GM is just another form of mutation breeding and is nothing to worry about
Mutation breeding brings its own problems and should be strictly regulated
Proponents often describe GM as just another form of mutation breeding, a method of plant breeding which they say has been successfully used for decades and is not controversial. They argue that mutation breeding is regulated no differently than conventional breeding, that genetic modification is just another form of mutation breeding, and that therefore, genetic modification should not be regulated any more stringently than conventional breeding.
However, scientific evidence exposes flaws in this logic.
1.3.1. What is mutation breeding?
The physical form of an organism’s genetic blueprint is the sequence of the four “letters” of the genetic alphabet structured within the DNA molecules. Mutations are physical alterations in the sequence of letters within the DNA. Mutation breeding is the process of exposing plant seeds to ionizing radiation (x-rays or gamma rays) or mutagenic chemicals in order to increase the rate of mutation in the DNA.
Just as you can change the meaning of a sentence by changing the sequence of letters in the sentence, you can change the “meaning” of a gene by changing the sequence of letters within the genetic code of the DNA of an organism. A mutagen is a physical or chemical agent that causes such changes.
This process of change in the DNA is known as mutagenesis. Mutagenesis can either completely destroy the function of a gene – that is, “knock out” its function, or it can change the sequence of letters of the genetic code in the gene, causing it to direct the cell to produce one or more proteins with altered function. The resulting plant is called a mutant.
1.3.2. Where did radiation-induced mutation breeding come from?
Mutation breeding using radiation was first seriously investigated in the 1950s, after the US atomic bombing of Japan at the end of World War II in 1945. In the wake of the devastation, there was a desire to find uses for the “peaceful atom” that were helpful to humanity. Atomic Gardens were set up in the US and Europe with the aim of creating high-yielding and disease-resistant crops. They were laid out in a circle with a radiation source in the middle that exposed plants and their seeds to radiation. This would cause mutations in the plants that it was hoped would be beneficial. To the lay population this was euphemistically described as making the plants “atom energized”. The results were poorly documented – certainly they do not qualify as scientific research – and it is unclear whether any useful plant varieties emerged from Atomic Garden projects.5
Today, radiation-induced mutation breeding is carried out in laboratories, but this branch of plant breeding retains strong links with the nuclear industry. The main database of crop varieties generated using radiation- and chemically-induced mutation breeding is maintained by the UN Food and Agriculture Organisation and the International Atomic Energy Agency.6 Many studies and reports that recommend radiation-induced mutation breeding are sponsored by organizations that promote nuclear energy.7,8
1.3.3. Is mutation breeding widely used?
Mutation breeding is not a widely used or central part of crop breeding, though a few crop varieties have apparently benefited from it. A database maintained by the UN Food and Agriculture Organisation and the International Atomic Energy Agency keeps track of plant varieties that have been generated using mutation breeding and by cross-breeding with a mutant plant.6 There are only around 3,000 such plant varieties. This number includes not only crop plants but also ornamental plants.9 It also includes not only the direct mutant varieties, but also varieties bred by crossing the mutants with other varieties by conventional breeding. Thus the actual number of primary mutant varieties is significantly lower than 3000.
Some commercially important traits have come out of mutation breeding, such as the semi-dwarf trait in rice, the high oleic acid trait in sunflower, the semi-dwarf trait in barley, and the low-linolenic acid trait in canola (oilseed rape).9,10,11
But conventional breeding, in contrast, has produced millions of crop varieties. The Svalbard seed vault in the Arctic contains over 400,000 seed varieties,12 which are estimated to represent less than one-third of our most important crop varieties.13 So relatively speaking, mutation breeding is of only marginal importance in crop development.
The reason mutation breeding is not more widely used is that the process of mutagenesis is risky, unpredictable, and does not efficiently generate beneficial mutations. Studies on fruit flies suggest that about 70% of mutations will have damaging effects on the functioning of the organism, and the remainder will be either neutral or weakly beneficial.14
Because of the primarily harmful effects of mutagenesis, the genetic code is structured to minimize the impacts of mutations and organisms have DNA repair mechanisms to repair mutations. In addition, regulatory agencies around the world are supposed to minimise or eliminate exposure to manmade mutagens.
In plants as well as fruit flies, mutagenesis is a destructive process. As one textbook on plant breeding states, “Invariably, the mutagen kills some cells outright while surviving plants display a wide range of deformities.”15 Experts conclude that most such induced mutations are harmful, and lead to unhealthy and/or infertile plants.15,16 Occasionally, mutagenesis gives rise to a previously unknown feature that may be beneficial and can be exploited.
