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The introduction of transgenic crops and foods into the existing food production
system has generated a number of questions about possible
negative consequences. People with concerns about this
technology have reacted in many ways, from participating
in letter-writing campaigns to demonstrating in the
streets to vandalizing institutions where transgenic
research is being conducted. What are the main concerns?
What scientific support is there for these concerns?
The issues surrounding objections to transgenic crops
can be broadly grouped into concerns about
These are complex issues and a thorough treatment of
each one would occupy volumes. For each topic we provide
a short discussion with a link to a longer discussion
and outside resources.
Allergenicity
The possibility that we might see an increase in the
number of allergic reactions to food as a result of
genetic engineering has a powerful emotional appeal
because many of us experienced this problem before the
advent of transgenic crops, or know of someone who did.
However, there is no evidence so
far that genetically engineered foods are more likely
to cause allergic reactions than are conventional
foods. Tests of several dozen transgenic foods for
allergenicity have uncovered only a soybean that
was never marketed and the now-famous StarLink corn.
Although the preliminary finding is that StarLink
corn is probably not allergenic, the scientific
debate continues. Every year some people discover
that they have developed an allergy to a common
food such as wheat or eggs, and some people may
develop allergies to transgenic foods in the future,
but there is no evidence that transgenic foods pose
more of a risk than conventional foods do.
More
on allergenicity |
Common sites for allergic reactions.
Source: FDA
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Horizontal transfer and antibiotic resistance
The use of antibiotic resistance markers in the development
of transgenic crops has raised concerns about whether
transgenic foods will play a part in our loss of ability
to treat illnesses with antibiotic drugs. At several
stages of the laboratory process, developers of transgenic
crops use DNA that codes for resistance to certain antibiotics,
and this DNA becomes a permanent feature of the final
product although it serves no purpose beyond the laboratory
stage. Will transgenic foods contribute to the existing
problems with antibiotic resistance?
Antibiotic pills.
Photo: www.molbio.princeton.edu |
One aspect of this topic
is the risk of horizontal gene transfer, that is,
transfer of DNA from one organism to another outside
of the parent-to-offspring channel. Transfer of
a resistance gene from transgenic food to micro-organisms
that normally inhabit our stomach and intestines,
or to bacteria that we ingest along with food, could
help those micro-organisms to survive an oral dose
of antibiotic medicine. Although horizontal transfer
of DNA does occur under natural circumstances and
under laboratory conditions, it is probably quite
rare in the acid environment of the human stomach. |
Another concern is that the enzyme product of the DNA
might be produced at low levels in transgenic plant
cells. While high processing temperatures would inactivate
the enzyme in processed foods, ingestion of fresh or
raw transgenic foods could result in the stomach containing
a small amount of an enzyme that inactivates an orally
administered dose of the antibiotic. This issue was
raised during the approval processes for Calgene's FlavrSavr
tomato and Ciba-Geigy's Bt corn 176. In both cases,
tests showed that orally administered antibiotics would
remain effective. While the risks from antibiotic resistance
genes in transgenic plants appear to be low, steps are
being taken to reduce the risk and to phase out their
use.
More
on antibiotic resistance
Eating foreign DNA
When scientists make a transgenic plant, they insert
pieces of DNA that did not originally occur in that
plant. Often these pieces of DNA come from entirely
different species, such as viruses and bacteria. Is
there any danger from eating this "foreign"
DNA?
Diagram of DNA. Source: Foodfuture,
Food and Drink Federation
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We eat DNA every time
we eat a meal. DNA is the blueprint for life and
all living things contain DNA in many of their cells.
What happens to this DNA? Most of it is broken down
into more basic molecules when we digest a meal.
A small amount is not broken down and is either
absorbed into the blood stream or excreted in the
feces. We suspect that the body's normal defense
system eventually destroys this DNA. Further research
in this area would help to determine exactly how
humans have managed to eat DNA for thousands of
years without noticing any effects from the tiny
bits that sneak into the bloodstream. |
So far there is no evidence that DNA from transgenic
crops is more dangerous to us than DNA from the conventional
crops, animals, and their attendant micro-organisms
that we have been eating all our lives.
