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GMOs struggle to stay one step ahead of evolution

October, 2016

Western corn rootworm

Western Corn Rootworm searching for pollen on corn silk. Credit: Wikimedia

While strawberries containing fish genes make big news, they haven't actually made an appearance at your local grocery store. In fact, few genetically modified organisms are currently sold as food in the US, but the exceptions are doozies: about 85 percent of the corn and soybeans grown in the US are genetically engineered. Most of those crops are engineered for two particular traits — to resist herbicides like Roundup (which is useful because it allows growers to use the substance against weeds in the same fields as their crops) and, in the case of corn, to kill the Western Corn Rootworm, a beetle larva that eats cornstalks from the ground up. Genetically modified corn kills rootworm with a gene originally found in the soil bacterium, Bacillus thuringiensis (i.e., Bt). This gene produces a protein that is toxic to rootworms, but not to humans and many other animals. So-called Bt corn has been on the market for a little over a decade; however, in recent years, the benefits of using this variety have dwindled as rootworm populations have evolved resistance to the Bt toxin. Now, scientists working for DuPont have announced the discovery of another gene from a different soil bacterium that could replace Bt once resistance is so prevalent that the Bt toxin is no longer effective.

Where's the evolution?

When inserted into the corn genome, the newly discovered gene (like Bt) produces a toxin that kills rootworm. And like Bt, the new gene will be vulnerable to the action of natural selection. If corn modified to carry the new gene is used widely, any rootworms that happen to carry genes that allow them to resist the toxin, even a little bit, will be more likely to survive and reproduce — and those offspring, in turn, are likely to carry the genes for resistance. Over the course of generations, the resistance genes will become common in a population. Ultimately, this evolutionary process leads to populations that are unaffected by the toxin to which they've been exposed.

In the case of the Bt toxin, studies showed that corn rootworm populations evolved some level of resistance in just three generations! And while widespread resistance has been somewhat slower to arise on farms (it took a little over 10 generations), it is clear that Bt corn's useful life is nearing its end and that the threat of corn rootworm ruining crops is on the rise. If corn engineered to carry this newly discovered toxin gene reaches the market, it would reset the whole process, giving farmers an easy way to protect their crops once again.

Hopeful as this advance is, it does come with a big question: how long will it take for rootworm populations to evolve resistance to the new toxin? After all, in terms of natural selection's ability to shape resistance in rootworm populations, there is little difference between the new toxin and Bt. What can we do to extend the usable shelf life of a new corn variety like this? Evolutionary theory points to a few strategies:

  1. Refuges. Traditionally, refuges were fields planted with regular corn that were adjacent to fields planted with genetically modified (GM) corn. The idea was that rootworms vulnerable to the toxin would survive in the refuge and mate with the few rootworm beetles surviving in the GM field — beetles that presumably carry the resistance genes. Based on the genetics of resistance, the offspring of these match-ups would likely be vulnerable to the toxin. If consistently implemented, this strategy could slow the evolution of widespread resistance by a lot. However, since the refuge fields may be decimated by pests, it's easy for farmers to see them as a waste. In the case of Bt, scientists recommended that 50% of a farmer's fields be devoted to refuge, but it was ultimately mandated that just 5% of fields be refuge, and not all farmers complied. Devoting more area to refuge would likely prolong the utility of any corn variety developed with a new toxin.
  2. Refuge in a bag. Seed companies have begun to sell seed mixes known as "refuge in a bag," which contain a mix of toxin-producing and vulnerable seeds. Instead of farmers devoting selected fields to refuge, they make every field a mix of the two plant types. If such mixes were made with higher percentages of vulnerable seed, it could provide an easy way to encourage compliance with refuge guidelines. On the other hand, there is some concern that placing vulnerable and toxin-producing plants so close to one another might make it more likely for slightly resistant individuals to survive, paving the way for highly resistant individuals to arise. Before this approach is used as a strategy to slow the evolution of resistance, further study is needed.
  3. High-dose GM crops. If plants can be engineered to produce very high doses of the toxin, this could slow the evolution of resistance. That's because a high-dose crop will kill nearly all of the pest population, even those few individuals that happen to carry genes that make them mildly resistant to the toxin. Individuals carrying all the mutations necessary to survive a high dose of toxin are much rarer than individuals carrying one of the many mutations that might confer mild resistance. Unfortunately, the Bt corn marketed as fighting corn rootworm did not produce a high-dose of toxin and this likely hastened the evolution of resistance.
  4. Pesticide pyramids. This refers to plants engineered to produce several different toxins. In terms of evolution, the pesticide pyramid strategy works somewhat as high-dose crops do. Individuals carrying all the mutations necessary to survive many different toxins are much rarer than individuals able to resist a single toxin. Natural selection cannot favor multi-toxin resistant individuals if none exist. Genetic engineers tried to use this strategy with the Bt toxin but failed in several ways. First, when pesticide pyramids were introduced, some populations of insects had already evolved resistance to one or more of the pesticides that the pyramided plants produced. Second, the toxins that these plants produced were not that different from one another, so, for example, insects resistant to toxin A were likely to be somewhat resistant to toxin B. This meant that the Bt pesticide pyramid varieties behaved more like a single toxin GM crop. Genetic engineers and agricultural regulators have a chance to avoid these mistakes if a new insecticide-producing GM corn variety comes to market.
  5. Traditional methods of pest control, like crop rotation. Crop rotation refers to changing the crop that is planted in a particular field from one year to the next. Corn rootworms specialize on corn and lay their eggs in the soil where they overwinter. So, if a cornfield is infested with rootworm one year, the soil will contain rootworm eggs. However, if that field is planted with another crop in the following year, when the eggs hatch, the pests will have nothing to eat and will starve. In fields planted with GM corn, crop rotation could help starve out the few resistant individuals that are able to survive from one year to the next, slowing the evolution of resistance.

