Head in starfish…

Starfish are undoubtedly one of the most elegant animals in the marine world. In addition to the bright colors they wear, their aesthetics depend largely on the perfect symmetry of their bodies, made up of five or more “arms.” At least that is the description we give in common language. Because these branches will not actually be “arms” or “legs.” This is the conclusion of a study conducted by Laurent Fourmery, a French researcher at Stanford University in California, with his colleagues. So what part of the body do starfish branches belong to? Scientists share their discovery by revealing the first complete map of starfish territories Patria MiniataInhabited by the Pacific Ocean.

The anatomy of a starfish and ours is very different. Like most animals, the human body visually exhibits bilateral symmetry, that is, it consists of two identical halves (right and left) with a head, torso, and limbs. On the contrary, these echinoderms have five-fold symmetry: their five branches radiate from a central point where the mouth is located, forming identical copies. It looks a bit like a five-petaled flower. But this special symmetry, called radial symmetry, does not appear at birth. At the beginning of its development, the starfish was a bilaterian larva that moved freely in the ocean. It acquires its radial symmetry only from the moment of its transformation and fall to the bottom of the sea. During the transition to adulthood, a radical change occurs at the body level with a constant question: where are the head, trunk and limbs?

To answer this question, the researchers looked at starfish genes, specifically “developmental genes.” ” [Ils] It is expressed in many animals, including insects. Laurent Formeri explains that its peculiarity is that it contains the genetic instructions necessary to form every part of the body. In other words, developmental genes determine which region of the body (trunk, head, limbs) each animal cell belongs to. By mapping its expression in sea stars, biologists hope to better understand the anatomical organization of these animals.

To do this, they first cut the specimen's branches into several “slices.” They then identified the developmental genes expressed in each of the obtained sections using a genomic sequencing technique, RNA tomography. This allowed them to build 3D computer models of developmental gene expression spread along the bodies of sea stars.

When I read the results, I was very surprised. Contrary to all expectations, genes necessary for stem growth, genes Hawks, is not expressed in any region of the ectoderm (the tissue that makes up the skin and nervous system) in animals. Which suggests that starfish completely lack trunks (and by extension limbs). On the other hand, genes involved in head formation were expressed not only in the central part of individuals, but even in all five branches.

Like the more terrifying mythological creatures, such as the three-headed Cerberus or the nine-headed Lernaean Hydra, that Hercules encountered during his Twelve Labors, is the starfish many-headed? “Not really,” confirms Laurent Formieri. We believe that these multiple heads are actually products of the central head. » A discovery that actually forces us to rethink the evolutionary history of these marine animals. One hypothesis is that sea stars evolved from a diploid ancestor by losing their trunks. “It is thought that at the time of their metamorphosis, instead of developing a trunk as their distant cousins ​​with bilateral symmetry do, these animals reorganize their frontal tissues corresponding to the head. This gives a trunkless animal with multiple brain extensions capable of crawling on the ocean floor,” the researcher explains.