Flatworms can either regrow lost heads or reproduce sexually, not both

Planarians, flatworms known for their goofy cross-eyed gazes, may have to make an evolutionary trade-off between regeneration and sex – across different species of planarians, the same gene signalling pathway facilitates the ability to either regrow heads or to produce eggs, depending on whether it is turned on or off. This could help answer the long-standing mystery of why some animals can regenerate lost body parts while others cannot.

This is “actually a really old question in the regeneration field”, says Jochen Rink at the Max Planck Institute for Multidisciplinary Sciences in Germany. He and his colleagues looked to planarians for answers.

They assembled what Rink calls a “zoo” of 40 different planarian species scooped up by hand from waterways spanning from Iceland to Japan to Australia. While they are closely related, some planarians regenerate remarkably well and others don’t at all. That presents a “really unique opportunity” to tease apart the ecological and biological factors that lead to loss of regeneration, says Rink.

They decapitated worms of each species to test their regenerative abilities. This revealed that several species had independently lost the ability to regenerate. The 18 wild species that lost some or most of their regenerative abilities tended to reproduce sexually, by laying eggs. Their regenerating cousins all reproduced asexually, by ripping themselves apart and growing new bodies from the shreds.

Rink’s team thinks the missing biological link between reproduction and regeneration could be a genetic signalling pathway called Wnt, which tells cells whether they are in an animal’s tail or head.

To regrow a head, Wnt needs to be turned off. In fact, the researchers found that artificially silencing Wnt reinstated regeneration in every species of non-regenerating or weakly regenerating planarians – those that can regrow only parts of their head, or regrow heads with defects depending on where they are cut – in their zoo. But they also found evidence that Wnt is important for sexual reproduction, specifically producing egg yolk, at least in some species.

The findings lead Rink and his team to think that worms living in environments where the genetic advantages of sex outweigh its risks and costs might have lost regeneration as a side effect.

Confirming that hypothesis will require digging deeper into the differences between species that regenerate and their relatives that don’t, says Peter Reddien at the Massachusetts Institute of Technology, who wasn’t involved in the study. But by considering so many species, he adds, this study will make it easier to do exactly that.

The research suggests that selection for sexual reproduction would have been “bad news for the capacity to regenerate”, says Reddien. The hypothesis is still in its early days, he says, but it is an intriguing one. Having assembled such a wide range of species sets the stage for future studies that could test this hypothesis and others, he says.

“If we can come to an understanding of how evolution has naturally selected for the capacity to regenerate across a whole range of organisms and species, we might have the knowledge needed to ask the question: what’s different in our wounds?”