by Heather Landry
Summary: The vast diversity in gene sequences are what create the large variety of plants and animals we see today. Genetic diversity is crucial for adapting to new environments, as more variation in genes leads to more individuals of a population having favorable traits to withstand harsh conditions. Low genetic diversity, on the other hand, can be very problematic during changing environments, as all individuals will react similarly. It is assumed that genetically engineered modifications may affect the genetic diversity of a population through crossbreeding or uncontrolled growth; therefore, many researchers are investigating whether this is true and how it might be prevented.
For billions of years, evolution has given rise to the diverse life forms on Earth today. This process has created species with wide-ranging traits and characteristics; however, producing desired agricultural products by natural evolution or selective breeding can be very slow. Now that researchers have a better understanding of genetic engineering, it has become possible to bypass evolution by introducing genetic modifications into plants and animals in the lab. These genetically modified organisms (GMOs) are advantageous for the food supply because they contribute to faster crop production (see this article and this article), pest resistance (see this article and this article), and more nutritious food sources (see this article). Despite these benefits of GMOs, it is imperative to first understand the risks of producing GMOs before introducing them into the wild.
A major concern of genetically modified organisms is that they will cause reduced genetic diversity of plants and animals in the environment. What this means is that the DNA, which codes for proteins in an organism, will become more similar between individuals of a species. Genetic diversity is directly related to biodiversity, the variability in the traits of organisms that make up an ecosystem, because diversity in DNA will inform the characteristics of the organisms that make up a population. Maintaining genetic diversity is important for the environment and agriculture because increased variability in DNA will provide a better opportunity for organisms to adapt to a changing environment.
One example of when a lack of genetic diversity contributed to a major agricultural problem is the potato famine that afflicted Ireland in the mid 1800s. At this time, Ireland was heavily dependent on potatoes for nutrition, and the type of potatoes they cultivated were not grown from seeds. Instead, they planted sections from a parent potato. In this way, all potatoes were clones of their parents and contained identical genetic information. The lack of genetic variability in these potato crops proved detrimental when an invasive pathogen, P. infestans, wiped out the entire population . Because all potatoes had nearly identical genes, there were no populations of potatoes with favorable traits that allowed them to evade P. infestans. Had Ireland grown different varieties of potato crops with more genetic diversity, it would have been more likely for a population of potatoes to contain genes that provide resistance to the pathogen. If a large enough percentage of potato crops in Ireland were resistant to P. infestans, perhaps this famine would not have been so catastrophic.
Figure 1. Reduced genetic diversity contributes to weak adaptation to changing environments. During the Irish potato famine, most potatoes were clones of their parents with nearly identical gene sequences. Had the population of cultivated potatoes been more genetically diverse (top panel), many potatoes would have had a greater opportunity to survive the deadly pathogen, P. infestans. However, because there was low genetic diversity in Irish potatoes at the time, a vast majority of potato crops were wiped out by the pathogen (bottom panel).
So how might GMOs affect genetic diversity? One possibility is that GMOs may crossbreed with wild plants or animals. A second is that favorable traits could allow GMOs to take over a population. It is easy to speculate how these situations would lead to changes in genetic diversity, but have they ever been observed with GMOs growing today?
When GMOs and non-GMOs mate
The ability for different species to mate, also known as hybridization, has allowed for the vast diversity of wild plant types we see in the environment today. However, this process is not restricted to wild plants and can occur between any type of plant, including wild crops and GMOs, if they are reproductively compatible. When GM plants are in close proximity with wild plants, they can cross-pollinate, producing a hybrid version of the two. Typically, natural hybridization has a positive impact on genetic diversity because it introduces new gene combinations into a population. However, opponents to GMOs are concerned that if modified genes are introduced into wild plant populations by hybridization, they could impart a fitness advantage in the hybrid species, meaning that the hybrid species would be better able to reproduce. This fitness advantage would lead to the engineered gene being maintained in the population and thus reduce the genetic diversity of the wild species.
One example of hybridization between GM crops and wild species has been documented in creeping bentgrass commonly used on golf courses. Scotts Miracle-Gro genetically engineered this grass to contain a gene that confers resistance to a common herbicide so that the golf course can be sprayed to kill weeds without harm to the grass. Because this grass is wind-pollinated, a perennial, and highly capable of outcrossing with related wild species, researchers decided to investigate wild grass in close proximity for the presence of the herbicide resistance gene. The researchers collected seeds from wild plants at varied distances from the origin of the grass farm and used herbicides to detect tolerant plants. They confirmed the presence of the herbicide resistant gene in wild grass up to 9 miles from its origin only one year after the grass was planted [2,3,4]. This distance is very surprising because most hybridization events have been reported between plants that are less than a mile apart. It is clear from this study that genetic modifications can be transferred to wild species through hybridization; however, future investigations will need to be performed in order to differentiate whether the genetic modification increases the fitness of wild species or if these hybridizations are a natural result of planting a large grass farm in close proximity to wild species.