The process of screening out undesirable traits and identifying desirable ones for further breeding has been likened to “finding a needle in a haystack”.15 The problem is that only certain types of mutations, such as those affecting shape or colour, are obvious to the eye. These plants can easily be discarded or kept for further breeding as desired. But other more subtle changes may not be obvious, yet may nonetheless have important impacts on the health or performance of the plant. Such changes can only be identified by expensive and painstaking testing.15
A report by the UK government’s GM Science Review Panel concluded that mutation breeding “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.”17
In retrospect, it is fortunate that mutation breeding has not been widely used because that has reduced the likelihood that this risky technology could have generated crop varieties that are toxic, allergenic, or reduced in nutritional value.
1.3.4. How does GM create mutations?
Just as mutation breeding is highly mutagenic, so is the process of creating a GM plant. The GM transformation process involves three kinds of mutagenic effects: insertional mutagenesis, genome-wide mutations, and mutations caused by tissue culture – described below.1,2
Genetic modification or genetic engineering of an organism always involves the insertion of a foreign gene into the genome (DNA) of the recipient organism. The insertion process is uncontrolled, in that the site of insertion of the foreign gene is random. The insertion of the GM gene (transgene) disrupts the normal sequence of the letters of the genetic code within the DNA of the plant, causing what is called insertional mutagenesis. This can occur in a number of different ways:
- The GM gene can be inserted into the middle of one of the plant’s natural genes. Typically this blocks the expression of (“knocks out”) the natural gene, destroying its function. Less frequently the insertion event will alter the natural plant gene’s structure and the structure and function of the protein for which it is the blueprint.
- The GM gene can be inserted into a region of the plant’s DNA that controls the expression of one or more genes of the host plant, unnaturally reducing or increasing the function of those genes.
- Even if the GM gene is not directly inserted into a host gene or its control region, its mere presence within an active host gene region can alter the ability of that region of the plant’s DNA to form chromatin (the combination of DNA and proteins that make up the contents of a cell nucleus) structures that influence the ability of any gene in that region to be expressed. The inserted gene can also compete with host genes for gene expression control elements (comparable to switches that turn the expression of a gene on or off) or regulatory proteins, resulting in marked disturbances in the level and pattern of gene expression.
Since the insertion of the GM gene is an imprecise and uncontrolled process, there is no way of predicting or controlling which of the plant’s genes will be influenced – or the extent of the changes caused by the inserted gene.
In most cases, the insertion process is not clean. In addition to the intended insertion, fragments of the GM gene’s DNA can be inserted at other locations in the genome of the host plant. Each of these unintended insertional events may also be mutagenic and can disrupt or destroy the function of other genes in the same ways as the full GM gene.
It is estimated that there is a 53–66% probability that any insertional event will disrupt a gene.1 Therefore, if the genetic modification process results in one primary insertion and two or three unintended insertions, it is likely that at least two of the plant’s genes will be disrupted.
Research evidence also indicates that the GM transformation process can also trigger other kinds of mutations – rearrangements and deletions of the plant’s DNA, especially at the site of insertion of the GM gene1 – which are likely to compromise the functioning of genes important to the plant.
Mutations caused by tissue culture
Three of the central steps in the genetic modification process take place while the host plant cells are being grown in a process called cell culture or tissue culture. These steps include:
(i) The initial insertion of the GM gene(s) into the host plant cells
(ii) The selection of plant cells into which the GM gene(s) have been successfully inserted
(iii) The use of plant hormones to induce cells selected in (ii), above, to develop into plantlets with roots and leaves.
The process of tissue culture is itself highly mutagenic, causing hundreds or even thousands of mutations throughout the host cell DNA.1,2 Since tissue culture is obligatory to all three steps described above and these steps are central to the genetic engineering process, there is abundant opportunity for tissue culture to induce mutations in the plant cells.
Given the fact that hundreds of genes may be mutated during tissue culture, there is a significant risk that a gene important to some property such as disease- or pest-resistance could be damaged. In another example, a gene that plays a role in controlling chemical reactions in the plant could be damaged, making the crop allergenic or reducing its nutritional value. The effects of many such mutations will not be obvious when the new GM plant is growing in a greenhouse and so genetic engineers will not be able to select them out.
In the process of insertion of a GM gene into the plant host DNA (step i, above), the GM gene is linked with an antibiotic resistance “marker” gene, which will later enable the genetic engineer to identify which plant cells have successfully incorporated the GM gene into their genome.
The host plant cells are then exposed simultaneously to the GM gene and the antibiotic resistance gene in the hope that some will successfully incorporate the GM gene into their genome.
