM plants be toxic to wildlife, and what might be the impacts?
6.4 Development of resistance
Could crops engineered with novel resistance genes lead to the emergence of new forms of
pests, diseases and weeds that are resistant to chemical sprays? Will new forms of insects
and diseases evolve which are able to bypass GM resistance genes?
6.5 New weed control strategies offered by GM herbicide tolerant crops
Will herbicide tolerant crops offer new weed control strategies and, if so, what are the likely
impacts, positive and negative?
6.6 Horizon scanning
Apart from herbicide tolerant crops, what are the major new traits that might give rise to
significant environmental impacts, positive or negative?
6.7 Changes in agricultural practice
Might GM crops change agricultural practice in the UK? If so, what might be the likely
6.8 Limitations of science
Is the science available to predict the environmental impact of GM plants?
Chapter 7: Gene flow, detection and impact of GM crops
7.2 Gene flow between crop varieties
Can the extent and consequences of gene flow from GM crops to other crop varieties (GM
and non-GM) be predicted and controlled? Is co-existence between GM and non-GM crops
possible and can we detect unintended GM presence?
7.3 Gene flow from GM crops to agricultural weeds and wild relatives
Can the extent and consequences of gene flow from GM crops to agricultural weeds and wild
relatives be predicted and controlled? Could gene flow from GM crops generate superweeds
or eliminate wild plant populations?
7.4 Can DNA from GM crops transfer to soil microbes?
In nature, how important and prevalent is horizontal gene transfer from plants to microbes
in the soil, and does the presence of transgenic DNA make this more likely to occur? To what
extent are the ecological effects of horizontal gene transfer from plants to soil microbes
7.5 Can genetic material in GM plants transfer to viruses?
Can plant-virus-derived transgenes recombine with, and be transferred to viruses? If
horizontal gene transfer is possible between GM plants and viruses could this result in new
viruses that could cause irrecoverable damage to the ecosystenm or to crops?
List of abbreviations
Annex I: Questions about GM (extract from Corr Willbourn report)
Annex II: Review process undertaken by ACRE in assigning
applications for the deliberate release of a GMO in England
Annex III: Description of the regulatory frameworks
Annex IV: Key UK decisions/actions in the Directive 2001/18 Part C
Annex V: European Commission proposals on GM food and feed
Annex VI: Information available on the GM Science Review website
Foreword by Professor Sir David King,
Government Chief Scientific Adviser
The GM Science Review was commissioned as part of the wider GM public dialogue by Mrs Margaret
Beckett, the Secretary of State for the Environment, Food and Rural Affairs; with the agreement of the
responsible Ministers in the devolved administrations. This report has therefore now been formally
submitted to Mrs Margaret Beckett MP, Mr Allan Wilson MSP at the Scottish Executive, and Mr
Carwyn Jones AM at the National Assembly for Wales to help inform Government decision making
on GM crops and food.
The Review has endeavoured to take an open look at the science relevant to GM crops and food, and
to do so in a way that recognises the interests and concerns of the public as well as the science
community. So I am sure this report will be of widespread interest. The Review Panel invites and
welcomes your comments on the report. Over the Summer, our Review website1 will be open to
receive them. We also continue to welcome scientific contributions to the website. All
contributions must be submitted by 15 October 2003.
The Panel will then reconvene in late Autumn to consider these comments together with the report of
the GM public debate “GM Nation?2”. In the light of these, we will wish to consider whether there are
any further issues we should address. We will also look to see if there have been significant
developments in GM science over the summer that we should report on, and will consider the results
of the farm scale evaluations of GM crops if these are available.
Those who attended our open Panel meetings will know that the Panel members cover a wide range of
expertise and of views on GM. I would like to pay tribute to all those members who have given real
commitment to the Review, expending a great deal of time and working extremely hard and
cooperatively to ensure that the issues we have considered have been fully explored. Whilst respecting
differences in views and recognising that Panel members do not individually cover all the areas of
expertise, I am pleased to say that a really good degree of consensus was reached on the basis of the
available science and that the Panel has collectively taken ownership of the review.
Finally, on behalf of the Panel, I would also like to thank all those who spoke at our open meetings
around the country, those who hosted them and, of course, those who came along and took part. Our
thanks go to the British Association for the Advancement of Science for organising this series of open
meetings, as well as the Royal Society and the Royal Society of Edinburgh. We would particularly
thank all those who contributed to the website; we have sought to take account of your submissions.
We have also valued our contacts with the those running the public debate and with the Prime
Minister’s Strategy Unit who have produced the report on the costs and benefits of GM crops and
food3. We are grateful to the Food Standards Agency and their advisory committees for their
comments. And I am sure that the Panel would wish to acknowledge the dedication of the Secretariat,
whose members have laboured mightily to bring this First Report to print.
21 July 2003
1 http://www.gmsciencedebate.org.uk For guidance on how to submit comments.
