For centuries, naturalists have puzzled over what might constitute the head of starfish. When looking at a worm, or a fish, it’s clear which end is the head and which is the tail. But with their five identical arms, it’s been anybody’s guess how to determine the front end of the organism from the back. This unusual body plan has led many to conclude that starfish perhaps don’t have a head at all. But now, biologists from Stanford University and elsewhere have found that the truth is closer to the absolute reverse. While they detected gene signatures associated with head development just about everywhere in juvenile starfish, expression of genes that code for an animal’s torso and tail sections were largely missing.
Gene expression data reveal the deployment of the patterning system in the sea star Patiria miniata. Image credit: Formery et al., doi: 10.1038/s41586-023-06669-2.
Starfish (sea stars) belong to a group of animals called echinoderms, which also includes sea urchins and sand dollars.
Echinoderms have a unique ‘fivefold symmetric’ body plan, which means that their body parts are arranged in five equal sections.
This is very different from their bilateral ancestors, which have a left- and right-hand side which mirror one another, as in humans and many other animals.
“How the different body parts of the echinoderms relate to those we see in other animal groups has been a mystery to scientists for as long as we’ve been studying them,” said University of Southampton’s Dr. Jeff Thompson.
“In their bilateral relatives, the body is divided into a head, trunk, and tail. But just looking at a starfish, it’s impossible to see how these sections relate to the bodies of bilateral animals.”
“This has been a zoological mystery for centuries,” said Stanford University’s Professor Chris Lowe.
“How can you go from a bilateral body plan to a pentaradial plan, and how can you compare any part of the starfish to our own body plan?”
In the new study, the researchers compared the molecular markers of the sea star Patiria miniata to other deuterostomes — a wider animal group that includes echinoderms and bilateral animals, like vertebrates.
They share a common ancestor, so by comparing their development, they could learn more about how echinoderms evolved their unique body plan.
The scientists used a variety of high-tech molecular and genomic techniques to understand where different genes were expressed during the development and growth of sea stars.
They used micro-CT scanning to understand the shape and structure of the animal in unprecedented detail.
Then, they used RNA tomography and in situ hybridization to create a three-dimensional map of gene expression in the sea star and find out where specific genes are being expressed during development.
Specifically, they mapped the expression of genes which control the development of the ectoderm, which includes the nervous system and the skin.
This is known to mark the anterior-posterior (front-to-back) patterning in the bodies of other deuterostomes.
They found this patterning was correlated with the midline-to-lateral axis of the sea star arms — with the midline of the arm representing the front and the outmost lateral parts more like the back.
In deuterostomes, there is a distinct set of genes expressed in the ectoderm of the trunk. But in the sea star, many of these genes are not expressed in the ectoderm at all.
“When we compared the expression of genes in a starfish to other groups of animals, like vertebrates, it appeared that a crucial part of the body plan was missing,” Dr. Thompson said.
“The genes that are typically involved in the patterning of the trunk of the animal weren’t expressed in the ectoderm.”
“It seems the whole echinoderm body plan is roughly equivalent to the head in other groups of animals.”
This suggests that sea stars and other echinoderms may have evolved their five-section body plan by losing the trunk region of their bilateral ancestors.
This would have allowed the echinoderms to move and feed differently than bilaterally symmetrical animals.
“Our research tells us the echinoderm body plan evolved in a more complex way than previously thought and there is still much to learn about these intriguing creatures,” Dr. Thompson said.
“As someone who has studied them for the last ten years, these findings have radically changed how I think about this group of animals.”
The results were published in the journal Nature.
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L. Formery et al. Molecular evidence of anteroposterior patterning in adult echinoderms. Nature, published online November 1, 2023; doi: 10.1038/s41586-023-06669-2