This is a very inefficient process because genomes are designed to exclude foreign genetic material – for example, invading viruses. So out of hundreds of thousands or even millions of host plant cells exposed to the GM gene, only a few will successfully incorporate the GM gene.
In order to identify and propagate the plant cells that have successfully incorporated the GM gene (step ii, above), biotechnologists usually use antibiotic resistance marker genes. This is because a cell that has successfully integrated the antibiotic resistance marker gene into its genome and expressed that gene is likely also to have integrated the GM gene into its genome and expressed that gene. Therefore, when the population of plant cells is exposed to the antibiotic, the vast majority of recipient plant cells die, leaving only the few cells that have incorporated and expressed the antibiotic resistance marker gene. In almost all cases these cells have also incorporated the GM gene.
Interestingly, this antibiotic-based selection process relies on the expression of the marker gene. This expression is required to make the plant resistant to the antibiotic. If this gene does not express its protein, it will not confer resistance to the antibiotic.
However, not all regions of the plant cell DNA are permissive for the gene expression process to take place. In fact, the vast majority of any cell’s DNA is non-permissive. Because the process of inserting the DNA that contains the GM gene and the antibiotic resistance marker gene is essentially random, most insertions will occur in non-permissive regions of the plant cell DNA and will not result in expression of either the marker gene or the GM gene. Cells in which such insertions have occurred will not survive exposure to the antibiotic. Only when the antibiotic resistance marker gene happens to have been inserted into a permissive region of the plant cell DNA will the cell express the marker gene and be resistant to the antibiotic.
Permissive regions are areas of DNA where genes important to the functioning of the recipient plant cells are present and active. Thus, selection for antibiotic resistance also selects for recipient cells in which the antibiotic marker gene (and by default the GM gene) have inserted into permissive regions of DNA. The consequence of this is an increased likelihood that the insertion of the GM gene and antibiotic marker gene may cause mutational damage to the structure or function of a gene or genes that are important to the function and even the survival of the recipient plant cell.
This means that the GM procedure maximises the likelihood that incorporation of the GM gene will result in insertional mutagenesis to – damage to – one or more genes that are active and important to the functioning of the plant host.
We conclude from this analysis of the mechanisms by which the GM process can cause mutations that it is not the elegant and precisely controlled scientific process that proponents claim but depends on a large measure of good fortune as to whether one obtains the desired outcome without significant damage.
1.3.5. Is GM technology becoming more precise?
Technologies have been developed that can target GM gene insertion to a predetermined site within the plant’s DNA in an effort to obtain a more predictable outcome and avoid complications that can arise from insertional mutagenesis.18,19,20,21,22
However, these GM transformation methods are not fail-safe. Accidental mistakes can still occur. For example, the genetic engineer intends to insert the gene at one particular site, but the gene might instead be inserted at a different site, causing a range of side-effects.
More importantly, plant biotechnologists still know only a fraction of what there is to be known about the genome of any crop species and about the genetic, biochemical, and cellular functioning of our crop species. That means that even if they select an insertion site that they think will be safe, insertion of a gene at that site could cause a host of unintended side-effects that could:
- Make the crop toxic, allergenic or reduced in nutritional value
- Reduce the ability of the GM crop to resist disease, pests, drought, or other stresses
- Reduce the GM crop’s productivity or compromise other agronomic traits, or
- Cause the GM crop to be damaging to the environment.
Moreover, because tissue culture must still be carried out for these new targeted insertion methods, the mutagenic effects of the tissue culture process remain a major source of unintended damaging side-effects.
These newer methods are also cumbersome and time-consuming, so much so that to date no GM crop that is currently being considered by regulators for approval or that is in the commercialisation pipeline has been produced using these targeted engineering methods.
1.3.6. Why worry about mutations caused in genetic engineering?
GM proponents make four basic arguments to counter concerns about the mutagenic aspects of genetic engineering:
“Mutations happen all the time in nature”
GM proponents say, “Mutations happen all the time in nature as a result of various natural exposures, for example, to ultraviolet light, so mutations caused by genetic engineering of plants are not a problem.”
In fact, mutations occur infrequently in nature.9 And comparing natural mutations with those that occur during the GM transformation process is like comparing apples and oranges. Every plant species has encountered natural mutagens, including certain types and levels of ionizing radiation and chemicals, throughout its natural history and has evolved mechanisms for preventing, repairing, and minimising the impacts of mutations caused by such agents. But plants have not evolved mechanisms to repair or compensate for the insertional mutations that occur during genetic modification. Also, the high frequency of mutations caused by tissue culture during the GM process is likely to overwhelm the repair mechanisms of crop plants.