3 Fieldwork: Weighing up the Costs and Benefits of GM Crops. http://www.strategy.gov.uk
Members of the Panel
Professor Sir David King FRS
(Chairman) Chief Scientific Advisor, HM Government
Professor Howard Dalton FRS
(Deputy Chairman) Chief Scientific Advisor, Department Environment, Food and
Dr Mark Avery Director of Conservation, Royal Society Protection of Birds,
Professor Janet Bainbridge Director Science and Technology, University of Teesside; Chair
of the Advisory Committee on Novel Foods and Processes
Dr Chitra Bharucha Consultant Haematologist; Chair of Advisory Committee on
Professor Dianna Bowles OBE Director of CNAP, Department of Biology, University of York
Dr Simon Bright Syngenta, Jealott's Hill International Research Centre, Berkshire
Dr Andrew Cockburn Monsanto, Trumpington, Cambridge
Professor Mick Crawley FRS Imperial College, Silwood Park, Berkshire
Professor Philip Dale John Innes Centre, Norwich
Professor Mike Gale FRS Deputy Director, John Innes Centre, Norwich Research Park,
Professor Mike Gasson Food Research Institute, Norwich
Professor Alan Gray OBE Director, NERC Centre for Ecology and Hydrology; Former Chair
of the Advisory Committee on Releases to the Environment
Professor John Gray Department of Plant Science, University of Cambridge
Professor Pat Heslop-Harrison Department of Biology, University of Leicester
Ms Julie Hill Programme Adviser, Green Alliance; Deputy Chair of the
Agriculture and Environment Biotechnology Commission
Dr Brian Johnson Head of Agricultural Technologies, English Nature, Somerset
Professor Chris Leaver FRS Head, Department of Plant Sciences, University of Oxford
Professor Jules Pretty Director of Centre for Environment and Society, University of
Revd. Professor Michael Reiss Institute of Education, University of London
Professor Bertus Rima MRIA Medical and Biological Centre, Queens University, Belfast
Professor Bernard Silverman
FRS Institute of Advanced Studies, University of Bristol
Dr Andrew Stirling Science Policy Research Unit, University of Sussex
Professor William Sutherland University of East Anglia, Norwich
Professor Michael Wilson FRSE Chief Executive, Horticulture Research International
Professor Peter Young Professor of Molecular Ecology, Department of Biology,
University of York
Dr Adrian Butt (Secretary) OST/DEFRA
Dr Louise Ball OST
Miss Maia Gedde OST
Mr Richard Pitts OST
Mr David Trew OST
Ms Rita Wadey OST
Ever since the beginnings of agriculture, some ten thousand years ago, people have been
selecting plants to develop into new crops. We now know that the process of plant breeding
builds on changes brought about in a plant's genetic structure, with the information being
encoded by genes (typically some 30,000 genes in each plant cell). Since the 1970s, it has
become possible to modify the genetic information of living organisms in a new way, by
transferring one or more gene-sized pieces of DNA directly between them. Such transfers have
become an everyday tool in biological research and are already the basis of a considerable
number of commercial applications in drug and food development that involve the genetic
modification of micro-organisms such as yeast and bacteria. When applied to the production of
crop plants, genetic modification can involve gene transfer from another plant species, or from a
completely different organism such as a bacterium or virus. The process shares some common
features with earlier plant breeding tools, as well as exhibiting unique differences.
World-wide, genetically modified (GM) crops occupy a relatively small proportion of the world’s
agricultural acreage. However, in 2002, GM crops were cultivated on some 59 million hectares
globally. Almost all (99%) of this was grown in only four countries: USA (66%), Argentina
(23%), Canada (6%) and China (4%). Three crops comprise 95% of the land under GM
cultivation: soybean (62%), maize (21%) and cotton (12%). Traits achieved by genetic
modification primarily involve herbicide tolerance (75%) and insect pest resistance (15%), or a
combination of both in the same crop.
No GM crops are currently grown commercially in the UK although they are grown to a limited
extent in some EU countries. There are, though, GM foods and animal feeds approved for
consumption in the EU and these include processed products from GM herbicide tolerant
soybean and maize, and oil from GM oilseed rape. Tomato paste made from slow-ripening GM
tomatoes is approved but is not currently available, although it was widely sold in the UK in the
late 1990s. Products made from GM micro-organisms are widely used in some sectors of the food
industry (e.g. as a processing aid in cheese manufacture) and in medicine. However, the issues
surrounding GM micro-organisms are not included in this science review which focuses
specifically on GM crops and their products.
THE SCIENCE REVIEW
Many claims have been made about potential benefits available from GM crops. At the same time
considerable reservations and concerns have been expressed. This review specifically addresses
the science surrounding GM crops, with a focus on topics shaped by public questions and
concerns. It differs from standard scientific reviews in its attempts to engage with the public and
explore different viewpoints. For instance, the remit for the review mandated that the work be
'driven' by public interests and concerns and that deliberate attention be given to ‘divergences of
view among scientists’ and ‘uncertainties, unknowns and gaps in knowledge’. Through a series
of public workshops and meetings and through a website, we have solicited and considered
concerns and interests of the public, whether or not professionally involved in science, agriculture
or the food industry.
The review does not aim to be exhaustive in surveying all that is known scientifically about the
various GM crops that have been developed to date. However, the review does aim to cover those
areas where there is evident public concern. As a result of the public consultation exercise and
input from the panel itself, seventeen topics were chosen for detailed analysis. In each case, the
topic was considered within a framework that aimed to: (1) summarise the range, quality and
degree of agreement of scientific studies that have investigated the issues; (2) ask whether the
topic is unique to the processes and products of genetic modification or whether there are
commonalities with crops bred conventionally; and (3) ask whether there are important scientific
uncertainties. Two other components of the framework involved ‘looking to the future’,
exploring relevant developments in scientific research, agricultural practice and also regulation.
