2023-11-08 07:00:02
Image of the nervous system of a starfish by the marking of acetylated tubulin proteins. LAURENT FORMERY/OLYMPUS/EVIDENT
It seems like a children’s game or an absurd question: where is the starfish’s head? In reality, this enigma has occupied specialists for more than a century as this animal and all its cousins from the echinoderm group (sea urchins, sea cucumbers, etc.) present an original organization. While all species from the deuterostome super-phylum display axial symmetry (right-left) with a so-called anteroposterior distribution (from head to tail), nothing like this in the starfish and its relatives. All exhibit radial symmetry of order five. As for determining where the head and tail are, no one knew until now.
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Researchers from American (Stanford, Berkeley, Columbia), British (Southampton) and Japanese (Okinawa) universities have just unraveled the mystery. “Basically, we can say that the starfish is just a head”summarizes Frenchman Laurent Formery, postdoctoral student at Stanford and first author of the magazine article Nature which announces this result. To establish this, scientists relied on molecular studies. Where anatomy had previously failed, genes provided closure.
Laurent Formery and his colleagues thus worked to map gene expression in animals. Thanks to two cutting-edge techniques – RNA tomography and in situ hybridization – they were able to track the activity of some of them and thus look at how the traditional head/trunk/tail distribution observed in species bilateral was reflected in sea stars. In fact, it is the same genes that “code” for these different large parts of the body throughout life.
An extraordinary shape
Completely unexpected results then appeared. The dominant hypothesis, which wanted each arm to reproduce in its length the classic deployment of the head towards the tail, was reduced to nothing. The researchers instead observed that head genes were expressed along the entire length of the animal’s arms. The head everywhere, but the trunk and tail nowhere. They did not in fact find any of the molecular markers associated with these parts of the body. On the other hand, by following the genes coding for the different parts of the head, they found the equivalent of an anteroposterior axis, extending from the center towards the periphery of each arm.
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This discovery will allow us to dive a little better equipped into two black boxes that have until now been completely airtight. First, to try to understand how the animal can move from a classically bilateral larval stage to this adult stage with symmetry of order 5. Then, to determine its evolution. We know, in fact, that our common ancestor, some 600 million years ago, was, like us, bilateral. Why and how did echinoderms branch off towards this extraordinary form? Laurent Formery, who did his thesis on sea urchins, is now looking for answers in sea cucumbers.
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