Natural recombination events that move large stretches of DNA around a plant’s genome do occur. But these involve DNA sequences that are already part of the plant’s own genome, not DNA that is foreign to the species.
“Conventional breeding is more disruptive to gene expression than GM”
GM proponents cite studies by Batista and colleagues23 and Ahloowalia and colleagues10 to claim that “conventional” breeding is at least as disruptive to gene expression as GM.24 They argue that if we expect GM crops to be tested extensively because of risks resulting from mutations, then governments should require conventionally bred plants to be tested in the same way. But they do not, and experience shows that plants created by conventional breeding are not hazardous. Therefore crops generated by conventional breeding and by genetic engineering present no special risks and do not require special testing.
This argument is based on what appears to be an intentional misrepresentation of the studies of Batista and Ahloowalia. These studies did not compare conventional breeding with GM, but gamma-ray-induced mutation breeding with GM.
The research of Batista and colleagues and Ahloowalia and colleagues actually provides strong evidence consistent with our arguments, above, indicating that mutation breeding is highly disruptive – even more so than genetic modification.
Batista and colleagues found that in rice varieties developed through radiation-induced mutation breeding, gene expression was disrupted even more than in varieties generated through genetic modification. They concluded that for the rice varieties examined, mutation breeding was more disruptive to gene expression than genetic engineering.23
Thus, Batista and colleagues compared two highly disruptive methods and concluded that genetic engineering was, in the cases considered in their study, the less disruptive of the two methods.
The GM proponents used the work of Batista and colleagues and Ahloowalia and colleagues to argue that, since mutation breeding is not regulated, genetic modification of crops should not be regulated either. The amusing part of their argument is that they represent the mutation-bred crop varieties as “conventionally bred”, not even mentioning that they were generated through exposure to high levels of gamma radiation. They then argue that, since these supposedly “conventionally bred” varieties are disrupted similarly to the GM varieties studied, it was not justified to require GM crop varieties to be subjected to safety assessment when “conventionally bred” varieties were not.24
Their argument only carries weight if the reader is unaware of the biotech proponents’ misrepresentation of mutation bred varieties as “conventionally bred”. When this fact comes to light, it not only causes their argument to disintegrate, but also exposes what appears to be a willingness to bend the truth to make arguments favouring GM technology. This in turn raises questions regarding the GM proponents’ motives and adherence to the standards of proper scientific debate.
Interestingly, the GM proponents’ conclusions were diametrically opposite to the conclusions that Batista and colleagues drew from their findings. The researchers concluded that crop varieties produced through mutation breeding and crops produced through genetic engineering should both be subjected to rigorous safety testing.23
In contrast, the GM proponents ignored the conclusions of Batista and colleagues and concluded the opposite: that as mutation-bred crops are not currently required to be assessed for safety, GM crops should not be subjected to such a requirement either.
We agree with the conclusions of Batista and colleagues. Although their study does not examine enough GM crop varieties and mutation-bred crop varieties to make generalised comparisons between mutation breeding and genetic engineering, it does provide evidence that both methods significantly disrupt gene regulation and expression, suggesting that crops generated through these two methods should be assessed for safety with similar levels of rigour. The fact that the risks of mutation breeding have been overlooked in the regulations of some countries does not justify overlooking the risks of GM crops.
We recommend that regulations around the world should be revised to treat mutation-bred crops with the same sceptical scrutiny with which GM crops should be treated. In fact, the Canadian government has reached a similar conclusion and requires mutation-bred crops to be assessed according to the same requirements as GMOs produced through recombinant DNA techniques.25
“Mutations occurring in genetic modification are no different from those that occur in natural breeding”
GM proponents say that in conventional breeding, traits from one variety of a crop are introduced into another variety by means of a genetic cross. They point out that the result is offspring that receive one set of chromosomes from one parent and another set from the other. They further point out that, during the early stages of development, those chromosomes undergo a process (sister chromatid exchange) in which pieces of chromosomes from one parent are recombined with pieces from the other.
They suggest that the result is a patchwork that contains tens of thousands of deviations from the DNA sequences present in the chromosomes of either parent. They imply that these deviations can be regarded as tens of thousands of mutations, and conclude that because we do not require these crosses to undergo biosafety testing before they are commercialised, we should not require GM crops, which contain only a few genetic mutations, to be tested.
But this a spurious argument, because sister chromatid exchange (SCE) is not the random fragmentation and recombination of the chromosomes of the two parents. Exchanges occur in a precise manner between the corresponding genes and their surrounding regions in the chromosomes donated by the two parents. SCE is not an imprecise, uncontrolled process like genetic modification.
