2023-11-01 18:17:22
When looking at a dog, a fish or even a worm (although you would have to get quite close to the latter), you can clearly recognize its head and tail. But what about the case of a starfish, where its five arms are identical and anyone can take the initiative to propel it through the water? This unusual body design has led some scientists to conclude that these animals may not have a head. However, a new study published in ‘Nature‘ refutes this extreme: asteroids not only have heads; but rather they evolved to become only heads, as indicated by their genes.
“It is as if the starfish is completely missing a trunk, and is best described as simply a head dragging along the seafloor,” explains Laurent Formery, lead author of the new study, who worked together with marine biologist and development Christopher Lowe of Stanford University and Daniel Rokhsar of UC Berkeley, an expert in the molecular evolution of animal species. “It’s not at all what scientists have assumed about these animals,” Formery says.
like a puzzle
Almost all animals, including humans, are bilaterally symmetrical: that is, they can be divided into two mirror halves along a single axis extending from head to tail. In 1995, the Nobel Prize in Physiology or Medicine was awarded to three scientists who had used fruit flies to demonstrate that the bilateral, head-to-tail body structure observed in most animals arises from the action of a series of molecular switches, encoded by genes, expressed in defined regions of the head and trunk.
Researchers have since confirmed that this same genetic programming is shared by the vast majority of animal species, including vertebrates such as humans and fish, and in many invertebrates such as insects and worms.
But the body plan of starfish has long confounded scientists’ understanding of animal evolution. Instead of showing bilateral symmetry, adult starfish (and related echinoderms such as sea urchins and sea cucumbers) have a five-fold axis of symmetry without a clear head or tail. And no one has been able to determine how genetic programming drives this unusual five-fold symmetry.
Some scientists have proposed that in starfish, the head-to-tail axis could extend from the animal’s armored back to its belly, which is covered by so-called tube feet. Others have suggested that each of the starfish’s five arms corresponds to a copy of a conventional head-to-tail axis. However, confirming these extremes has been complicated, since methods to detect gene expression had been developed for a small number of model organisms, such as mice and flies.
Innovative technology
The answer came with an innovative technique for sequencing massive amounts of genetic material using postage-stamp-sized chips packed with millions of individual chemical reactors, each set to simultaneously read long stretches of DNA captured within.
The researchers found that none of the prominent hypotheses about the structure of the starfish’s body plan were correct. Instead, they saw that gene expression corresponding to the forebrain in humans and other bilaterally symmetric animals was located along the midline of the starfish arms, with gene expression corresponding to that of the human midbrain toward the outer edges of the arms. While genes marking different subregions of the head in humans and other bilaterians were expressed in the starfish, only one of the genes typically associated with the trunk in animals was expressed, at the very edges of the starfish’s arms. of sea
“These results suggest that echinoderms, and starfish in particular, have the most dramatic example of uncoupling of the head and trunk regions that we know today,” Formery says, adding that some starfish ancestors of similar appearance Strangely preserved in the fossil record it appears to have had a trunk. “It just opens up a lot of new questions that we can now start to explore.”
A repeating pattern?
The questions the team hopes to address next concern whether the genetic pattern observed in starfish also appears in sea urchins and sea cucumbers. For his part, Formery also wants to investigate what the starfish can teach us about the evolution of the nervous system, something that, according to him, no one fully understands in echinoderms.
“It’s certainly more difficult to work on organisms that are less frequently studied,” Rokhsar said. “But if we take the opportunity to explore unusual animals that operate in unusual ways, that means we are broadening our perspective on biology, which will eventually help us solve both ecological and biomedical problems.”
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