In a recent study in the journal Nature Ecology & Evolution titled "Evolutionary dynamics of whole-body regeneration across planarian flatworms," a group of researchers looked into the limitations of regeneration capabilities in planarians They wanted to know why some flatworms demonstrate the ability to regrow their body parts, while others simply don't. This led the scientists to look at the role of Wnt signalling in flatworm RNA, and they discovered that this little bit of genetic code impacts both the planarians' regenerative abilities, as well as their sexual potential.
In other words, the worms have a choice: they can either get laid, or grow their heads back. But they can't do both.
From the study:
Our finding that Wnt signalling has multiple roles in the reproductive system of the model species Schmidtea mediterranea raises the possibility that a trade-off between egg-laying, asexual reproduction by fission/regeneration and Wnt signalling drives regenerative trait evolution. Although quantitative comparisons of Wnt signalling levels, yolk content and reproductive strategy across our species collection remained inconclusive, they revealed divergent Wnt signalling roles in the reproductive system of planarians.
Or, as New Scientist summed up the findings:
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.
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.
As part of their experiment, the researchers were able to artificially "silence" the Wnt signal, which they observed as reinstating regenerative capabilities. But they also found that the same genetic code apparently plays a role in egg yolk production. Which, I guess sort of makes sense if I twist my brain and think about it too hard?
Evolutionary dynamics of whole-body regeneration across planarian flatworms [Miquel Vila-Farré, Andrei Rozanski, Mario Ivanković, James Cleland, Jeremias N. Brand, Felix Thalen, Markus A. Grohme, Stephanie von Kannen, Alexandra L. Grosbusch, Hanh T.-K. Vu, Carlos E. Prieto, Fernando Carbayo, Bernhard Egger, Christoph Bleidorn, John E. J. Rasko & Jochen C. Rink / Nature Ecology & Evolution]
Flatworms can either regrow lost heads or reproduce sexually, not both [Elise Cutts / New Scientist]