The review panel included both specialist and non-specialist scientists and social scientists from a
wide range of backgrounds. The institutions from which panellists were drawn included
universities, specialist research institutes, research groups associated with biotechnology
companies, and organisations with particular environmental concerns. Whatever their background
and current employment and interests, all panel members acted as individuals in their own right,
with a shared vision of producing a balanced, accurate and well-informed review. We hope that
this review will enable debates and decisions to be informed by sound scientific evidence.
The review is specifically concerned with the potential use of GM crops in the UK. Assessing the
implications of the adoption of GM technologies in other countries is beyond its scope, although
issues with regard to the use of GM crops elsewhere, particularly in developing countries, were
raised in the consultation exercise and discussed by the review panel. We hope that the approach
we have used, and the scientific material we have brought together, may be of use in other
countries in clarifying issues and generally informing debate.
THE SCIENTIFIC PROCESS
The bedrock of this Report is peer-reviewed published scientific literature in the relevant areas,
but other sources of appropriate scientific evidence have also been considered where appropriate.
Good scientific results have a sound basis in terms of existing knowledge and stand up to careful
experimental and observational investigation. A good scientific paper explains clearly its claimed
advance in knowledge and the evidence for it. When submitted for publication, the paper is read
carefully by other experts in the field to see whether its conclusions are justified, and this process
of ‘quality control’ is called peer review. A paper that passes this test is published in the scientific
literature and becomes part of the public body of knowledge on which future scientific work can
be based. No single peer-reviewed paper should be believed uncritically, and if a paper makes a
surprising claim or a substantial advance, it becomes an obvious candidate for further scientific
investigation. The aim of this whole system, which has grown up over more than three hundred
years, is that knowledge should continually be challenged, refined and improved, through a
developmental process based on appropriate evidence, valid inference and the work of a large and
open scientific community.
Some of the questions asked about GM crops are purely scientific, whilst others are not of a
scientific nature at all, but may be economic, social, ethical or even personal. For science, as for
other areas, the answers given may often depend on the way the question is asked and be open to
divergent interpretations. Accordingly, scientific issues represent only a part, albeit an important
one, of the wider debate over GM crops. Being ‘rational’ is not enough to make a question
scientific; the question and/or its potential solution must be amenable to objective testing. Of
course, there are many questions that are not wholly or even primarily scientific, but are such that
scientific understanding can make an important contribution to their resolution.
STRUCTURE OF THE MAIN REPORT
The first two chapters describe the scope and methodology of the review. Chapter 3 discusses the
role of science in regulation. Chapter 4 discusses the reliability of GM plant breeding compared
with conventional methods. Seventeen topics reflecting issues of public concern are then grouped
into three chapters, broadly covering food, feed and animal safety, environmental impact, and
HOW RELIABLE IS GM PLANT BREEDING? (CHAPTER 4)
Concern has been expressed that GM plant breeding is too unreliable and imprecise for crops to
be grown and consumed safely, or at least without more extensive testing. One argument
presented is that it is necessary to produce about 100 GM plants to obtain one that has the
desirable characters for its use as a basis of a new GM crop variety. There is also evidence that
genes introduced by genetic modification vary in their effects depending on precisely where they
insert into the host plant’s genetic material
To address such concerns it is important to place GM crop breeding in the context of non-GM
crop breeding methods such as gene transfer by pollination, mutation breeding, cell selection and
induced polyploidy. Most of these so-called conventional plant breeding methods have a
substantially greater discard rate. Mutation breeding, for instance, involves the production of
unpredictable and undirected genetic changes and many thousands, even millions, of undesirable
plants are discarded in order to identify plants with suitable qualities for further breeding. The
success of all methods of breeding relies on careful testing and evaluation and on rejection of
plants with undesirable qualities. The rejection rate is substantially higher for most non-GM crop
breeding methods than it is for GM crop breeding.
All plant breeding methods, however, have unique features and the main special feature of GM
plant breeding is that it allows a wider choice of genes for modifying crops in novel ways. No
other plant breeding technique permits the incorporation of genetic material from such diverse
biological sources. Inevitably this raises the possibility that some new consequences of GM plant
breeding may be unexpected. This presents challenges for their regulation and management in the
future that will need to be managed carefully and intelligently.
THE SAFETY OF FOOD AND ANIMAL FEED DERIVED FROM GM
CROPS (CHAPTER 5)
A number of issues of public concern are considered in detail. Might GM crops result in more
food allergies? Could GM foods be less nutritious or more toxic than their conventional
counterparts? More generally, could DNA from GM crops harm people, either through being
consumed directly in GM-derived food, or by entering the food chain through animal feed?
Possible nutritional and toxicological differences in GM food (5.2)
All novel food in the UK, which includes food produced by GM organisms, is subject to an EU-
based and internationally determined regulatory regime, with procedures for safety assessment
and risk analysis. The regime recognises that the consumption of food is not risk-free and
requires any novel (including GM) food to be at least as safe and nutritious as any traditional
food it replaces or complements.
