Without genetic change we'd be nowhere—well perhaps just unicellular blobs kicking around in ponds. Alterations in DNA, such as point mutations, duplications, rearrangements and insertions from microbial neighbors, have helped humans and our deep-time ancestors climb out of the swamps and, in our case at least, start swimming in backyard pools.
But these basic tools of evolution don't entirely explain how we and other organisms have evolved to be so complex. Recent research has shown that a process called RNA editing, which tweaks how certain enzymes are made without requiring alterations in basic genetic blueprints, has allowed living organisms to regulate important functions, such as nervous system function and development.
Now, octopuses have provided the first evidence that this sort of micro-tinkering can happen in response to external environmental cues rather than just internal developmental ones. In a paper published online Thursday in Science Express, two researchers explain how RNA editing has allowed octopuses to adapt to the warm waters of Puerto Rico down to the icy depths of the Southern Ocean near Antarctica.
"RNA editing gives an organism options," Joshua Rosenthal, of the University of Puerto Rico Medical Sciences Campus's Institute of Neurobiology and co-author of the new paper, explained to me over the phone from South Carolina, where he was attending the Society for Integrative and Comparative Biology meeting. "It gives them a pretty cool repertory of tricks and tools they can use to adapt and acclimate." The new find specifically helps to explain how octopuses have been able to flourish in tropical shallow seas as well as some 2,400 meters down, around the deep-sea hydrothermal vents off the coast of Antarctica.
Cephalopods, it turns out, seem to be doing a lot of RNA editing, Rosenthal says. And probably for good reason. Animals with the same basic DNA makeup can use RNA editing to fine-tune various processes, such as communication among neurons. As opposed to a hard-wired genetic mutation, RNA editing "gives you much better options because it lets you decide whether you want to use it," Rosenthal explains. Not only that, but "you can decide how much you want to edit, so you can have a graded response."
Rosenthal and his colleague Sandra Garrett, a doctoral researcher at the University of Puerto Rico, found that octopuses are using RNA editing to adapt to the temperature of the water around them. We warm-bloods don't have to worry much about temperature affecting our neurons because we keep our bodies at a nice steady 37 degrees Celsius. But for poikilotherms, such as octopuses, temperature differences can wreak havoc on their neural networking. Communication in the nervous system—for movement and thought—is controlled by the rapid pace of neuron firing. A sodium-ion channel starts the firing and a potassium-ion channel shuts it down. Both of these functions slow down in cool temperatures—to say nothing of the 1.8-degree Celsius waters that the Pareledone octopus lives in—but the potassium portion slows down much more than the sodium side. So without something to balance these functions out, the neural signals could get thrown way out of whack. At the nearly freezing Antarctic temperatures, "channels would open about 14 times slower and close about 60 times slower" than they typically do among their warm-water Octopus vulgaris cousins, Rosenthal and Garrett explained in their paper. And that's where these animals' RNA editing comes in handy. One of the Antarctic octopus's editing locations (I321V) "more than doubled the rate" of the potassium channel's closing, which, along with other editing tweaks, would help bring the two channels closer to the same rate, the two researchers noted in their paper.
Rosenthal and Garrett checked other species of octopuses to see if they had the same pattern of RNA editing to control neuron firing for their respective temperature environments. Sure enough, two species of Arctic octopus, collected from water temperatures that approached 0 degrees Celsius, and two species of tropical octopus, collected in waters off Puerto Rico and Baja California, also showed extensive editing of the I321V area—as well as plenty of other edits.
Temperature adaptation is probably just one small piece of octopuses's use of RNA editing to respond to their environment, Rosenthal says. They could "change protein function for anything: starvation, heat stress, learning—I think these are all plausible."
Octopuses and squid seem to be particularly promising animals in which to study RNA editing. Although the process has been found in organisms ranging from coral to humans, most scientific searches for editing sites turn up just tens or hundreds after a scan of thousands of locations. In the cephalopods, however, Rosenthal and Garrett have already found some 100 editing sites just by looking at eight messenger RNAs. "The cephalopods have really taken the editing to heart," Rosenthal says.
In fact they even seem to be editing the editing RNA, which makes for an even larger diversity of editing enzymes. This meta-editing could be a clue as to how these invertebrates, whose mollusk relatives include snails and scallops, became so bewitchingly complex.
Illustration courtesy of Ivan Phillipsen