Of course, the new GM corn variety still faces a lot of regulatory hurdles before it is approved for market. DuPont scientists need to do testing to show that the new variety is safe for people and for the environment. But if it is approved, heeding the evolutionary lessons learned from the rapid rise and fall of Bt corn could keep this new variety pest free for much longer.

Read more about it

Primary literature:

  • Schellenberger, U., Oral, J., Rosen, B. A., Wei, J., Zhu, G., Xie, W., ... Liu, L. (2016). A selective insecticidal protein from Pseudomonas for controlling corn rootworms. Science. DOI: 10.1126/science.aaf6056 read it
News articles:

Understanding Evolution resources:

Discussion and extension questions

  1. Review the process of natural selection. Use the four steps described on that page to explain how Bt resistance became so common among rootworms feeding on Bt cornfields.
  2. In your own words, explain how pesticide pyramids can help slow the evolution of resistance to Bt.
  3. In your own words, explain how crop rotation can help slow the evolution of resistance to Bt.
  4. Advanced: Do you think it is possible for rootworms to evolve "resistance" to crop rotation — i.e., for traits to evolve that help the rootworms survive in areas where crops are rotated each season? Explain your reasoning, and if you answered yes, explain how such mutations might operate.
  5. Advanced: Compare and contrast this press release from DuPont about the discovery to this article from NPR. Describe two similarities and two differences. How do you think each article was influenced by the perspective of its author?
Related lessons and teaching resources

References

  • Gassmann, A. J., Petzold-Maxwell, J. L., Clifton, E. H., Dunbar, M. W., Hoffmann, A. M., Ingber, D. A., and Keweshan, R. S., (2014). Field-evolved resistance by western corn rootworm to multiple Bacillus thuringiensis toxins in transgenic maize. Proceedings of the National Academy of Sciences USA. doi: 10.1073/pnas.1317179111
  • Gassmann, A. J., Petzold-Maxwell, J. L., Keweshan, R. S., Dunbar, M. W. (2011). Field-evolved resistance to Bt maize by Western Corn Rootworm. PLoS One. 6: e22629.
  • National Agricultural Statistics Service. (June, 2016). Acreage. Retrieved October 5, 2016 from http://usda.mannlib.cornell.edu/usda/current/Acre/Acre-06-30-2016.pdf
  • Schellenberger, U., Oral, J., Rosen, B. A., Wei, J., Zhu, G., Xie, W., ... Liu, L. (2016). A selective insecticidal protein from Pseudomonas for controlling corn rootworms. Science. DOI: 10.1126/science.aaf6056. http://science.sciencemag.org/content/early/2016/09/22/science.aaf6056.full