To date world-wide there have been no verifiable untoward toxic or nutritionally deleterious
effects resulting from the cultivation and consumption of products from GM crops. However,
absence of readily observable adverse effects does not mean that these can be completely ruled
out and there has been no epidemiological monitoring of those consuming GM food. Some
reason that the absence of evidence of harm should not be treated as evidence of the absence of
harm. This argues for greater reliance on scientific research and epidemiological monitoring.
Others reason that the combination of testing by developers to demonstrate safety equivalence to
commercial crops in order to satisfy regulatory requirements for clearance and extensive use
around the world over long time periods and large exposed populations and absence of evidence
of harm, does provide important experience of safety. The long-term assessment of the health
effects for whole foods and feeds is considerably more difficult than the post-marketing
monitoring and surveillance of a simple substance such as a single medicine. Countries are
working to develop post-marketing surveillance to detect potential human health effects of food
in general, but at present there is nothing yet available for GM foods in any country.
Safety assessment technologies such as screening and profiling techniques will need to continue
to evolve, incorporating data on all possible entry-points for new hazards and to cope with
uncertainties and gaps in knowledge. The complexity of the safety assessment process is likely to
increase with the development of ‘second generation’ GM crops. These crops and their products
aim to: decrease levels of anti-nutritional factors (e.g. toxins); increase levels of health promoting
factors (e.g. antioxidants); and modify levels of macro or micronutrients (e.g. vitamins).
Food allergies from GM crops (5.3)
Changes in allergenicity during the breeding of conventional crops are not assessed in a
regulatory framework and are not formally evaluated.
GM technology enables a particular gene construct for a new protein to be introduced, and the
potential allergenic effect of that protein is a focal point for safety assessment. In addition, the
regulatory process, with its case-by-case approach, must take account of possibly increasing
exposure to a GM protein, especially if it is expressed in a diversity of different GM plants, and
thus introduced into a diverse range of foodstuffs. In the hypothetical case, where an GM allergen
was not recognised in regulatory screening, and its effects only emerged in the longer term,
avoidance of the allergenic protein by the consumer could be difficult, because they would not be
able to recognise its presence in the foodstuffs. The likelihood of this scenario is very low for a
number of reasons. However, avoidance in a GM or non-GM case would depend on the relative
effectiveness of labelling, traceability and recall systems and it would be for the regulatory
system to ensure that any GM allergen once known, with a potentially significant effect on any
consumer, should be labelled in a fail-safe way or withdrawn from the marketplace.
It is probably easier to evaluate the risk of introducing allergenic proteins and altering the
allergenic composition of the target crops after use of GM than with some conventional breeding
There is an accepted approach, based on a standard set of safety tests, to the assessment of the
allergic potential. But there is some contention over the value of specific tests and if, and how
they can be improved. These tests are under continuous evaluation and improvements are
considered in the scientific and regulatory literature.
The GM foods consumed at present (by large numbers of people for up to seven years) do not
appear to have elicited allergic reactions. The same arguments for and against the significance of
this are the same as for nutritional and toxicological effects (see 5.2 above). Our relative lack of
knowledge about factors that are important in sensitisation and the elicitation of an allergic
response suggest that we should continue to exercise caution when assessing all new foods,
including foods and animal feeds derived from GM crops.
The fate of transgenic DNA (5.4)
The food we consume from conventionally bred crops contains large quantities of DNA, since
DNA is a universal component of all living organisms and is not typically removed by the
extraction and processing technologies used by the food and drinks industry. Some processes,
such as sugar purification and the production of refined oils, remove most, sometimes all, of the
DNA from a product before it is consumed. Other processes, such as heat treatment, whilst not
removing DNA entirely, cause extensive inactivation and breakdown. The consumption of raw
vegetables and fruits does, of course, mean that intact DNA is ingested.
DNA, like other large molecules in food, is very largely degraded (broken down to smaller
molecules) in the gut, but this process of structural degradation whilst inactivating the DNA’s
genetic information, is not 100% efficient. Fragments of ingested DNA have been found
throughout the digestive system and elsewhere in the body, including the blood stream. Our guts
contain very large numbers of bacteria which help us to digest the food we consume. Whilst it is
possible that these bacteria take up DNA from their environment (i.e. our digestive systems and
the foods they contain) there are a series of well-established barriers in place to prevent the
genomic integration and expression of foreign genes. This process is unlikely to be of biological
significance unless: (1) the bacterial cells can use at least some of the genetic information that the
DNA encodes; and (2) that information confers a selective advantage, leading to an increase in
the proportion of the bacteria that contain this new DNA.
In GM food, the introduced DNA will have the same fate as DNA present in conventional food
and will be inactivated and increasingly degraded as the food progresses through the digestive
system. If the food originates from a GM crop in which bacterial DNA is part of the transgene,
then, whilst still likely to be a rare occurrence, there is increased opportunity for that DNA to
transfer into gut bacteria. This possibility makes it essential, in the achievement of maximum risk
reduction, for the regulatory process to consider each GM crop as an individual entity with its
own potential risks.
