For centuries naturalists have asked: Where could his head be? sea stars. When you look at a worm or a fish, you can tell which is the head and which is the tail. But with five identical arms, any one of them could lead the starfish along the seafloor. No one knows how to determine which of these organisms is the back and which is the front. Such an unusual body design led many scientists to conclude that the starfish could not have a head or legs.
But now laboratories at Stanford University and UC Berkeley have published a study that puts this mystery to rest. In short, while the team detected genetic signatures associated with head development nearly everywhere in the baby starfish, expression of genes that code for an animal’s body and tail parts was largely missing.
Researchers used a variety of high-tech molecular and genomic techniques to understand this. Where different genes arise during the development and growth of the starfish. A team from Southampton used micro-CT scanning to understand the animal’s shape and structure in unprecedented detail.
Another discovery made is this: Molecular signatures typically associated with the anteriormost part of the head were located toward the middle of each arm of the starfish.and these signs become increasingly posterior and directed towards the edges of the arms.
Actually it’s all heads
And the research published on November 1 Naturesuggests: Far from being headless, starfish lost their bodies throughout evolution, becoming only heads.
“It’s as if the starfish were completely missing a trunk, and the best way to explain it is this: a head crawling on the seabed“said Laurent Formery, a postdoctoral researcher and lead author of the new study. “This is definitely not what scientists assumed about these animals.”
Two of the study’s three co-authors, marine biologist Christopher Lowe of Stanford University and UC Berkeley’s Daniel Rokhsar, an expert on the molecular evolution of animal species, have been collaborating for a decade.
Almost all animals, including humans, are bilaterally symmetricalthat is, they can be divided into two mirrored halves along a single axis running from head to tail. In 1995, the Nobel Prize in Physiology or Medicine was awarded to three scientists for using fruit flies to show that the bilateral, head-to-tail body structure observed in most animals results from the action of a set of encoded molecular switches. genes.
Researchers have since confirmed that the same genetic programming is shared by the vast majority of animal species, including vertebrates such as humans and fish, as well as many invertebrates such as insects and worms.
Fivefold symmetry instead of double
But the starfish’s body design has long puzzled scientists studying animal evolution. Instead of exhibiting bilateral symmetry, adult starfish (and echinoderms such as sea urchins and sea cucumbers) have fivefold axes of symmetry without a clear head or tail. And no one has been able to determine how genetic programming engineered this unusual fivefold symmetry.
Some scientists have suggested that the starfish’s head-to-tail axis may extend from the animal’s armored back to its belly, which is covered with parts called tube feet. Others have suggested that each of the starfish’s five arms corresponds to a copy of the traditional head-tail axis.
However Efforts to conclusively confirm such hypotheses have encountered several obstacles.This is largely because methods for detecting gene expression developed in a small number of model organisms such as mice and flies do not work well in young starfish tissue. For years, Lowe and his colleagues have wanted to provide genetic information that would solve the question by mapping genetic activity in developing starfish. But without the sophisticated genetic tools developed over decades of research for typical model organisms, such a comprehensive analysis was daunting.
innovative technology
Lowe found a solution to this problem at one of Biohub’s regular researcher meetings in San Francisco; Another researcher suggested he contact PacBio, a Silicon Valley-based company that produces genome sequencing devices. For the past five years, PacBio has been developing a technique for sequencing large amounts of genetic material. Postage stamp-sized chips filled with millions of individual chemical reactors, each simultaneously prepared to read long sections of DNA captured inside.
Unlike traditional sequencing, which requires chopping genetic material into small pieces to ensure accuracy, PacBio’s approach, called HiFi sequencing, can obtain highly precise data from intact gene-sized strands of DNA. makes the process much faster and cheaper. That’s exactly what Lowe and his team needed to create a process for studying starfish genetics from scratch.
“The sorting that would take months can now be done in a few hours.”“These advances meant that we were essentially able to start from scratch compared to five years ago,” said David Rank, former PacBio scientist and co-senior author of the new study. We examine an organism that is not normally studied in the laboratory and perform a detailed study that was impossible 10 years ago.
A new genetic analysis system made it possible to see traces of the brain in the body of the star
This technology allowed researchers to sequence starfish genomes and use a system called spatial transcriptomics. They were able to identify which starfish genes were active in specific parts of the organism. To look for patterns indicating the head-to-tail axis, the researchers examined differences in gene expression in three different directions along the body: from the starfish’s center to the arm tips, from bottom to top, and from one point to another. one side of your arms to the other.
Then, to take a closer look at how certain key genes behaved, they labeled them individually with fluorescent dyes, creating a detailed map of their distribution throughout the starfish’s body.
brains in arms
Researchers found that none of the common hypotheses about starfish body structure are correct. Instead they saw this In humans and other bilaterally symmetrical animals, gene expression corresponding to the forebrain was located along the midline of the arms. The starfish’s genetic expression corresponds to that of the human midbrain towards the outer edges of the arms.
While genes marking different head subregions in humans and other bilateral creatures have been detected in starfish, Only one of the genes typically associated with the trunk in animals was detected, specifically at the edges of the stars’ arms.
“These results suggest that echinoderms and Starfish, in particular, are the clearest example we know of where the head and body regions are separated.“This opens up a lot of new questions that we can now begin to explore,” Formery said, adding that some odd-looking starfish ancestors preserved in the fossil record appear to have had a trunk.
The door to new discoveries
The next question the team hopes to address is whether the genetic pattern observed in starfish also appears in sea urchins and sea cucumbers. Formery also wants to investigate what starfish can teach us about the evolution of the nervous system; According to him, it’s something no one fully understands about echinoderms.
Learning more about starfish and their relatives will not only help unravel fundamental mysteries of animal evolution, but Can inspire innovations in medicine, researchers said. Starfish walk by moving water along thousands of tubular legs and digest their prey by extending their stomachs outside their bodies. It makes sense that these unusual creatures would also have developed completely unexpected strategies to stay healthy. These are strategies that, when studied properly, could expand our approaches to combating human diseases.
“Less studied organisms are definitely more difficult to work with,” Rokhsar said. “But if we get the opportunity to discover unusual animals that work in unusual ways, it means we are expanding our perspective on biology, and this will eventually help us solve both ecological and biomedical problems.”
Reference work: DOI: 10.1038/s41586-023-06669-2. www.nature.com/articles/s41586-023-06669-2
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