by Sophie Westbrook
It’s not hard to tell frightening stories about genetically modified crops. These days, there is even a formula to follow: the soulless company creates dangerous variants, silences the protests of right-thinking environmentalists, and sends biodiversity and public health down the drain. This scenario’s proponents tend to be horrified by transgenic organisms. Unfortunately, this can polarize their conversations with any agricultural scientists responsible for “Frankenstein’s monster.” The fiery controversies often marginalize a key idea: genetically modified crops are part of the biosphere. This has more complex implications than the popular “they’ll drive everything else extinct” hypothesis. We cannot understand what transgenic variants will do to—or for—us without examining when, how, and why the organisms around them react to invasion.
Genetically modified (“GM”) crops were a cornerstone of the 1980s push for more efficient agriculture. Initial experiments with pest and disease resistance were quickly followed by qualitative modifications: engineered crops could grow quickly into big, attractive specimens with a predetermined chemical makeup.1 Almost immediately, the technology spawned concerns rooted in food safety, economics, and environmental impact. A number of these issues are still with us. In particular, scientists and citizens alike struggle to understand the implications of resistance evolution, gene flow between hybrid and natural populations, and competitive advantages.2 A nuanced discussion of these topics is critical to developing a modern crop management plan.
To GM proponents, pest resistance is one of the technology’s best success stories. Modified crops can repel not only bacterial invaders but also undesirable insects and weeds. This trait improves output by increasing survival rates and facilitating “double-cropping,” planting twice a year and letting growth extend into insect-heavy seasons.3 It also has the potential to reduce non-point source pollution: GM plants need less protection from sprayed pesticides and herbicides. These developments have produced higher yields at lower costs worldwide.
Naturally, the introduction of “hostile” GM organisms into the environment has consequences. Shifting land use patterns can drive “pest” species away from areas used for GM crop cultivation. This is worth keeping in mind as the amount of land used for GM crops continues to grow. In 2010, GM crops covered an estimated 140 million hectares (346 million acres).3 Larger-scale cultivation could destabilize the ecosystems surrounding cultivation sites by removing key producers and primary consumers.
There are more immediate concerns, though. Where anti-insect crops grow, some insect species are developing resistances to their artificially introduced toxins. For instance, corn plants modified with Bt toxins are increasingly unable to repel caterpillars. This effect has been observed globally.2 Adding increasingly poisonous compounds would only prompt more evolution by the insect species. Such ideas also raise questions about health impacts and environmental contamination.
Other GM crops are not themselves toxic, but have been engineered to resist cheap, relatively safe herbicides. Glyphosate-resistant crops in the United States are a notable example. Since their introduction and initial success, they have been viewed as an easy solution to weed problems.4 Some staple crop species, like corn and soybeans, are seldom cultivated without glyphosates. Now, these herbicides are increasingly ineffective (and consequently applied in increasing concentrations). This is evidence that weed species are experiencing strong selective pressures to develop their own herbicide resistance.
The fact that GM crops prompt pest evolution is neither shocking nor devastating. After all, unmodified plots also promote shifting gene frequencies; some organisms are better suited to take advantage of crop growth than others. However, the GM era has seen an unusually violent “arms race” between scientists and pests. Acknowledging that native insects and weeds can always evolve in response to invading species’ biochemistry means investigating alternative, multi-layered management solutions.2
Plants share DNA. Gene flow, the transfer of genetic material from one population to another, is one of their fundamental mechanisms for generating biodiversity. When this happens between GM and natural crops, it can lead to transgene introgression: the fixture of an “invasive” modified gene into a native species’ genome.5 Transgene introgression is never sure to happen. At a minimum, it tends to require population compatibility on a sub-genomic scale, strong selective pressure to retain the transferred trait, time, and luck.5 Even “useful,” artificially inserted genes have a relatively low probability of leaping to nearby organisms.
There are two key barriers to spreading transgenes. First, many modern crops lack genetically compatible wild types, so they simply cannot spread their modifications. Second, “domestication genes” are frequently unsuccessful in natural populations.2
That said, transgene introgression does occur. One of the most famous cases took place between maize populations in Oaxaca, Mexico.6 There was widespread alarm when remotely situated wild maize contained artificial genes. It called into question our ability to safeguard unmodified plant varieties, which would become critical if a commonplace GM species proved unviable or unsafe.
Oaxaca has been analyzed extensively. Unfortunately, data on specific events cannot help us prevent transgene introgression everywhere. The process depends heavily on which species are involved, so one-size-fits-all policies for discouraging gene flow are inadequate.7 A more specialized understanding would help us to manage the possibility of dangerous escapee genes and better answer questions about legal and ethical responsibility.
When functionally similar invasive and native species do not hybridize, they often compete for the same resources. If the native species is wholly out-classed, it may be driven to extinction. This is the idea behind discussions about GM crops’ threat to biodiversity. Biodiversity is indisputably necessary: it is the foundation of stability and flexibility in an ecosystem. Allowing a single variant to overcome its peers would leave any community more vulnerable to stresses like disease, climate change, and natural disaster.
Do GM crops have an advantage over natural ones in the wild? They tend to incorporate some traits, such as fast growth and temperature tolerance which promote greater survivorship. However, as mentioned above, they are primarily adapted to living in cultivated areas. This means that they lack characteristics like seed dormancy and phenotypic plasticity (the ability to take different forms) that would make them more effective, invasive, weeds.8
Looking forward, extreme crop modifications mean GM variants may entirely lose metabolic capabilities they would need to survive in nature.7 This suggests that they should become increasingly unlikely to succeed after accidental dispersal. Nonetheless, hard-to-predict factors such as mutations within modified crops could always lead to the loss of native populations. Once a species—natural or transgenic—becomes invasive, it is nearly impossible to recapture. Unfortunately, GM plot isolation is a difficult proposition, especially given the crops’ prevalence throughout the developing world.
Wild organisms have a surprisingly diverse menu of responses to transgenic invaders. They may evolve in response to the crops’ new traits, hybridize to access the traits themselves, or try to outcompete the variants through their other weaknesses. The strategy adopted depends primarily on the native species’ ecological position, but also on the characteristics of the invader. To develop a comprehensive understanding of the ways GM crops affect the communities they enter, we need to analyze these relationships in all their variety. This examination may lay the groundwork for a safer, more sustainable food supply in the future.
Sophie Westbrook ‘19 is a freshman in Hurlbut Hall.
- Nap, J. P. et al. Plant J. 2003, 33, 1-18.
- Goldstein, D. A. J. Med. Toxicol. 2014, 10, 194-201.
- Barrows, G. et al. J. Econ. Perspect. 2014, 28, 99-120.
- Beckie, H. J.; Hall, L. M. Crop Prot. 2014, 66, 40-45.
- Stewart, C. N., Jr. et al. Nat. Rev. Genet. 2003, 4, 806-817.
- Quist, J.; Chapela, I.H. Nature 2001, 414, 541-543.
- Messeguer, J. Plant Cell Tiss. Org. 2003, 73, 201-212.
- Conner, A. J. et al. Plant J. 2003, 33, 19-46.
Categories: Fall 2015