By Luke Dunning, University of Sheffield
Are octopuses so clever because they ignore their genetic programming? Research has shown that octopuses and other cephalopods edit the messages sent from their DNA instead of following them almost exactly like most living things usually do.
Previously, scientists thought this process of molecular Chinese whispers was largely insignificant in animal evolution. But a new study published in the journal Cell shows this is certainly not true for these tentacled ocean dwellers.
It suggests that genetic editing may directly contribute to cephalopods’ remarkable intelligence, which enables them to solve complicated puzzles and visually communicate by changing their skin colour, making them the smartest of all invertebrates. However, the ability to alter genetic messages may come at a price, potentially reducing other more common forms of adaptive evolution.
DNA is the blueprint of life. This genetic instruction manual is written in a four-letter language and describes how to build all the different proteins an organism can construct. To synthesize a particular protein, an enzyme first transcribes the recipe from DNA into a message in a similar molecule called RNA. Part of this message is eventually translated into chains of amino acids, the building blocks of protein.
For most organisms, including humans, the whole process of protein synthesis remains remarkably faithful to the original instructions laid out in the DNA. This premise has underpinned much of our understanding of genetics since we discovered that genetic information is stored as DNA. But recent research has shown that octopuses, squid, and cuttlefish do not always follow their genetic instructions to the letter.
After the RNA message is copied accurately from the DNA, it can be altered in a process called RNA editing. This edited message then produces proteins that also have modifications. RNA editing was first described in 1986 by researchers studying single-celled parasites closely related to the microbe responsible for causing sleeping sickness.
Since then, RNA editing has been seen as relatively unimportant in animals. Humans, for example, have only a handful of RNA editing sites in their protein coding sequences. What’s more, the changes from RNA editing we knew about were thought to have little effect, and very few of them were shared across groups of closely related species, including mammals.
However, in the remarkably intelligent squid, RNA editing affects most genes associated with the nervous system. This suggests that, in certain groups of organisms, the practice may play an important role. By comparing how often RNA editing occurs within the cephalopod family, the new study identified when in evolutionary history this ability to alter genetic messages appeared.
The researchers showed that widespread RNA editing evolved in the common ancestor of the exceptional behaviourally sophisticated octopuses, squid and cuttlefish. And that it is lacking in the relatively simple-minded nautilus, a more distant relative. Because neural proteins exhibit some of the most intensive RNA editing, the researchers suggest that this process may contribute to cephalopods’ remarkable intelligence.
RNA editing is thought to give cephalopods an evolutionary advantage by increasing the variation in the proteins they produce. This enables them to rapidly adapt to changing environments. For example, we know RNA editing allows octopuses to deal with dramatic changes in temperature and so survive in frigid polar waters.
But this recent study also suggests that there may be a trade-off between this ability to adapt quickly and the long-term process of Darwinian DNA evolution.
RNA editing is performed by an enzyme, and this enzyme requires a specific target site written into the genetic code. This long stretch of molecules surrounds the point in the RNA message that needs editing. Any mutation in this target marker will disrupt the whole process, and the enzyme will no longer be able to edit the RNA message. To avoid this, cephalopods have evolved DNA that is less likely to mutate.
However, mutation is the basis for conventional molecular DNA evolution and is needed to permanently change the code for a protein in a way that improves its function. So the prevalence of RNA editing in cephalopods appears to have substantially limited its potential for adaptive DNA evolution. This pattern may go some way towards explaining why this process is not more prevalent across the animal kingdom.
The widespread use of RNA editing in cephalopods means we may have to rethink our understanding of its importance in nature. In the case of cephalopods, the ink the DNA instructions are written in appears to be less permanent than we originally thought.