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The heads of most animals are easily recognizable, but scientists haven’t been able to say the same for sea stars until now.
The starfish has five identical arms with a layer of “tube feet” underneath, which can help the marine creature move along the sea floor, leading naturalists to wonder whether sea stars have specific front and rear ends, and whether they have heads at all.
But new genetic research suggests otherwise, that sea stars are largely heads that lack a trunk or tail, and likely lost those features evolutionarily over time.
The strange fossils of sea star ancestors, which appear to have a trunk, make more evolutionary sense in light of the new findings, the researchers said.
The results were published on Wednesday in the journal nature.
“It’s as if the starfish is missing a torso entirely, and is best described as just a head crawling along the seafloor,” Laurent Formery, the study’s lead author and a postdoctoral researcher at Stanford University and the University of California, Berkeley, said in a statement. . “This is not at all what scientists assumed about these animals.”
The discovery, made possible by new methods of genetic sequencing, could help answer some of the biggest remaining questions about echinoderms. Including their common ancestors with humans and other animals that look nothing like them.
Unique body plan
Starfish belong to a group called echinoderms, which includes sea urchins, sand dollars and sea cucumbers. The unusual animals have unique body plans arranged in five equal sections that differ significantly from the symmetrical head-to-tail bodies of bilateral animals, in which the left and right sides mirror each other.
Sea stars start out as fertilized eggs that hatch into larvae that float in the ocean, like plankton, for weeks to months before settling on the ocean floor. There, they undergo the process of converting the binary body into a star-shaped, or pentagonal, body.
“This has been a mystery in zoology for centuries,” Christopher Lowe, one of the study’s senior authors and a marine and developmental biologist at Stanford University, said in a statement. “How do you go from a twofold body plan to a fivefold body plan, and how do you compare any part of a starfish to our own body plan?”
The bilateral body plan in most animals stems from genetic actions at the molecular level that can be traced to the head and trunk or main body regions, which is why vertebrates, such as humans, and many invertebrates, including insects, share similar genetic programming. This discovery won an award Nobel prize in Physiology or Medicine in 1995.
But echinoderms also share a common ancestor with bilaterian animals, adding to the mystery researchers are trying to solve.
Study co-author Dr Geoff Thompson, a lecturer at the University of California, said: “How the different body parts of echinoderms relate to those we see in other groups of animals has been a mystery to scientists throughout their study.” “In their bilateral relatives, the body is divided into a head, torso and tail,” Southampton said in a statement. “But just looking at a starfish, it is impossible to see how these parts relate to the animals’ bilateral bodies.”
The researchers behind this new study used micro-computed tomography to capture an unprecedented 3D look at the shape and structure of sea stars.
Next, team members used advanced analytical techniques to determine where genes are expressed within tissues and identify specific RNA sequences within cells. Gene expression occurs when information within a gene becomes active.
Specific molecular markers act like body blueprints, directing each cell to the region of the body to which it belongs.
“If you strip the skin of an animal and look at the genes involved in determining head from tail, the same genes code for these body regions in all groups of animals,” Lowe said. “So we ignored the anatomy and asked: Is there a molecular axis hidden beneath all this strange anatomy and what is its role in the starfish’s formation of the five-rayed body?”
Shown here is the nervous system of a starfish during analysis.
Together, the data created a 3D map to determine where genes are expressed as sea stars develop and grow. The team was able to identify genes that control the development of the starfish’s ectoderm, which includes its skin and nervous system.
Genetic signatures associated with head development have been detected throughout sea stars, especially concentrated in the center of the star and the center of each limb. But gene expression for the trunk and tail parts was largely absent, revealing that sea stars “have the most dramatic example of the separation of head and trunk regions that we know of today,” said Formeri, who is also a Chan researcher. Zuckerberg BioHub, a nonprofit research organization in San Francisco.
The research was funded by the Chan Zuckerberg BioHub, co-founded in 2021 by Dr. Priscilla Chan and Mark Zuckerberg, as well as NASA, the National Science Foundation and the Leverhulme Fund.
The researchers stained the starfish’s genetic material with fluorescent markers, enabling scientists to map the behavior of the animals’ genes.
“When we compared gene expression in starfish to other groups of animals, such as vertebrates, an important part of the body plan appeared to be missing,” Thompson said. “The genes normally involved in the formation of the animal’s trunk are not expressed in the ectoderm. The entire body plan of echinoderms appears to be roughly equivalent to the head in other groups of animals.
Sea stars and other echinoderms likely developed their unique body plans once their ancestors lost their trunk region, allowing them to move and feed differently from other animals.
“Our research tells us that the body plan of echinoderms evolved in a more complex way than we previously thought, and there is still a lot to learn about these interesting creatures,” Thompson said. “As someone who has studied these animals for the past 10 years, these findings have radically changed the way I think about this group of animals.”
Animal research largely targets those that share similarities with humans. But studying groups like echinoderms could solve some of the most complex mysteries about the evolution of life on Earth.
“Most animals don’t have amazing nervous systems, and they hunt their prey out there. They’re humble animals that live in burrows in the ocean. People aren’t generally attracted to these animals, yet they probably represent how much life began,” Lowe said.
Understanding how animals such as sea stars evolve can also allow insight into the diverse ways in which different species maintain their health.
“It is certainly difficult to work with organisms that are not frequently studied,” study co-author Daniel Rokhsar, a professor of genetics, genomics, evolution and development at the University of California, Berkeley, and a researcher at the Chan Zuckerberg BioHub, said in a statement.
“But if we seize the opportunity to explore unusual animals that work in unusual ways, it means we are broadening our perspective on biology, which will ultimately help us solve environmental and biomedical problems.”
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