Octopus
and Squid Evolution Is Officially Weirder Than We Could Have Ever Imagined
Just
when we thought octopuses couldn't be any weirder, it turns out that they
and their cephalopod brethren evolve differently from nearly every other
organism on the planet.
In
a surprising twist, scientists have discovered that octopuses, along with some
squid and cuttlefish species, routinely edit their RNA (ribonucleic acid)
sequences to adapt to their environment.
This
is weird because that's really not how adaptations usually happen in multi cellular
animals. When an organism changes in some fundamental way, it typically starts
with a genetic mutation - a change to the DNA.
Those
genetic changes are then translated into action by DNA's molecular sidekick,
RNA. You can think of DNA instructions as a recipe, while RNA is the chef that
orchestrates the cooking in the kitchen of each cell, producing necessary
proteins that keep the whole organism going.
But
RNA doesn't just blindly execute instructions - occasionally it improvises with
some of the ingredients, changing which proteins are produced in the cell in a
rare process called RNA editing.
When
such an edit happens, it can change how the proteins work, allowing the
organism to fine-tune its genetic information without actually undergoing any
genetic mutations. But most organisms don't really bother with this method, as
it's messy and causes problems more often that solving them.
"The
consensus among folks who study such things is Mother Nature gave RNA editing a
try, found it wanting, and largely abandoned it," Anna Vlasits reports
forWired.
But
now it looks like cephalopods didn't get the memo.
In
2015, researchers discovered that the common squid has edited more than 60
percent of RNA in its nervous system. Those edits essentially changed its brain
physiology, presumably to adapt to various temperature conditions in the ocean.
Now
the team is back with an even more startling finding - at least two species of
octopus and one cuttlefish do the same thing on a regular basis. To draw evolutionary
comparisons, they also looked at a nautilus and a gastropod slug, and found
their RNA-editing prowess to be lacking.
"This
shows that high levels of RNA editing is not generally a molluscan thing; it's
an invention of the coleoid cephalopods," says co-lead
researcher,, Joshua Rosenthal of the US Marine Biological Laboratory.
The
researchers analysed hundreds of thousands of RNA recording sites in these
animals, who belong to the coleoid subclass of cephalopods. They
found that clever RNA editing was especially common in the coleoid nervous
system.
"I
wonder if it has to do with their extremely developed brains," geneticist
Kazuko Nishikura from the US Wistar Institute, who wasn't involved in the
study, told Ed Yongat The Atlantic.
It's
true that coleoid cephalopods are exceptionally intelligent. There are countless
riveting octopus escape artist stories out there, not to mention evidence
of tool use, and that one eight-armed guy at a New Zealand aquarium who
learned to photograph people. (Yes, really.)
So
it's certainly a compelling hypothesis that octopus smarts might come from
their unconventionally high reliance on RNA edits to keep the brain going.
"There
is something fundamentally different going on in these cephalopods," says
Rosenthal.
But
it's not just that these animals are adept at fixing up their RNA as needed -
the team found that this ability came with a distinct evolutionary tradeoff,
which sets them apart from the rest of the animal world.
In
terms of run-of-the-mill genomic evolution (the one that uses genetic
mutations, as mentioned above), coleoids have been evolving really, really
slowly. The researchers think that this has been a necessary sacrifice - if you
find a mechanism that helps you survive, just keep using it.
"The
conclusion here is that in order to maintain this flexibility to edit RNA, the
coleoids have had to give up the ability to evolve in the surrounding regions -
a lot,"says Rosenthal.
As
the next step, the team will now be developing genetic models of cephalopods so
they can trace how and when this RNA editing kicks in.
"It
could be something as simple as temperature changes or as complicated as
experience, a form of memory," says Rosenthal.