The Paper: van Oppen, M.J.H., Oliver, J.K., Putnam, H.M., Gates, R.D. Building coral reef resilience through assisted evolution. PNAS. Doi.10.1073.pans.14223011112.
Recently, we have been witnessing a disturbing decline in coral health and coral reef coverage worldwide. Raising sea temperatures, pollutants (like microplastics! Read yesterday’s post for more info) storm events, and extensive trawling are just a few of the factors that can be attributed to the worsening state of coral reefs. Many of these issues are at least partly caused by human actions. While organisms are able to evolve to deal with environmental changes, the process takes a long time. Right now major environmental changes (like ocean warming and ocean acidification) are occurring at a relatively fast pace, posing threats to corals reefs.
When stressed by increased water temperatures, corals tend to undergo bleaching events. Essentially, bleaching occurs when the coral expels its algal symbionts. These symbionts live within the coral, providing the coral with nutrients in exchange for a place to live. Because these symbionts provide the coral with food, a coral left without its little helpers for too long may not be able to adequately feed itself and is more susceptible to disease. Ocean acidification is also predicted to hinder the growth rate and diminish health of corals by making it more difficult for them to build their skeleton. For a primer on how ocean acidification impacts the ocean, watch this short video:Ocean-acidification explained with a soda maker . With changes like these occurring at a relatively fast rate, there is concern that corals will not be able to keep pace with the environmental changes and will suffer extreme population depletions.
Some scientists, such as the authors of this paper, have proposed a novel coral management strategy which would help corals evolve at a faster pace than they would under natural conditions. This technique is called human-assisted evolution. This phrase may sound a bit frightening at first, but believe it or not, human-assisted evolution is nothing new!
Humans have been utilizing evolutionary processes to create beneficial alterations to organisms for thousands of years. Who loves their golden retriever? You can thank selective breeding for the wide variety of dog breeds present today (Fig 1a). Selective breeding is a process by which humans chose specific desirable traits and selectively breed individuals to, overtime, create a new dog (or horse, or cat, etc). A variety of animals and even plants and microbes have been selectively breed for human benefit.
More recently, techniques like acclimatization (a process by which an organism adapts to changes in the environment over the course of its own lifetime), trans-generational acclimatization (where the environmental changes can force adaptations that are actually handed down to the next generation), and genetic modification have been utilized in areas such as agriculture and farming. These techniques have given rise to crops that are able to withstand more stressful environments, plants capable of increased nutrient values (Fig 1b), and increased growth rates and meat yields in some livestock (Fig 1c). Though controversial, current technology allows us to alter organisms at a faster rate than would normally occur through the natural processes of evolution.
The Current State of Coral Reef Management:
Coral reef management is of high priority and ongoing research is aimed at effectively protecting degraded reef habitats. Currently, the preferred method is coral “gardening” (Fig 2). Coral clippings are taken from healthy reef systems and are grown up until they are larger. Once the coral reaches an optimal size, the coral colonies are transplanted onto a degraded reef. With some luck, the gardened coral colonies will settle happily in their new home and can help restore the degraded coral reef. The authors agree the current restoration strategies should continue and that additional research on management strategies is needed. Moreover, they propose the novel use of human-assisted evolution to further aid the restoration of diminishing reef populations.
Assisting Coral Reef Evolution:
Based on the biology of corals and the current human-assisted evolutionary protocols used in terrestrial organisms, the authors propose four main types of human-assisted evolution to be investigated.
- Exposing natural coral stocks to stresses to induce acclimatization and transgenerational acclimatization. By forcing the native corals to undergo stresses similar to those that they may experience under changing environmental conditions, the corals may boot out their current symbionts and replace them with new symbionts that are more beneficial under altered environmental conditions. This strategy will take less time than natural genetic adaptation and (due to the mechanism by which symbionts are transferred from parent to offspring in coral) may last for several generations- hopefully allowing the corals to live to fight another day in the face of rapid environmental changes.
- Actively modifying the symbionts associated with coral reefs. There are numerous types of coral symbionts, some of which have become specially adapted to certain environments (like warmer areas for instance), but may not be found in every coral reef habitat. This technique would actively take strains of the symbionts which have naturally developed to tolerate specific environmental conditions and introduce them into coral colonies in need of the specific environmental tolerance. (for example, take those symbionts that have evolved in warmer oceans, and introduce them into an area where coral are dying due to warming temperatures and hope that the change in symbiont allows the coral to survive).
- Genetically altering the coral symbionts to be better suited to changing environmental conditions. This differs from strategy #2 because instead of utilizing naturally occurring symbionts, the research would focus on actively changing the genetic structure of these symbionts to be better suited for a variety of environmental conditions. These would be “lab-grown” strains of symbionts that would then be introduced into wild coral populations. So for example, researchers may be able to induce changes to the symbiont’s genetic structure, making them more tolerant to environmental stressors (i.e. by exposing the symbiont to a mutagen like UV light). The mutated, stress-tolerant symbiont can then be introduced into natural coral populations, hopefully allowing them to be better adapted to environmental change.
- Selectively breeding coral strains that are better suited for specific environmental conditions. This is a lot like breeding dogs- start out with your generic dog. If you want an extremely tall dog, you take the two tallest puppies you can find and you breed them. That litter should produce a higher percentage of taller dogs.You then take your two tallest puppies from that litter and breed them to produce a litter with an even higher percentage of taller individuals. You keep doing this for multiple generations and BANG! you have yourself a Great Dane! Same for corals- breed corals under high temperature, for instance, and take those that survive best in that condition and selectively breed those individuals for generations until you have ideally produced a line of coral that can withstand higher temperatures.
Human-assisted evolution is still a controversial topic for coral reef management. The authors do not discuss the potential issues that may be presented by trying to alter the course of evolution in organisms. They merely bring the potential management strategies to the table to try and start a discussion. The purpose of their article is not to urge scientists to begin altering natural coral populations in the wild. Instead, they encourage scientists to begin looking into the possibilities, the pros, and the cons of aiding coral evolution through human assistance as a potential management and restoration technique.