MicroRNA in octopus brains could have evolved to make them extra brainy

Octopuses (no not octopi) aren’t an ocean creature to mess with. They build cities, like to sling mud at each other, and can escape from almost anywhere you put them

A new study published in Science Advances has tried to decode what makes octopus so much smarter than other invertebrates and stumbled across a curious feature – they’ve got way more microRNA than you’d expect.

“This is the third-largest expansion of microRNA families in the animal world, and the largest outside of vertebrates,” says first author, geneticist Grygoriy Zolotarov from the Centre for Genomic Regulation in Barcelona, Spain.

“To give you an idea of the scale, oysters, which are also molluscs, have acquired just five new microRNA families since the last ancestors they shared with octopuses – while the octopuses have acquired 90!”

MicroRNA (or miRNA) is a small type of RNA which can silence or regulate other types of RNA. This sounds sort of counterintuitive, but it helps cells control the kinds and amounts of proteins they make.

Despite being a bit mysterious, miRNA are incredibly important – animal studies have shown that removing miRNA genes can create defects in the development of a variety of body parts or can cause epilepsy, deafness or cancer.

The new study suggests that not only are miRNAs required for vertebrates to not fall apart, but they could also have something to do with our smarts.

Diagram of miRNA action with mRNA. Credit: Kelvinsong/Wikimedia/CC BY 3.0

“We show that the major RNA innovation of soft-bodied cephalopods is an expansion of the miRNA gene repertoire,” the team writes in their new paper.

“The notable explosion of the miRNA gene repertoire in coleoid cephalopods may indicate that miRNAs and, perhaps, their specialised neuronal functions are deeply linked and possibly required for the emergence of complex brains in animals.”

The team sequenced 18 tissues in adult octopuses, analysed data sets from multiple octopus species, and then looked at how the evolution of miRNAs have occurred, all the way back to when those of us who have bilateral symmetry (bilaterians) split from sponges and the like (cnidarians).

They discovered that the evolutionarily new miRNAs in octopuses were mostly focused on the development of brain tissue, suggesting that this could be the secret sauce of octopus intelligence.

Read more: How the octopus got its smarts

“This is what connects us to the octopus!” says Professor Nikolaus Rajewsky, a biologist at the Max Delbrück Center for Molecular Medicine, Germany.

Humans also have also evolved a lot of miRNA, as do most vertebrates, so the team could be onto something.

The researchers are now hoping to create new ways to be able to analyse octopus intelligence. But trying to convince an octopus to do something is harder than it looks.

“If you do tests with them using small snacks as rewards, they soon lose interest,” says Rajewsky.

The researchers are now planning to apply a technique which will make the cells in octopus tissue visible at a molecular level.

“Since octopuses aren’t typical model organisms, our molecular-biological tools were very limited,” says Zolotarov.

“So, we don’t yet know exactly which types of cell express the new microRNAs.”

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