Coral reefs are poster organisms for ecosystem services, aiding fisheries, promoting biodiversity and protecting the land from heavy waves. Unfortunately, we seem to repay them by killing them. Our warming oceans are causing coral bleaching and death, rising sea levels will force them to move, and the acidification of our oceans will make it harder for them to form reefs. It would be great if we could help them, but interventions are difficult to design if you don’t know enough about coral biology.
Now scientists have announced that a new tool is available to study corals: genetic editing by the CRISPR/Cas9 system. The ability to selectively eliminate genes could help us understand how corals function normally and could ultimately be a tool to help them overcome climate change.
You might think we would have a pretty good understanding of coral biology given the amount of study reefs receive. But much of that study focused on coral reefs as an ecosystem, rather than coral as an organism. And that’s a big barrier to helping these reef builders survive in our changing world. To give an example, coral bleaching is caused by a heat-induced breakdown in the symbiosis between coral and a photosynthetic alga that provides the coral with food. Corals living in warmer waters are clearly able to form partnerships with heat-tolerant algae, but the precise mechanics of which species partner with which algae is not well understood.
There are similar gaps in our knowledge about things like the reef building activities, which vary in speed and the shapes they produce. Corals also tolerate varying depths; some species are specialized for specific environments, while other species can find homes in different habitats. While we have an idea of the genes that may be involved in many of these adaptations, the authors of the new paper describe them as “plausible hypotheses.” The problem? We can’t really test them. That’s partly because corals grow slowly and have complex lifestyles that are difficult to replicate in the lab. And it’s partly because we can’t manipulate them on a genetic level, eliminate a gene and see if the process we think it’s involved in is brought about.
Now it seems more “can’t” than “can’t”. Thanks to a system called CRISPR/Cas9, gene editing, which allows genes to be turned off or changed, has taken off in various organisms. The system evolved to help bacteria knock out viruses that have infected them and works by having a short piece of RNA guide an enzyme to a matching DNA sequence, which the enzyme then cuts. People eventually figured out that if you give it a piece of RNA that matches a gene, it will happily cut the gene for you. A cell’s DNA repair process begins, either removing a short piece of the gene (and turning it off) or repairing it with a matching sequence you provide.
So a small international team of researchers decided to test whether the process could work in coral. That’s a bit of a challenge, as corals usually reproduce at specific times of the year through what’s called “broadcast spawning.” That is basically releasing sperm and eggs into the water column and hoping they crash into each other. Still, the researchers were able to identify a species that could reproduce under laboratory conditions and obtained single-cell embryos for injecting with a solution containing the CRISPR/Cas9 system.
DNA analysis showed it worked, but things were a bit complicated. For starters, just like us, corals have two sets of chromosomes, one from each parent. And the system seemed to only edit one of the two chromosomes. As a result, the corals developed normally, although they knocked out a copy of what appears to be a very important gene, because they still had a good copy. If we’re really going to study their genetics, this will have to be optimized, otherwise we’d have to figure out how to keep the corals working in the lab for two generations (the researchers note, “there’s little immediate prospect” of that happening) .
The other problem is that some of the gene editing happened after the cells of the embryos started dividing. This led to organisms that were a complicated mix of healthy and partially processed cells, with slightly different editing in many of the cells. This carries the risk of creating complicated genetics that are difficult to interpret.
So we’re not there yet when it comes to editing the genes of corals, but the early results are promising, and CRISPR/Cas9 seems to be a pretty flexible system, considering all the other organisms it’s optimized for. So it seems quite plausible that we can make this system work well enough to understand how corals manage their complex biological behavior. And, perhaps more importantly, this raises the prospect that we might eventually be able to manipulate that behavior, by splicing gene variants that allow corals to maintain a symbiosis more resistant to heat, for example. to give.
Whether that would be enough to preserve reefs in light of the changes we’re likely to see isn’t clear. And it shouldn’t be seen as an excuse not to try to prevent even worse changes. But one thing is clear: protesters will no doubt try to stop the release of GMO coral if we ever get there.
PNAS2018. DOI: 10.1073/pnas.1722151115 (About DOIs).