Hundreds of millions of years ago, at a time when back-boned animals were just starting to crawl onto land, one such creature became infected by a virus. It was a retrovirus, capable of smuggling its genes into the DNA of its host. And as sometimes happens, those genes stayed put. They were passed on to the animal’s children and grandchildren. And as these viral genes cascaded through the generations, they changed, transforming from mere stowaways into important parts of their host’s biology.
One such gene is called Arc. It’s active in neurons, and plays a vital role in the brain. A mouse that’s born without Arc can’t learn or form new long-term memories. If it finds some cheese in a maze, it will have completely forgotten the right route the next day. “They can’t seem to respond or adapt to changes in their environment,” says*Jason Shepherd*from the University of Utah, who has been studying Arc for years. “Arc is really key to transducing the information from those experiences into changes in the brain.”
Shepherd’s team has now shown that Arc is a repurposed viral gene. It creates proteins that assemble into hollow spheres which, under a microscope, look a lot like viruses. Neurons can use these capsules to transfer genetic material to other neurons. It’s not quite that our brain cells are making their own viruses to infect other brain cells—but “it’s not that far off either,” says Shepherd. When someone learns from new experiences, when they form new memories, when their brain adapts to the world around them… all of this happens with help of a domesticated virus.
Despite its importance, Arc has been a very difficult gene to study. Scientists often work out what unusual genes do by comparing them to familiar ones with similar features—but Arc is one-of-a-kind. Other mammals have their own versions of Arc, as do birds, reptiles, and amphibians. But in each animal, Arc seems utterly unique--there’s no other gene quite like it. And Shepherd’s team found that these Arc genes are similar to a group of fish genes called gypsy retrotransposons, which are also descended from ancient domesticated viruses. That similarity helped the team to nail down Arc’s ancestry: it descends from a viral gene called gag, which retroviruses like HIV use to build the protein shells that enclose their genetic material.