Antibiotic resistance is not only widespread as a consequence of antibiotic and feed additive
usage, but because it is highly selected for in microbes in the wild. Bacterial genes conferring
antibiotic resistance have been a commonly used tool for selection in GM technology, but
alternatives have now been developed and it is possible to eliminate antibiotic resistance gene
markers following GM plant construction. So, the presence of antibiotic resistance genes can now
be avoided in GM plants intended for food use. The use of antibiotic resistance genes in plants
remains controversial, with differing views on its potential impact. There is a scientifically well-
supported argument that any rare resistance gene transfer event from a GM plant or food would
have no impact as antibiotic resistance is already widespread as a consequence of antibiotic usage
in medicine and animal feed.
The effect of GM derived feed on the food chain (5.5)
Animal feed is a major product of conventional agriculture, and of crops developed using GM
technology. The processing of crops into animal feed often completely degrades such
constituents as DNA and proteins, but this cannot be assumed always to be the case. Most DNA
is degraded in the gut, but some survives and there is evidence that some DNA fragments from
feed ingested by poultry and livestock can appear in the blood and other tissues.
However, food and feed safety studies have been unable to find introduced feed DNA or its gene
products in milk, meat or eggs produced from animals fed GM crops. Many millions of people,
particularly in the United States, Canada and Argentina, have for up to seven years been eating
food products derived from animals fed on GM diets and no substantiated ill effects have been
reported. There is a similarly lack of evidence for any adverse effects of GM feed on the health,
welfare and productivity of livestock.
However, as mentioned in relation to nutritional and toxicological differences, the absence of
readily observable adverse effects in humans or animals does not mean that these can be
completely ruled out for any crop GM or non GM, existing or novel. For example, rare, mild or
long-term adverse effects are not easy to detect and could in future be the subject of post-
marketing monitoring and surveillance. The safety assessment of crops with significantly altered
nutritional qualities will need careful consideration where there may not be historical knowledge
of assumed safe use.
THE ENVIRONMENTAL IMPACT OF GM CROPS (CHAPTER 6)
There has long been concern about the ways that GM crops might affect the environment, and
this was reflected in the public consultation exercise. In addition to direct environmental impacts,
there could be indirect effects, for example in the ways that cultivation of GM crops might
change agricultural practices and rural landscapes. The latter seems most likely and could bring
benefits as well as risks. The great majority of all GM crops currently in cultivation are grown in
the USA, Canada and Argentina. In each of these countries the crops tend to be grown on large-
scale farms that are geographically quite isolated from wilderness areas. More recently, many
smallholder farmers in China have also adopted GM crops. The circumstances are distinct from
the many smaller-scale farms embedded in the countryside that are characteristic of the UK and
the rest of Europe. These differences in scale and farming practices must be considered in
harm/benefit analyses of the potential environmental impacts of GM crops in different parts of
the world. It is also essential to compare the environmental impact of the GM crop with other
current and evolving practices in conventional agriculture.
Could GM plants become more widely invasive or persistent? (6.2)
Notwithstanding the case-by-case approach taken by the regulatory authorities in evaluating
invasiveness, there are two principal models that have been influential in considering the
potential for GM crops to become more invasive of natural habitats than their conventional
counterparts. One is the alien species model. The hypothesis is that roughly 0.1% of introduced
GM plants would become pests, because that was the rate of invasive alien plants species (some
15 problem plants out of an estimated 15,000 alien species introduced into the UK). The other is
the crop model, which argues that GM crops will behave in much the same way as conventional
crop plants except for the GM trait that may influence fitness. Conventional annual crop plants
generally do not prosper outside arable fields. Although escaped plants of crop species are found,
they do not tend to increase in abundance but are replenished each year by fresh ‘escapes’.
Detailed field experiments on several GM crops in a range of environments have demonstrated
that the transgenic traits investigated do not significantly increase the fitness of these plants in
semi-natural habitats, and therefore they behave in a similar way to non-GM crops.
We do not have an exact understanding of what changes in a plant’s life history will affect its
invasiveness. More knowledge on the potential effects of releasing GM plants with traits such as
pest and disease resistance and stress tolerance is required since these may significantly alter a
crop plant’s ability to survive outside the agricultural environment. In particular, we need to
know whether GM for fitness-affecting traits like growth rate, longevity, plant size, or
survivorship in plant species with potentially more invasive life histories (e.g. woody plants,
perennial grasses, thicket-forming herbs) is consequential.
Could GM crops be toxic to wildlife, and what might be the impacts?
Crop breeding, whether through genetic modification or ‘conventional’ methods, has the
potential to alter levels of plant toxins or create novel compounds that are toxic to some wildlife.
Such effects are unusual but they are a key element of the risk assessment process for
experimental and commercial release of GM crops. The principal risks arise for crops that have
been deliberately bred to contain toxins to control key pests or diseases. GM pest- and disease-
resistant crops are unlikely to be grown commercially in the UK in the near future. Nevertheless,
evidence from the USA and China indicates that for some, but not all, GM pest-resistant crops
there have been significant reductions in pesticide use. In every case when attempting to
determine the effects of pest-resistance, it is necessary to judge the crop-pesticide combination as
a ‘system’ rather than simply considering the ecological impacts of the crop in isolation
There is little scientific dispute about the fact that GM plants engineered to produce toxins can
sometimes be toxic to non-target wildlife, since even in nature toxins are rarely species-specific.
