A study of the activity of RNA, a type of genetic material, in the bodies of octopuses suggests that the brains of cephalopods evolved greater complexity in the same way as vertebrate brains did 鈥� by using a lot more regulatory RNAs called microRNAs听(miRNAs) to control gene activity.
鈥淲e show that the major RNA innovation of soft-bodied cephalopods is a massive expansion of the miRNA gene repertoire,鈥� states a study yet to be formally peer reviewed, led by at the Max Delbr眉ck Centre for Molecular Medicine in Berlin. 鈥淭he only comparable miRNA expansions happened, strikingly, in vertebrates.鈥�
The relatively high intelligence of octopuses, squid and cuttlefish has long intrigued biologists, not least because it is so unusual among molluscs and invertebrates generally. It is widely assumed that high intelligence requires a more complex brain, so various groups have been looking at how cephalopod brains evolved to be more complex.
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Rajewsky鈥檚 team found that the number of different miRNAs in cephalopods is comparable with the number in vertebrates. In particular, more than 50 miRNAs arose in the ancestors of squid and octopuses, and have been conserved ever since these lineages split more than 300 million years ago. The fact that they have been conserved suggests their function is important. No other invertebrates evolved so many miRNAs.
The main RNA molecules made by cells are known as messenger RNAs. These are copies of the genes in our genome and they carry instructions for making proteins to a cell鈥檚 protein-making factories. However, miRNAs are small pieces of RNA that don鈥檛 code for proteins. Instead, they regulate the activities of many different genes, typically by interacting with many different messenger RNAs.
Having a higher number of different miRNAs would allow cephalopods to generate more types of neurons and could thus underlie greater brain complexity. While the team hasn鈥檛 shown this is definitely the case, it did show that many of these miRNAs are active in the developing brains of octopus hatchlings.
鈥淲e propose that miRNAs are intimately linked to the evolution of complex animal brains,鈥� states the study. The authors didn鈥檛 want to discuss their findings prior to publication in a peer-reviewed journal, but other researchers say the results are fascinating.
鈥淲e certainly agree that a dramatic expansion of the miRNA repertoire may have contributed to the development of the complex nervous system,鈥� say at the Marine Biological Laboratory in Woods Hole, Massachusetts, and at Tel Aviv University in Israel. But they think it might not be the only mechanism involved.
In 2017, a team including Rosenthal and Eisenberg suggested cephalopod brain complexity is due to a听process called RNA editing. RNA editing refers to changes made to messenger RNAs, which can include alterations to the genetic sequences in these molecules.
RNA editing happens in all complex cells, but Rosenthal and Eisenberg听 in cephalopods. They proposed that this enables single genes to generate a wider range of different proteins, and that this is what led to greater brain complexity.
Rajewsky鈥檚 team didn鈥檛 find any evidence that RNA editing is especially important in octopuses. However, Rosenthal and Eisenberg , saying the two ideas aren鈥檛 mutually exclusive.
The idea that RNA editing plays a part can鈥檛 be dismissed, says at the University of Chicago. He says his team has found much higher levels of RNA editing in nervous systems of squid than in other tissues, in work yet to be published.
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