More on
eating DNA
CaMV promoter
When scientists use transgenic technology to put a new
gene into a plant, they put in additional pieces of
DNA to direct the activity of that gene. One of these
pieces is the "promoter" that turns the gene on.
Cauliflower mosaic virus infection
in canola.
Source: Institute National de la Recherche
Agronomique, Versailles-Grignon
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The most
widely used promoter is the cauliflower mosaic virus
35S promoter, often abbreviated as the CaMV promoter
or the 35S promoter. This promoter was obtained
from the virus that causes cauliflower mosaic disease
in several vegetables, such as cauliflower, broccoli,
cabbage, and canola. There are concerns that the
CaMV promoter might be harmful if it were to invade
our cells and turn on our genes. |
A multi-step chain of events would have to occur for
the CaMV promoter to escape the normal digestive breakdown
process, penetrate a cell of the body, and insert itself
into a human chromosome. While there have been no tests
to determine whether the CaMV promoter has invaded human
tissues, experiments with mice indicate that normal
body defenses eliminate stray fragments of foreign DNA
that sneak into the blood stream from the digestive
tract.
There is some evidence that the CaMV promoter poses
little threat to human health. People have been eating
it in small quantities for hundreds of years when we
eat vegetables that are infected with the disease. Although
vegetables heavily infected with CaMV are unappetizing,
there have been no documented negative effects on health
from eating the virus or its promoter.
More
on the CaMV promoter
Changed nutrient levels
How do genetically engineered foods compare with conventional
foods in nutritional quality? This is an important issue,
and one for which there will probably be much research
in the future, as crops that are engineered specifically
for improved nutritional quality are marketed. However,
there have been only a few studies to date comparing
the nutritional quality of genetically modified foods
to their unmodified counterparts.
The central question for GE crops that are currently
available is whether plant breeders have accidentally
changed the nutritional components that we associate
with conventional cultivars of a crop. Because isoflavones
are thought to play a role in preventing heart disease,
breast cancer, and osteoporosis, the isoflavone content
of RoundupReady soybeans has been investigated by several
researchers.
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The studies completed so far do not resolve the
issue of whether RoundupReady soybeans have isoflavone
levels comparable to conventional varieties, but
the differences found in experiments appear to
be small or moderate in comparison with natural
variation in isoflavone levels. Additional evidence
may clarify the arguments for and against Roundup
applications as a risk factor in soybean cultivation.
Industry studies submitted in support of applications
for permission to sell transgenic crops indicate
that the nutritional components that are commonly
tested are similar in transgenic foods and conventional
foods.
More
on nutrient levels
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Source: Marck L. Tucker, USDA/ARS |
Monarch butterfly
Photo: Marlin E. Rice
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The
suggestion that Bt corn pollen might kill Monarch
butterfly larvae galvanized public interest in the
effect of transgenic crops on the environment. We
present a full discussion of this issue under Hot
Topics: Monarch
Butterfly. |
Crop-to-weed gene flow
Hybridization of crops with nearby weeds may enable
weeds to acquire traits we wish they didn't have, such
as resistance to herbicides. Research results indicate
that crop traits may escape from cultivation and persist
for many years in wild populations. Genes that provide
a competitive edge, such as resistance to viral disease,
could benefit weed populations around a crop field.
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Many cultivated crops have sexually compatible
wild relatives with which they hybridize under
favorable circumstances. The likelihood that transgenes
will spread can be different for each crop in
each area of the world.
For example, there are no wild relatives of corn
in the United States or in Europe for transgenic
corn to pollinate, but such wild relatives exist
in Mexico.
Soybeans and wheat are self-pollinating crops,
so the risk of transgenic pollen moving to nearby
weeds is small. However, that small risk must
balanced against the fact that there are wild
relatives of wheat in the United States.