However, no significant adverse effects on non-target wildlife resulting from toxicity of GM ‘Bt
‘ plants, for example, have so far been observed in the field. This suggests that Bt crops are
generally beneficial to in-crop biodiversity in comparison to conventional crops that receive
regular, broad-spectrum insecticide applications. Despite this, benefits would probably be
restricted (or even negated) if Bt crops required insecticide applications to control target or
secondary pests that were not sufficiently controlled by the Bt toxin. Studies on the impacts of
GM crops on soil processes have shown some differences in soil microbial community structure,
but so far there does not seem to be any convincing evidence to show that GM crops could
adversely affect soil health in the long term. The differences in soil microbial communities
observed beneath GM crops have been within the range of variation in microbial community
structure and of the order of magnitude of the differences observed under different crops of even
different cultivars of the same crop. However, almost all this data is drawn from small-scale,
short-term studies and there is a need for larger, more agronomically realistic studies to be
undertaken to demonstrate absence of harm to non-target organisms.
There tends to be scientific disagreement about the amount of information needed to demonstrate
that growing GM pest and disease-resistant crops is environmentally sustainable in the long term.
Some scientists argue that current evidence of reductions in pesticide use and increases in
biodiversity compared to conventional crops are sufficient to demonstrate absence of adverse
impacts, while others advocate the need for a greater fundamental understanding of the
Most of the possible negative impacts of GM crops on biodiversity are likely to be reversible, so
small-scale field trials to test for impacts on relevant ecosystems are unlikely to pose any long-
term environmental risks. After a crop has been approved for commercial use, the monitoring
systems required for GM crops grown in the EU provide a valuable mechanism to collect
ecologically relevant data. This will be useful to enhance our understanding of the impacts of GM
pest-resistant crops on non-target species.
Could GM crops lead to particular problems in the development of
resistant insects, weeds and diseases? (6.4)
A key long-standing target of ‘traditional’ plant breeding, including some uses of genetic
modification, has been the development of crop varieties that are resistant to pests and diseases.
Widespread, uniform cultivation of these varieties, together with any agrochemicals applied to
reduce the incidence of disease or to kill weeds, provide a strong selection pressure for the
emergence/evolution of resistant target organisms (pests, pathogen and weeds) that can attack the
new variety or survive the pesticide application. The time it takes for a resistant target organism
to emerge depends on the nature of the toxins and how they are expressed, the ecology, genetics
and mating behaviour of the target organism(s), the mode of action of the toxin, and on the
effectiveness of the crop management techniques deployed by farmers.
Current widespread scientific opinion is that ‘single dominant resistance gene’ mechanisms are
less durable than resistance controlled by several genes. However, some sources of GM
resistance, including Bt genes that confer resistance to a narrow range of target insects, appear to
be particularly robust. However, there is no a priori reason to suppose that resistance genes
introduced by GM will be any less susceptible to ‘breakdown ‘ than those introduced by slower
conventional breeding methods.
Over 120 species of weeds have been recorded worldwide that have become resistant to various
herbicides in association with herbicide-tolerant crops, irrespective of whether tolerance was
obtained by GM or conventional breeding technologies. Weeds that are closely related and
hybridise freely with the cultivated herbicide-tolerant crop variety have the added possibility of
obtaining tolerance gene(s) directly from the crop. However, unless the weed is exposed to the
herbicide in question, this does not pose any ecological or selective advantage.
Therefore, although resistance-breaking strains of pathogens, pests or weeds can be expected to
emerge, there is no reason to expect different responses depending on whether a crop's resistance
was introduced by GM or by conventional breeding methods.
However, since GM has frequently employed genes which confer resistance to common
herbicides and pesticides (e.g. glyphosate and Bt) in its weed and pest control strategies, impacts
on agriculture and possibly biodiversity could be significant if some target organisms developed
resistance to these compounds. The extent and possible severity of impacts on the environment
are difficult to quantify and subject to much debate.
Will herbicide-tolerant crops offer new weed control strategies and if
so what are the likely positive and negative impacts? (6.5)
GM herbicide-tolerant (GMHT) crops enable new weed control strategies. The key possibility is
the replacement of existing approved but persistent, toxic herbicides by those with a more benign
environmental profile. They may also enable farmers to spray crops less frequently and to relax
weed management practices for conventional crops at different stages in the rotation. Hence they
are an attractive option for farmers wishing to simplify crop management. It may also be possible
to delay the date of herbicide application, avoid pre-sowing weed treatments and so leave
emerging weeds in the fields for longer. Such a result might have benefits for biodiversity,
though this claim is largely speculative and is not strongly supported by the current small-scale
experimental studies. Similarly, evidence from the USA indicates that tillage can be reduced in
HT crops, which provides environmental benefits that may not necessarily be relevant to the UK.
Fifty years of agricultural intensification has undoubtedly led to a decline in farmland
biodiversity, but the role of herbicides in this decline is unclear. Broad spectrum herbicides used
in conjunction with GMHT crops are known to provide highly efficient and reliable weed control
in comparison to many ‘conventional’ herbicide regimes, and if their use resulted in fewer weed
seeds and further declines in weed populations then organisms depending on those weeds during
part of their life cycle could be adversely affected. We do not yet have sufficient evidence to
predict what the long-term impacts of GM HT crops might be on weed populations. An important
uncertainty is how farmers will apply this technology in the field.
