EMBARGO Friday November 25 1900 GMT | Saturday 26 November 0600 AEDT
Our glorious little blue marble of a planet is filled with an amazingly diverse array of life forms, but some are definitely more than just weird others.
This is especially true of the octopus, an animal so odd that it regularly invites comparisons to extraterrestrials.
According to British neuroscientist Anil Seth, if there’s a creature on Earth strange enough to have evolved elsewhere, it’s the octopus. Some fringe theories even suggest that octopuses might be aliens.
However, there is plenty of evidence firmly tying octopus evolution to Earth, and a team of scientists led by systems biologist Nikolaus Rajewsky of the Max Delbrück Center for Molecular Medicine has just found new one.
And it’s really fascinating.
It’s a property that octopus brains actually share with human brains and the brains of other vertebrates: a vast repertoire of microRNAs in their neural tissue.
“That,” says Rajewsky, “is what connects us to the octopus!”
Octopuses are weird in many ways. They are also smart, as are other cephalopods such as squid. And it has been found that squid brains are almost as complex as dog brains. There is even evidence that squid can dream – rarely confirmed in invertebrates.
Unlike other intelligent animals, their nervous systems are highly distributed, with a significant portion of their roughly 500 million neurons spread across their arms. Each arm is capable of making independent decisions and can continue to respond to stimuli even after it has been severed.
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The complex nervous system and intelligence of cephalopods has been a mystery. These features are relatively common in vertebrates, but are particularly noticeable in invertebrates.
There’s something else really weird about octopuses and other cephalopods. Their bodies can edit their RNA sequences on the fly to adapt to their environment. Customization doesn’t usually work that way; It usually starts with DNA, and these changes are passed down to RNA.
This led Rajewsky to wonder what other RNA secrets octopuses might be hiding.
Rajewsky and his team analyzed 18 samples of dead squid – supplied by the marine research institute Stazione Zoologica Anton Dohrn in Italy – and sequenced mainly RNA from Octopus vulgaristhe common octopus. Also included in the study was a whole California two-spotted octopus (Octopus bimaculoides) and a Hawaiian bobtail squid (Euprymna scolopes).
Sequencing provided a profile of the messenger RNAs and small RNAs it contained. And the results were a surprise.
“There was indeed a lot of RNA editing, but not in areas that we think are interesting,” explains Rajewsky.
What the team found was that squid have lots of microRNA, or miRNA. They found 164 miRNA genes grouped into 138 miRNA families in the common octopus and 162 miRNA genes grouped into the same 138 families in the California two-spotted octopus. And 42 of the families were new, mostly in brain and nerve tissue.
miRNA are non-coding RNA molecules that are heavily involved in regulating gene expression and bind to larger RNA molecules to help cells fine-tune the proteins they make.
The fact that these miRNA families, like the RNA-binding sites, have been conserved in the octopus suggests that they still play a role in octopus biology, although scientists do not yet know what role this role plays or in which cells the miRNAs are involved with.
“This is the third largest expansion of microRNA families in the animal world and the largest outside of vertebrates,” says biologist Grygoriy Zolotarov, now at the Center for Genomic Regulation in Spain, formerly in Rajewsky’s lab.
“To give you an idea of the magnitude, oysters, which are also mollusks, have acquired only five new microRNA families since the last ancestor they shared with squid — while the squid acquired 90!”
The only comparable expansions have occurred in vertebrates, although the extent is somewhat different. The human genome encodes approximately 2,600 mature miRNAs. But the number of octopus miRNA families is comparable to that of animals like chickens and frogs.
The discovery, the researchers say, suggests that complex intelligence, including that of cephalopods, may be related to this miRNA expansion.
Interestingly, this is not the only similarity between octopus brains and those of vertebrates. Scientists previously found that both human and octopus brains contain large numbers of a type of cell called transposons. It seems like there’s a lot more going on inside an octopus’ head (and arms) than we understand.
The next step for Rajewsky’s team is to figure out what exactly these miRNAs do.
“The remarkable explosion of miRNA gene repertoires in coleoid cephalopods may suggest,” the researchers write, “that miRNAs, and perhaps their specialized neuronal functions, are intimately linked and may be required for the emergence of complex brains in animals.”
The research was published in scientific advances.
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