There are no wild relatives of soybean in the
United States, but such wild relatives exist in
China. Thus each crop must be evaluated individually
for the risk of gene flow in the area where it
will be grown.
More on crop-to-weed gene flow
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Corn tassels shedding pollen. Source:
www.rockandrollagronomy.com
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Antibiotic resistance
There is also concern that transgenic plants growing
in the field will transfer their antibiotic resistance
genes to soil micro-organisms, thus causing a general
increase in the level of antibiotic resistance in the
environment. However, many soil organisms have naturally
occurring resistance as a defense against other organisms
that generate antibiotics, so genes contributed occasionally
by transgenic plants are unlikely to cause a change
in the existing level of antibiotic resistance in the
environment.
More on antibiotic resistance
Leakage of GM proteins into soil
Source: Rural Life Center, Kenyon
College, Gambier, Ohio.
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Many plants leak chemical compounds into the
soil through their roots. There are concerns that
transgenic plants may leak different compounds
than conventional plants do, as an unintended
consequence of their changed DNA.
Speculation that this may be happening leads
to concern about whether the communities of micro-organisms
living near transgenic plants may be affected.
The interaction between plants and soil micro-organisms
is very complex, with the micro-organisms that
live around plant roots also leaking chemical
compounds into the soil. Much more research must
be done before we understand the relationships
that occur between micro-organisms and conventional
crops. Attempts to discover whether transgenic
plants are changing the soil environment, and
whether they are changing it in good ways or bad
ways, are hindered by our lack of basic scientific
knowledge.
More
on leakage of proteins into soil
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Reductions in pesticide spraying: are they real?
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One of the most appealing arguments in favor
of transgenic plants is the potential for reducing
the damage we do to our environment with conventional
methods of farming. Pest-resistant crops such
as Bt corn and Bt cotton have been promoted as
a means to reduce the spraying of pesticides,
while herbicide-tolerant crops such as RoundupReady
soybeans are said to reduce the application of
herbicides. Large reductions in chemical spraying
have been claimed to result from the introduction
of these transgenic varieties. Are the claims
true?
Bt cotton is the only crop for which claims of
reduced spraying are clear. Analysts paint a mixed
picture on the results of planting RoundupReady
soybeans. Bt corn and herbicide-tolerant cotton
and corn have not resulted in clear reductions
in the spraying of chemicals.
More
on pesticide spraying
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Cotton boll. Source: www.mahyco.com
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Long-term effects of Roundup use
coming soon
Crop-to-crop gene flow
Hybridization of transgenic crops with nearby conventional
crops raises concerns about separation distances to
ensure purity of crops and about who must pay if unwanted
genes move into a neighbor's crop. As "Identity Preservation"
and segregation of GM from non-GM crops become factors
in marketing products, it will be important to ensure
that hybridization is not occurring in the field.
Experimental plots of transgenic
canola,
which has been shown to hybridize with
canola in neighboring fields.
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Many factors influence
the potential for gene flow from crop to crop. Some
crops are highly outcrossing, with pollen carried
to other fields by wind and by insects. Other species
are highly self-pollinating, with little potential
for pollen transfer to neighboring plants. Because
of the differences among crops species, every case
must be evaluated individually for potential to
contribute to gene flow from transgenic to conventional
crops. |
If GM pollen pollinates plants in a neighboring field,
then the issue of genetic trespass may arise. What level
of GM presence, if any, should be allowed in products
that are sold as organic or conventional? Should GM
farmers and companies bear responsibility for preventing
gene flow, or should conventional and organic farmers
pay to protect their products from gene flow? Should
GM versions of outcrossing plants be banned as too risky,
while GM versions of self-pollinating plants are permitted?
These issues have already prompted several lawsuits
and they will continue to be a factor in the development
and use of trangenic plants for years to come.
More
on crop-to-crop gene flow
Development of resistance to herbicides and Bt
coming soon
coming soon
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