Natural mechanisms at work within the nucleus of the fertilized egg result in precise recombination events between the copy of the maternal copy of gene A and the paternal copy of gene A. Similarly, thousands of other precise recombination events take place between the corresponding maternal and paternal genes to generate the genome that is unique to the new individual.
This is not an example of random mutations but of the precision with which natural mechanisms work on the level of the DNA to generate diversity within a species, yet at the same time preserve, with letter-by-letter exactness, the integrity of the genome.
When a fertilised ovum undergoes sister chromatid exchange as part of conventional breeding, the chromosome rearrangements do not take place in a random and haphazard way, but are precisely guided so that no information is lost. There can be defects in the process, which could lead to mutations. But the process works against defects occurring by employing precise cellular mechanisms that have evolved over hundreds of thousands of years to preserve the order and information content of the genome of the species.
Genetic engineering, on the other hand, is an artificial laboratory procedure that forcibly introduces foreign DNA into the cells of a plant. Once the engineered transgene is in the nucleus of the cells, it breaks randomly into the DNA of the plant and inserts into that site. Furthermore, GM plants do not contain only a few mutations. The GM transformation process produces hundreds or thousands of mutations throughout the plant’s DNA.
For these reasons, conventional breeding is far more precise and carries fewer mutation-related risks than genetic engineering.
“We will select out harmful mutations”
GM proponents say that even if harmful mutations occur, that is not a problem. They say that during the genetic engineering process, the GM plants undergo many levels of screening and selection, and the genetic engineers will catch any plants that have harmful mutations and eliminate them during this process.
As explained above, the process of gene insertion during the process of genetic modification selects for engineered GM gene insertion into active gene regions of the host (recipient) plant cell. This means that the process has a high inherent potential to disrupt the function of active genes present in the plant’s DNA.
In many cases, the disruption will be fatal – the engineered cell will die and will not grow into a GM plant. In other cases, the plant will compensate for the lost function in some way, or the insertion will occur at a location that seems to cause minimal disruption of the plant cell’s functioning. This is what is desired. But just because a plant grows vigorously does not mean that it is safe to eat and safe for the environment. It could have a mutation that causes it to produce substances that harm consumers or to damage the ecosystem.
Genetic engineers do not carry out detailed screening that would catch all potentially harmful plants. They introduce the GM gene(s) into hundreds or thousands of plant cells and grow them out into individual GM plants. If the gene insertion process has damaged the function of one or more plant cell genes that are essential for survival, the cell will not survive this process. So plants carrying such “lethal” mutations will be eliminated. But the genetic engineer is often left with several thousand individual GM plants, each of them different, because:
- The engineered genes have been inserted in different locations within the DNA of each plant
- Other mutations or disturbances in host gene function have occurred at other locations in the plants through the mechanisms described above (1.3.4).
How do genetic engineers sort through the GM plants to identify the one or two that they are going to commercialise? The main thing that they do is to verify that the trait that the engineered transgene is supposed to confer has been expressed in the plant. That is, they do a test that allows them to find the few plants among the many thousands that express the desired trait. Of those, they pick one that looks healthy, strong, and capable of being bred on and propagated.
That is all they do. Such screening cannot detect plants that have undergone mutations that cause them to produce substances that are harmful to consumers or lacking in important nutrients.
It is unrealistic for GM proponents to claim that they can detect all hazards based on differences in the crop’s appearance, vigour, or yield. Some mutations will give rise to changes that the breeder will see in the greenhouse or field, but others give rise to changes that are not visible because they occur at a subtle biochemical level or only under certain circumstances. So only a small proportion of potentially harmful mutations will be eliminated by the breeder’s superficial inspection. Their scrutiny cannot ensure that the plant is safe to eat.
Some agronomic and environmental risks will be missed, as well. For instance, during the GM transformation process, a mutation may destroy a gene that makes the plant resistant to a certain pathogen or an environmental stress like extreme heat or drought. But that mutation will be revealed only if the plant is intentionally exposed to that pathogen or stress in a systematic way. Developers of GM crops are not capable of screening for resistance to every potential pathogen or environmental stress. So such mutations can sit like silent time bombs within the GM plant, ready to “explode” at any time when there is an outbreak of the relevant pathogen or an exposure to the relevant environmental stress.
An example of this kind of limitation was an early – but widely planted – variety of Roundup Ready® soy. It turned out that this variety was much more sensitive than non-GM soy varieties to heat stress and more prone to infection.26