Head or egg: an evolutionary trade-off

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The native planarian Crenobia alpina found in alpine streams (collage). © Credit
The native planarian Crenobia alpina found in alpine streams (collage). © Credit: Bernhard Egger/Fabian Oswald
Would you rather lay lots of eggs - or grow your own head back? In the evolutionary history of flatworms, some species apparently had to choose one of the two abilities. This is what a team of scientists found out when they investigated the regenerative ability of planarians. Bernhard Egger, who heads the Regeneration Group at the Institute of Zoology at the University of Innsbruck and was involved in the study, reports on the amazing properties of these animals.

Why can some animals regrow entire body parts, but not others? This question remains largely unanswered in biology. To test the evolutionary principles behind the ability to regenerate, a team of scientists studied planarian species from around the world. These animals from the flatworm phylum are usually only a few centimeters in size. In research, the species Schmidtea mediterranea in particular is used as a model organism because almost every part of its body can regrow a complete animal.

The study was led by Jochen Rink and Miquel Vila-Farre from the Max Planck Institute for Multidisciplinary Natural Sciences in Göttingen, where over 40 planarian species were studied. The University of Innsbruck also contributed five species from its extensive planarian collection. Bernhard Egger and Alexandra Grosbusch from the Institute of Zoology also studied two species of close relatives of the planarians in which regeneration is only weakly developed-these are no longer able to regenerate after a certain body part.

Other research institutions from Germany, Spain, Brazil and Australia were also involved. The results of the study were published in the scientific journal Nature Ecology & Evolution.

Behind becomes in front

The study on the different planarian species showed that there is a strong correlation between the Wnt signaling pathway and the regenerative capacity. In biology, a signaling pathway is a sequence of chemical processes by which an organism responds to external stimuli. The Wnt signaling pathway is a very old mechanism that is responsible for numerous functions in organisms, including the formation of body axes.

When the production of a specific component of the Wnt pathway was inhibited, namely the protein beta-catenin1, planarian species with poor or non-existent regenerative capacity were suddenly able to recover their heads.

"Beta-catenin1 is particularly abundant in the tail of animals," Egger explains. "If it is switched off, then the hind end loses its tail identity and a head can develop there instead."

It follows that the ability to regenerate the head is basically present in all planarians, but is inhibited by beta-catenin1. But why would a species give up its ability to regenerate again? This is related to an important function of beta-catenin.

A "nice to have

"At first glance, regeneration is something very practical. In nature, however, losing one’s head is a comparatively rare event" says Egger. "That’s why one hypothesis is that the ability to regenerate is treated neutrally in evolution - like a "nice to have" that is not absolutely necessary. You can think of it like animals that live in lightless caves. For them, it doesn’t matter whether the eyes work or not, and so this ability is either lost or it remains."

However, beta-catenin1 not only blocks head regeneration, but at the same time is also conducive to yolk formation. So in the case of some planarians, the ability to regenerate the head was in competition with the ability to lay more eggs, and possibly this is why the ability to regenerate the head has been diminished in some planarian species. "In a sense, it’s a kind of evolutionary trade-off to be able to lay more eggs."

Publication: Probing the evolutionary dynamics of whole-body regeneration within planarian flatworms. Miquel Vila-Farré, Andrei Rozanski, Mario IvankoviÄ, James Cleland, Jeremias N. Brand, Felix Thalen, Markus Grohme, Stephanie von Kannen, Alexandra Grosbusch, Han T-K Vu, Carlos E. Prieto, Fernando Carbayo, Bernhard Egger, Christoph Bleidorn, John E. J. Rasko, Jochen C. Rink. Nature Ecology & Evolution 2023. DOI: 10.1038/s41559’023 -02221-7.




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