Worms found to share "memories" by swapping RNA to warn of danger

Sharing information is crucial for species to survive, but how do simple lifeforms like worms spread the word? New research from Princeton has found that tiny roundworms can transfer “memories” to their neighbors, and pass them down to their offspring for generations.

C. elegans is a roundworm species common to both lab experiments and soil environments. It eats bacteria, but not every bug it encounters is food – a bacteria called P. aeruginosa can make them very sick, and if it doesn’t kill them the worms learn not to go back for seconds. But it’s not a very efficient system if every individual worm has to learn this lesson the hard way.

A few years ago, the Princeton team discovered that C. elegans has a shortcut. Worms that have previously eaten P. aeruginosa will genetically pass down an avoidance behavior to their offspring, for four generations. On closer inspection, they found that worms that had eaten the bacteria absorbed a small RNA called P11, which triggered a signal in their germline reproductive cells. Future baby worms born of this adult will then have this same signal transmitted to a specific neuron that directs their behavior.

Teaching your kids is one thing, but do the worms have a way to alert other adults to the dangers? In the new study, the researchers discovered that C. elegans can transfer the same genetic “memories” to others around them, spreading the avoidance behavior through the community.

“We found that one worm can learn to avoid this pathogenic bacterium and if we grind up that worm, or even just use the media the worms are swimming in, and give that media or the crushed-worm lysate to naive worms, those worms now ‘learn’ to avoid the pathogen as well,” says Coleen Murphy, lead author of the study.

Intriguingly, the worms who were taught to avoid the bacteria also passed the lessons down to their offspring for four generations, suggesting the same mechanism was at work. The researchers investigated exactly what the signal was composed of, and found that it seemed to be a virus-like genetic element called a retrotransposon that they have absorbed from their environment at some stage.

“What we discovered is that a retrotransposon called Cer1 that forms viral-like particles seems to carry a memory not only between tissues (from the worm's germline to its neurons) but also between individuals,” says Murphy. “We think that Cer1 may give worms an advantage in their battle with pathogens, even though acquiring Cer1 in its genome can be deleterious for the worm under non-pathogenic conditions.”

To confirm the suspicion, the researchers experimented by disabling Cer1 in the genome. Sure enough, these worms didn’t learn to avoid P. aeruginosa through P11, and they didn’t pass the avoidance behavior down to their offspring or share it with nearby worms. It seems that the recipient worms also need to have Cer1 in their genomes to learn the behavior too.

This study builds on a growing body of work showing how animals can learn these genetic memories from each other. A few years ago, scientists injected a memory – also in the form of RNA – between two snails. The donor had been trained to respond to a stimulus in a particular way, and the recipient reacted in a similar way, despite never having been directly exposed to the stimulus itself.

The new research was published in the journal Cell.

Source: Princeton University