Worm study shows behaviors can be passed down several generations
It's basic biology that physical traits are passed from parent to offspring, but what about behaviors? Biologists at Princeton have shown that in worms, learned behaviors can be passed down as many as four generations, with younger worms instinctively avoiding bacteria they've never encountered themselves.
More and more scientific attention is turning towards the "nurture" side of the nature versus nurture debate. Put an organism under stressful conditions – say, in an environment that's a bit too hot or has very little food – and the expression of certain genes will change. Prolonged exposure to that environment can make the changes permanent enough to be passed down to the organism's offspring, to give them the best chance of survival. It's evolution in action.
This is known as epigenetic inheritance, and it's increasingly understood to be at play in worms, mice and maybe even humans. Last year, a study revealed some of the biological mechanisms behind the process in mice, and controversial studies in humans have even suggested that great trauma like child abuse or surviving the Holocaust could influence the mental health of the next generation.
For the new study, the Princeton team wanted to investigate if learned behaviors could be inherited. In a previous study, scientists managed to inject RNA "memories" from one snail into another, with the receiving snail reacting to stimuli the same way the donor did – even though the second snail had never been directly taught to react that way.
In this study the team pulled off a similar stunt, albeit one that's more natural. This time they experimented with C. elegans, a simple worm species that's often used in genetic studies. The stimuli took the form of pathogenic bacteria that individual worms often learn to avoid after an unpleasant experience with them.
"In their natural environment, worms come into contact with many different bacterial species," explains Coleen Murphy, an author of the study. "Some of these are nutritious food sources, while others will infect and kill them. Worms are initially attracted to the pathogen Pseudomonas aeruginosa, but upon infection, they learn to avoid it. Otherwise they will die within a few days."
Sure enough, the team found that when mother worms learned to avoid the bad bugs, their babies instinctively did too. Not only that, but the lesson was passed down through four whole generations. By the fifth generation, however, it seemed to wear off, with those worms rediscovering an attraction to P. aeruginosa.
Interestingly, it doesn't seem to be a universal thing – the worms weren't able to epigenetically inherit an avoidance of a different dangerous bacteria, Serratia marcescens. And even with P. aeruginosa, the team found that the original worms needed to actually get sick from them for the genes to be passed down. Just smelling them wasn't enough.
Next, the team went looking for the biological mechanism behind the epigenetic transference. They identified several genes associated with neurons that were upregulated in the mothers and their descendants that were bacteria-averse, as well as a particular molecule called daf-7 that seems to be expressed in higher amounts.
Crucially, the team also found an explanation for why that avoidance wears off after a few generations. The daf-7 expression stays elevated for four generations, then drops off, which appears to be, counterintuitively, a survival mechanism. P. aeruginosa is safe for the worms to eat at lower temperatures, and only becomes pathogenic in warmer environments. Nature seems to be giving them time to cool off before the younger worms can try again.
The research was published in the journal Cell.