Coral reefs are the poster-organisms for ecosystem services, aiding fisheries, promoting biodiversity, and protecting land from heavy waves. Unfortunately, we seem to be repaying 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.
Now scientists have announced a new tool is available to study corals: genetic editing provided by the CRISPR/Cas9 system. The ability to selectively eliminate genes could help us understand how corals function normally and could eventually provide a tool that lets us help them ride out climate change.
Coral complexities
You might think that we’d have a pretty good grasp of coral biology, given the amount of study that reefs receive. But much of that study has 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 one example, coral bleaching is caused by a heat-driven breakdown in the symbiosis between coral and a photosynthetic algae that provides the coral with food. Corals that live in warmer waters are clearly able to form partnerships with heat-tolerant algae, but the precise mechanics of which species partner with what algae aren’t 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 a variety of depths; some species are specialized for specific environments, while others can find homes in a variety of habitats. While we’ve got some idea of the genes that may be involved in many of these adaptations, the authors of the new paper describe these as “plausible hypotheses.” The problem? We can’t really test any of them. That’s in part because corals are slow growing and have a complex lifestyle that’s hard to replicate in the lab. And it’s in part because we can’t manipulate them at the genetic level, eliminating a gene and seeing if the process we think it’s involved in are effected.
Now it seems like it’s more “couldn’t” than “can’t.” Gene editing, in which genes can be disabled or modified, has taken off in a variety of organisms thanks to a system called CRISPR/Cas9. The system evolved to help bacteria disable viruses that infected them and acts by allowing a short piece of RNA to guide an enzyme to a matching DNA sequence, which the enzyme then cuts. People eventually figured out that if you provide it with a piece of RNA that matches a gene, it’ll happily cut the gene for you. A cell’s DNA repair process kicks in, either deleting a short piece of the gene (and disabling it) or repairing it with a matching sequence that you provide.
Testing
So, a small international team of researchers decided to test whether the process could work in coral. That’s a bit of a challenge, given that corals mostly reproduce at specific times of the year through what’s called “broadcast spawning.” That’s basically releasing sperm and eggs into the water column and hoping they bump into each other. Still, the researchers were able to identify a species that could breed under lab conditions and obtained single-celled embryos to inject with a solution containing the CRISPR/Cas9 system.
DNA analysis showed that it worked, but things were somewhat complicated. To begin with, corals, like us, have two sets of chromosomes, one from each parent. And the system only seemed to edit one of the two chromosomes. As a result, even though they knocked out a copy of what seems to be a very important gene, the corals developed normally, since they still had a good copy. If we’re actually going to study their genetics, this will have to be optimized, since otherwise we’d have to figure out how to get the corals to go for two generations in the lab (the researchers note “there is little immediate prospect” of that happening).
The other issue is that some of the gene editing took place after the embryos’ cells started dividing. This led to organisms that were a complicated mix of healthy and partly edited cells, with somewhat different edits in many of the cells. That runs the risk of creating some complicated genetics that are difficult to interpret.
So, we’re not there yet when it comes to editing the genes of corals, but the initial results are promising, and CRISPR/Cas9 appears to be a pretty flexible system, given all the other organisms it has been optimized for. So, it seems quite plausible that we’ll be able to get this system working well enough to understand how corals manage their complex biological behaviors. And, perhaps more importantly, this raises the prospect that we could eventually be able to manipulate those behaviors, splicing in gene variants that allow corals to maintain a symbiosis that’s more heat tolerant, to give one example.
Whether that would be enough to maintain any reefs in the face of the changes we’re likely to see isn’t clear. And it shouldn’t be viewed as providing us an excuse for not trying to prevent even worse changes from happening. But one thing is obvious: protesters will undoubtedly try to stop the release of GMO coral should we ever get there.
, 2018. DOI: 10.1073/pnas.1722151115 (About DOIs).
