GMO – theory, practice and risk assessment
Srećko Jelenić
Department of Molecular Biology, Division of Biology, Faculty of Science,
University of Zagreb, Rooseveltov trg 6, HR-10000 Zagreb, Croatia
(sjelen@zg.biol.pmf.hr)
A discussion of genetically modified crops can best start by reviewing conventional
agriculture and what humankind has done with the food supply for thousands of years
because it creates a context within which to talk about modern biotechnology. It is
important to recognise that we have been genetically modifying the food supply for
thousands of years. We have been using techniques like conventional breeding,
radiation and chemical breeding, and wide crosses to improve the genetic background
of our crops.
For several thousand years, farmers have been altering the genetic makeup of the
crops they grow. Human selection for features such as faster growth, larger seeds or
sweeter fruits has dramatically changed domesticated plant species compared to their
wild relatives. It is interesting to compare the original ancestor of corn, teosinte, with
hybrid seed corn. Teosinte has a very small cob with only a few kernels. The dramatic
differences when compared to modern field corn have been achieved using
conventional breeding methods. The art of recognising valuable traits and
incorporating them into future generations is very important in plant breeding.
Breeders have traditionally scrutinised their fields and travelled to foreign countries
searching for individual plants that exhibit desirable traits. The potential of wild
species as a source of genetic variation to bring about crop improvement was
recognised early in the twentieth century. However, the use of these exotic genetic
resources in breeding programmes was a time-consuming and laborious process that
often ended in failure. The transfer of traits from poorly adapted germplasm that
carries many undesirable genes into elite lines required many backcrosses, an efficient
selection procedure and much luck. The problems that stood, and still stand, in the
way of progress when crossing wild and domesticated species include: cross
incompatibility between the wild species and the cultivated crop; hybrid sterility;
reduced recombination between the chromosomes of the two species; and genes of
negative effect being tightly linked to the trait of interest (so-called ‘linkage drag’).
Desirable genetic alterations occasionally arise spontaneously through a process
called mutation, but the natural rate of mutation is too slow and unreliable to produce
all the plants that breeders would like to see. In the late 1920s, researchers discovered
that they could greatly increase the number of these variations or mutations by
exposing plants to X-rays. "Mutation breeding" accelerated after World War II, when
the techniques of the nuclear age became widely available. Plants were exposed to
gamma rays, protons, neutrons, alpha particles, and beta particles to see if these would
induce useful genetic modifications. Chemicals, too, such as sodium azide and ethyl
methanesulphonate, were used to cause mutations. Mutation breeding was particularly
popular during the 1970s. Although interest has waned somewhat in recent years,
occasional varieties continue to be produced using these methods. For example, the
new herbicide-resistant wheat variety Above was developed using exposure to sodium
azide. Mutation breeding efforts continue around the world today. Of the 2,800
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officially released mutation breeding varieties, 1,400 have been released during the
last 15 years.
Genetic engineering of crops became available in the early 1980’s as genetic
engineering techniques were being perfected. Transgenic plants were first created by
four groups working independently at Washington University in St. Louis, Missouri,
the Rijksuniversiteit in Ghent, Belgium, Monsanto Company in St. Louis, Missouri,
and the University of Wisconsin. Genetic engineering provides the means to make
even more distant "crosses" than were previously possible. Organisms that have until
now been completely outside the realm of possibility as gene donors can be used to
donate desirable traits to crop plants. For example, a single insect-resistance gene
from the bacterium Bacillus thuringiensis can be transferred to a corn plant to make
Bt corn. The advantage of genetic engineering is that it allows the transfer of a single
gene, or a couple of genes, in a much more precise and controllable way than is
achievable with conventional breeding. Conventional breeding involves random
mixing of the tens of thousands of genes present in a plant with the ten of thousands of
genes present in another plant. In contrast, genetic engineering is a much more precise
way of improving crops. In addition, improvements can be achieved in much shorter
time frames. In the past, it took 10 to 15 years to introduce a new variety of corn. This
time has been cut in half using the tools of genetic engineering.