The publication of the UK farm-scale evaluations of GMHT crops will clarify some of these
uncertainties. Inevitably others will remain. The question would become more complex if farmers
were to grow two or more herbicide tolerant crops in rotation.
Apart from herbicide tolerance, what are the major new traits that
might give rise to significant environmental impacts? (6.6)
Over the next ten years, there is the possibility of introducing GM crops resistant to attack by
insects, nematodes, fungi, bacteria or viruses. In all cases, we would expect these to enable
reductions in pesticide use. There are potential negative impacts on non-target organisms, but in
the case of insect resistance, field studies on commercially grown Bt crops have failed to identify
any adverse effects. In addition, subject to regulatory approval, there will be imports of GM food,
feed and fibre, with improved shelf life or nutritional quality, but these are not expected to affect
the UK environment.
Further ahead, it becomes more difficult to make confident predictions about the
commercialisation of GM crops and their possible environmental impacts. The horizon scan has
identified the paucity of baseline data and models at different scales, from field to landscape
scale, which is needed as a basis for future assessment of large-scale environmental effects. Many
of the issues foreseen are not unique to GM crops and will be driven by economic, social and
political rather than purely scientific factors. Current research points to GM crops for certain non-
food purposes: pharmaceuticals, speciality and bulk chemicals and biomass for energy. These
could provide renewable resources for industry, provide new medicines and could diversify rural
landscapes and economies. Conversely, there could be undesired effects on wildlife caused by the
way these crops might be managed and/or changes in patterns of land use. Another longer-term
possibility is the development of traits aimed at improving crop production in marginal
environments (e.g. tolerance of drought, heat or salt) with obvious advantages to certain growers
in these environments. However, such crops could become more successful as weeds, there could
be economic pressure to cultivate areas with wildlife and conservation value, and there might be
adverse socio-economic and political consequences, for example with regard to optimal farm
Might GM crops change agricultural practice is the UK? If so, what
might be the likely consequences? (6.7)
It is widely acknowledged that modern (non-GM) agriculture has already had significant negative
impacts on biodiversity and the wider environment in the UK. Large changes over the last
century, including recent decades, in the way farmland is managed have resulted in a decline in
farmland plant, invertebrate and bird abundance and diversity.
The consequences of commercial growing of GM crops in the UK would depend on the nature of
each individual technology and the decisions made by farmers, the public and policy makers. For
example, some GM technologies could increase agricultural intensification, to produce more
from the same area of land, while other niche and specialist GM crops could increase the
diversity of the landscape. Some GM crops would lead to reduced agrochemical use while others
would have the opposite effect.
Each potential agricultural application of genetic modification must, therefore, be examined on a
case-by-case basis, taking careful account of the physical, social and political environments
within which it would be deployed. There is a major need for policy makers to understand how
these factors are likely to interface with the new technologies, to enable prediction of
environmental outcomes and thus delivery of environmental targets because they will predict
outcomes from the environment if targets are to be delivered.
What are the limitations of the science available to predict the
environmental impact of GM plants? (6.8)
There are several approaches for determining the ecological consequences of GM crops.
Examples include extrapolations from experience with comparable traits or with other crop
varieties that are in some or all ways ‘equivalent’, laboratory and field experiments, experience of
GM crops, and ecological modelling. In practice it is usually necessary to use a number of these
methods in combination.
Most of the environmental issues raised by growing currently available GM crops do not differ
qualitatively from conventional crops. In both the GM and conventional context, we are limited
in our ability to predict ecological changes within complex systems. This applies to a wide range
of ecological issues and to many aspects of agriculture: modern intensive, organic or
conventional. Important gaps in knowledge include the possible rate of uptake of GM crops in the
UK; detailed knowledge of farmland ecology; soil ecology.
GENE FLOW, DETECTION AND IMPACT OF GM CROPS (CHAPTER 7)
Gene flow is the movement of genes from one organism to another, and is something that takes
place in nature all the time. There are various mechanisms by which gene flow can occur and
various natural barriers to minimise its effects. None of these mechanisms is specific to GM
plants; therefore a great deal of evidence from conventional agriculture is relevant.
Gene flow between crop varieties (7.2)
Genes can move between different varieties of the same species by the spread of seed and by
cross-pollination. The complete genetic isolation of crops grown on a commercial scale, either
GM or non-GM, is not practical at present. However, gene flow can be minimised, as currently
happens in the case of oilseed rape varieties grown for food, feed or industrial oils. The levels at
which gene flow can be maintained for different crop varieties are significant in determining
whether co-existence of different types of agriculture is feasible. However, political decisions
may ultimately affect whether co-existence is practical, in particular what thresholds are set for
maximum GM presence in non-GM crops (and their products), whether conventional or organic.
For some crops, maintaining thresholds of gene flow may be relatively straightforward, by
employing separation distances and, more importantly, by reducing gene flow through seed.
However, in other cases it may be difficult, if not impossible, to grow certain crops or use some
existing farming practices (e.g. using farm-saved oilseed rape seed on farms where both GM and
non-GM varieties are grown).
