Eight-legged evolution exploits editing

The frigid waters of the Southern Ocean don’t slow Antarctic octopuses down, even though their nervous systems are governed by the same genetic instructions as their tropical counterparts. Now scientists know why: Edits to the creatures’ genetic instructions tweak the octopuses’ nerve cells for smooth operating in the numbing polar waters.

The discovery is the first report of such editing actually helping an organism adjust to its environment, scientists report online January 5 in Science.

Because nerve cells can’t send signals as quickly in the cold, scientists decided to compare genes from Pareledone, an octopus that lives in the icy waters off Antarctica, with those of the warm-water species Octopus vulgaris. To the researchers’ surprise, the genetic instructions were pretty much the same.

“It was a real disappointment at first,” says molecular neurobiologist Joshua Rosenthal of the University of Puerto Rico Medical Sciences Campus in San Juan. “We thought there was going to be a difference in their genes, but they were basically identical. It was puzzling.”

Rosenthal and graduate student Sandra Garrett figured something must alter the way the DNA instructions are relayed to and interpreted by the body’s nerve cell–building machinery.

DNA always stays put in the nucleus of cells, and sections of it are copied when instructions for functioning or building parts are needed. So the researchers looked at this genetic photocopy, the mRNA. It turns out that an enzyme that specializes in editing mRNA alters the blueprints for the octopuses’ nerve cells in both polar and tropical species, the researchers discovered.

The mRNA edits slightly change the way that nerve cells open and close gates to produce electrical impulses. In the Antarctic octopus species, the edits speed up the closing of one gate, while in the tropical species, the edits make that gate close more slowly. Because cold slows the gates down, these editing tweaks nicely counteract the effects of temperature and keep the gates in sync with other parts of the nervous system.

“This is amazing, wonderful work,” says molecular biologist Yi-Tao Yu of the University of Rochester in New York. Researchers used to think that dialing down or amping up the activity of genes through DNA mutation was evolution’s main trick for fine-tuning the body’s machinery. But there’s a downside: Change the DNA and you can’t easily go back.

“If you just modify the mRNA it’s very economical and convenient,” Yu says.

Rosenthal and Garrett also examined another polar species from the frosty northern waters off Svalbard, Norway, another tropical species and some in-between species. The researchers found evidence that the same kind of RNA editing happens in those octopuses as well.

The various species that the researchers examined come from very different branches on the octopus family tree, says cephalopod expert Louise Allcock of the National University of Ireland, Galway. This suggests that the RNA-editing trick, or the ability to turn it on when needed, evolved multiple times.

While the discovery is the first report of such editing helping an organism adapt to its environment, many questions remain. The scientists don’t know yet, for example, if a cold-water octopus could change the way it edits RNA in a warm tank. But they’re working on experiments to test just that.