The early transgenic plants were laboratory specimens, but subsequent research has
developed transgenic plants with commercially useful traits such as resistance to
herbicides, insects, and viruses. Almost all of the world’s major crops are now being
improved using genetic engineering, although initial efforts have focused on
commodity crops - primarily soybeans, corn, cotton and canola. The first genetically
modified crops were commercially grown in 1996. In 2003, GM soybean occupied
41.4 million hectares (61% of global GM area), GM maize was planted on 15.5
million hectares (23% of global GM area), transgenic cotton was grown on 7.2
million hectares (11% of global GM area) and GM canola occupied 3.6 million
hectares (5% of global GM area). During the eight-year period 1996 to 2003, global
area of transgenic crops increased 40 fold. The 67.7 million hectares of GM crops in
2003 was grown by 7 million farmers in 18 countries. The USA grew 42.8 million
hectares (63% of global total), followed by Argentina with 13.9 million hectares
(21%), Canada 4.4 million hectares (6%), Brazil 3 million hectares (4%), China 2.8
million hectares (4%) and South Africa 0.4 million hectares (1%). During the eightyear period, herbicide tolerance has consistently been the dominant trait followed by
insect resistance. In 2003, herbicide tolerance, deployed in soybean, maize, canola
and cotton occupied 73% or 49.7 million hectares of the global GM 67.7 million
hectares, with 12.2 million hectares (18%) planted to Bt crops. The two dominant GM
crop/trait combinations in 2003 were: herbicide tolerant soybean occupying 41.4
million hectares or 61% of the global total and grown in seven countries; and Bt
maize, occupying 9.1 million hectares, equivalent to 13% of global transgenic area
and grown in nine countries.
While it seems logical to express a concern such as “I don’t know what I am eating
with GM foods!” it must be remembered that we really never had that information
before with classically bred crops. With GM crops, at least we know what new
genetic material is being introduced, so we can test for predictable and even many
unpredictable effects. Consider, for example, how conventional plant breeders would
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develop a disease-resistant tomato. They would introduce chromosome fragments
from its wild relative to add a gene for disease resistance. In the process, hundreds of
unknown and unwanted genes would also be introduced, with the risk that some of
them could encode toxins or allergens, armaments that wild plants deploy to survive.
Alternatively, by mutation breeding hundreds of unknown genes would be modified.
Yet we never routinely tested most conventionally bred varieties for food safety or
environmental risk factors, and they were not subject to any regulatory oversight.
We have always lived with food risks, but in the last few decades we have become
increasingly more adept at asking questions. To address the concern about long-term
health consequences of GM foods, it is instructive to recognise that we worried little
about such impacts when massive amounts of new proteins (and unfamiliar
chemicals) were introduced into our foods from wild species or when unknown
changes were created through mutation breeding. When new foods from exotic crops
are introduced, we often assimilate them easily into our diets. What’s more we rarely,
if ever, before asked the same questions that we now pose about GM crops. Many so
called functional foods, health foods, and nutraceuticals have been entering
into the mainstream diet lately, with little or no regulation or testing. We do not
question the long-term health implications of these food supplements, even though
they involve relatively large changes in our food intake. In contrast, the GM foods
currently on the market have been tested extensively. Food safety evaluation issues of
foods derived from GM crops comprise: 1) Molecular characterisation of the
introduced genetic fragment and resulting new proteins or metabolites (in addition, an
increasing number of states routinely ask for characterisation of the insertion point of
the transgenic fragment); 2) Analysis of the composition of the relevant plant parts
with respect to key nutrients and anti-nutrients, including natural toxins and potential
allergens; 3) Potential for gene transfer of specific genes from the GM food to –
particularly – microorganisms in the human and animal gastrointestinal tract; 4)
Potential allergenicity of the new gene products, or alteration of the intrinsic
allergenicity of the GM food organism; 5) Estimated intake levels of the newly
introduced proteins as well as of the final product, including any altered constituent;
6) A toxicological and nutritional evaluation of the resulting data; and 7) Additional
toxicity testing (of the whole food) where necessary. In addition, genetically
engineered crops are required to undergo extensive environmental safety assessment,
whereas conventionally modified crops are not. Key issues in the environmental
assessment of GM crops are putative invasiveness, vertical or horizontal gene flow,
other ecological impacts and effects on biodiversity.
If the current regulatory standards imposed on GM crops were to be invoked for crops
obtained through traditional and mutation breeding, how many of them would meet
their requirements?
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