Octopi, squid, and cuttlefish all have the ability to rearrange their RNA bases, one of the rarest traits in the animal kingdom.
Unlike most animals, cephalopods do not follow the genetic instructions in their DNA. Rather, they pick out different adenosine RNA bases that code for protein and replace them with bases known as Inosine. This process — called RNA editing — is rarely used across the animal kingdom. However, octopi and their relatives edit RNA base pairs in over half of their transcribed genes.
An international team of scientists made this discovery by conducting a number of experiments to see how often the animals went through the editing process as well as how the process first developed in the aquatic animals. This revealed that the genetic strategy has both affected and constrained the evolution of the cephalopod genome.
Octopi typically use RNA editing to quickly adapt to temperature changes, and squid neurons contain those edits as well. While almost all vertebrates are capable of this process, it is rarely used. For example, humans have 20,000 genes, but only a few dozen can freely act as editing sites. In contrast, 11,000 of the squids 20,000 proteins are able to edit.
“Basically, this is a mechanism to make proteins that are not encoded in the DNA. They are not present in the genomic sequence,” says study co-author Eli Eisenberg, a biophysicist at Tel Aviv University in Israel, in a statement. “With these cephalopods, this is not the exception. This is the rule. The rule is that most of the proteins are being edited.”
The wide use of this editing is so rare that it goes against the fundamental rules of genetics. Though the team did not quite believe the occurrence when they first saw it, analysis across different species showed it remained consistent in two species of octopus, the common cuttlefish, and one species of squid. All of those are a part of the “coleoid” subclass within cephalopods, which are known for their complex hunting and social behaviors.
Octopi are some of the most intelligent animals on Earth. Not only are they one of the few groups that can learn through observation, research has shown they also have the ability to escape from jars, use coconut husks to hide, and signal to others by changing the color of their skin. That, combined with the fact that most heavily edited RNAs code for key neural proteins, has researchers wondering if rapid RNA editing is somehow connected to the animals’ higher intelligence.
“They’re the only taxon out there that approaches vertebrates in terms of behavioral complexity,” said study co-author Joshua Rosenthal, a cephalopod neurobiologist at the Marine Biological Laboratory in Woods Hole, MA. “These behaviorally complex coleoids all have this tremendous RNA editing, particularly in their nervous system, where they’re recoding the messenger RNAs that encode for the very things that are important for electrical excitability.”
This research is published in the journal Cell.