Studies of creeping bentgrass uncovered clear hybridization events between GM crops and wild grass; however, it is important to recognize that cross-pollination is not equally likely for all crops. Many crops commonly cultivated in the US, such as corn, soybeans, and cotton, are not perennials and do not have wild relatives growing in close proximity. It is clear from this study that understanding the reproductive behavior of a GM crop and the function of its genetic modification is very important before introducing it into the wild.
Figure 2. Spreading of GM creeping bentgrass into wild populations. To investigate potential hybridization events between GM creeping bentgrass (A. stolonifera) and the closely related species, A. gigantea, researchers tested wild A. gigantea plants at 39 locations surrounding the GM bentgrass farm. Hybridization was tested by growing seeds from wild A. gigantea plants and testing the resulting grass for 1) resistance to herbicide and 2) presence of the herbicide resistance gene. The researchers found seeds with the genetic modifications at 13 of the 39 locations (red dots), including one location 9 miles from the GM bentgrass farm. The GM positive seeds at all locations were less than 0.5% of the entire population. Adapted from Watrud et al., 2004 .
Genetically engineered traits may be too advantageous
In addition to crossbreeding, GMOs can also affect genetic diversity through uncontrolled growth of a genetically engineered population. If advantageous genes are introduced into GMOs, it may allow them to become more fit than their wild relatives. This situation would be detrimental because the GMOs would grow faster and reproduce more often, allowing them to take resources away from non-GMO relatives if they inhabit the same environment.
One GM animal where uncontrolled growth is a concern is a fast-growing Atlantic salmon engineered by AquaBounty technologies to reach market weight in half the time as their standard relatives. AquaBounty introduced two sequences of DNA into these salmon. The first codes a growth hormone from the related Chinook salmon that stimulates growth, and the second is a sequence that activates the growth hormone year-round and not just in warm weather [4,6]. The combination of these two DNA sequences allows these fish to develop at a dramatically increased rate, and many are concerned about what would happen if they escaped into the wild. Some believe that engineered salmon will continue to grow at a faster rate in the wild. However, others suspect that because the engineered salmon have traits that were not developed by natural selection, they will not be perfectly adapted to the wild environment, resulting in similar or even reduced fitness compared to wild relatives in their natural habitat. In one study performed in a similar transgenic salmon, it was shown that fish with a genetically modified growth hormone actually grow at a much closer rate to wild fish in a tank that simulates a natural stream-like habitat compared to a conventional hatchery tank . Despite studies like this, it is not possible to conclude whether this is the case for all types of genetically engineered salmon or whether it will hold true in the wild . Therefore, to prevent GM salmon from propagating in the wild, AquaBounty uses both physical and biological methods of containment. AquaBounty’s salmon are all female, grown in land-based tanks, and are sterile due to increased chromosome number. The Food and Drug Administration (FDA) has an ongoing review of AquaBounty’s genetically engineered salmon and no decision has yet been made about whether this salmon will be the first genetically modified animal to enter the US food supply.
Figure 3. Growth of genetically engineered salmon depends on the environment. One study with genetically engineered coho salmon investigated the growth of GM and non-GM salmon in hatcheries verses a simulated natural environment. They found that the GM salmon are almost three times the length of non-GM salmon reared for one year in a hatchery environment. However, after one year of growth in a simulated natural environment, GM salmon are only about 1.25 times the length of non-GM salmon. Adapted from Sundström et al., 2007 .
What can be done to protect genetic diversity?
Although there is little evidence that GMOs have impacted genetic diversity in today’s environment, scientists and ecologists are very aware of the potential influence that GMOs have on biodiversity. Therefore, researchers are investigating how to better prevent crossbreeding and spreading of GMOs, similar to the physical and biological containment strategies used for AquaBounty’s salmon. Some approaches for preventing hybridization of plants involve methods that make the second generation of seeds sterile or dependent on a chemical for fertility. Other approaches prevent the spread of genetically modified material by requiring that two GMO plants must be crossed in order for the offspring to contain the advantageous trait. Some researchers have even completely recoded the genome of bacteria to include synthetic amino acids that are not present in the wild [9,10]. This approach, if adapted to crops treated with synthetic amino acids in an enclosed area, is a powerful tool because it significantly limits the chance that GM plants will escape dependence on the synthetic amino acids or be able to share its DNA with non-GM species.
We have learned from evolution that organisms are capable of developing a large variety of advantageous traits through natural genetic mutations and hybridizations. By manipulating this system, scientists are still uncovering how genetically engineered modifications affect the natural environment. Many of the concerns with genetic diversity in agriculture are not restricted to GMOs, as standard crop cultivation faces very similar issues. Therefore, it is imperative that researchers continue to study the impact of GMOs and agricultural practices on genetic diversity and discover new ways to minimize their influence on biodiversity.
Heather Landry is a Ph.D. candidate in the Biological and Biomedical Sciences Program at Harvard University.
This article is part of the August 2015 Special Edition, Genetically Modified Organisms and Our Food.
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