Gene flow from GM crops that have been approved for commercial release can be detected but
unapproved GMOs present difficulties. Gene flow may be detected if commonly used transgenic
DNA is present, but the actual source of the GM presence will be difficult, maybe impossible, to
identify. Detection methods are very sensitive but they cannot guarantee a total absence of
transgenic content. Equally, false positives may indicate that transgenic DNA is present when it
‘Gene stacking’ is the accumulation of genes conferring a range of traits as a result of cross-
pollination between different varieties. It is not unique to GM crops. However, if GM crops are to
be grown commercially in the UK, assessments of the potential consequences of such gene
stacking may well become a more prominent consideration for regulators. GM crops that produce
non-food, non-feed products such as pharmaceuticals, bioplastics or biofuels pose different
regulatory issues and would, as for all GN crops have to be judged on a case-by-case basis. In
any case, such crops would (certainly, should) be designed and/or grown in ways that would
preclude gene flow to food and feed crops.
More information is needed about the mechanisms and management of seed dispersal in
agricultural systems, along with diagnostic and sampling methodologies for determining the
extent of gene flow early in the production/supply chain. In the longer term, it is possible that
gene containment systems will be developed that significantly reduce gene flow.
Gene flow from GM crops to agricultural weeds and wild relatives (7.3)
Gene flow can occur from GM crops to sexually compatible wild relatives and to agricultural
weeds. Cross-pollination will occur to an extent that depends on the closeness of the relationship
between the species and on other conditions. However, the key issue is whether any resulting
hybrid plants survive, grow and reproduce successfully allowing the new gene to be introgressed
(stably introduced into the new population). Hybridisation seems overwhelmingly likely to
transfer genes that are advantageous in agricultural environments, but will not prosper in the
wild. This general view is supported by specific studies on oilseed rape and on sugar beet, where
there has been little or no detectable gene flow to semi-natural habitats even though there can be
hybridisation within a field. Furthermore, no hybrid between any crop and any wild relative has
ever become invasive in the wild in the UK.
Within current agricultural practice, more than 120 non-GM herbicide-resistant species have
emerged worldwide in the last 40 years. In most, but not necessarily all cases, such plants are at a
disadvantage away from agricultural conditions. This disadvantage has also been found in
experiments carried out on GM plants. There have been some instances in Canada, where there is
complete freedom to grow several herbicide-tolerant varieties, e.g. oilseed rape, of tolerance
being transferred to weeds or stacked through hybrids in one variety. However, if herbicide-
tolerant crops are carefully managed, this should delay, or even prevent, the emergence of any
herbicide-tolerant weed problem.
Genes associated with resistance to pests and diseases have greater potential than herbicide-
resistant genes to lead to the local expansion of a plant population. However, there are other
natural constraints that could prevent an increase in population growth rates in such cases.
Overall, genes for pest- and disease-resistance inserted into crops by conventional breeding have
not produced invasions of wild relatives in semi-natural habitats.
However, there are gaps in our understanding of the potential consequences of gene flow, and the
effect of particular traits on the fitness of the weed or wild relative, which may receive them, is
an important target of ongoing research. In addition, several technological solutions to containing
or reducing gene flow from GM crops have been proposed.
Can genetic material in GM plants transfer to soil microbes? (7.4)
Most plant DNA is degraded during the natural processes of decay, but there is a small possibility
that genes in plant DNA could be acquired and expressed by environmental microbes. There is no
evidence from complete bacterial gene sequences that genes from plants have successfully
established during bacterial evolution, but bacterially derived transgenes in current use may have
a higher probability of transfer to soil bacteria than average plant DNA. No such transfer under
field conditions has yet been observed. However, there are limited tools, and there have been
limited attempts to test the phenomenon under field conditions.
Most current transgenes are of bacterial origin. They are therefore unlikely to have any
significant novel effect on bacteria that have already been exposed to them by gene transfer from
other bacteria, though their similarity to bacterial DNA may increase the chance that bacteria
acquire them. Inserting transgenes in plastids (i.e. chloroplasts) may increase the chance of
horizontal gene transfer (HGT) to bacteria because of the increased copy number (several 100
copies per cell instead of 1 or 2 copies of nuclear DNA) and closer relationship to prokaryotic
gene structure. Careful design of transgenes can greatly reduce the potential for HGT to bacteria.
In future, inserted genes may encode proteins not found naturally. Although these will be less
easily acquired by bacteria, their effects may need to be explicitly tested in representative
HGT to other microbes, (e.g. fungi and protists), has not been as well researched as for bacteria.
As with bacteria, there is some indication that the rate may not be zero. Since these are
eukaryotes, some further consideration should be given to the likelihood of incorporation and
expression of the transgenic DNA used in GM plants, as the work directed at bacteria will not be
Initially, a gene transfer event affects a single microbial cell. It will have no ecological impact
unless the transgene confers an advantage on its recipient that causes it to become widespread in
the microbial population. For most genes that may be used in GM crops, this is unlikely. A
potential transgene should be assessed by first asking whether it could be expressed in microbes
and could confer an advantage on them. In some cases, this may require direct testing, and high-
throughput methods coul