Is it the middle, a special arm – or do starfish have no head at all? A genetic study has now revealed that your entire body consists only of this part of your body. According to this, the genes that usually lead to the formation of heads in animals are active throughout the entire body of the starfish. However, programs that lead to the development of a torso do not take place during their development. This suggests that the evolution of these creatures’ unusual body structure was based on a reduction of anatomy to the head, say the scientists.
Normally the front and back are clearly visible and the bodies are structured in mirror symmetry: from worms to humans, most animals from the Bilateria group have a characteristically symmetrical anatomy with a head and torso. But starfish and some other representatives of the echinoderms are an exception that has been causing biologists headaches for generations. Because their bodies consist of several identically structured appendages and a division into head and torso is not recognizable. So far it has not been possible to clarify to what extent certain parts of these animals correspond to the typical structures of other representatives of the Bilateria and how the strange star-shaped body structure could have developed in the course of evolution.
Anatomy as reflected in genetics
Since studies of anatomical features and tissue structures have not yet provided any clear clues, a team of British and US researchers has now investigated the mystery using genetic methods. This approach is based on the fact that it has been known since the 1990s that in the Bilateria, certain genetic genes are responsible for the formation of the various body parts during embryonic development: the activity of certain genes is characteristic of the formation processes of the head, while that of others is characteristic of the head Development of the trunk or tail. This means: A section of an animal body can also be assigned to the respective body segments based on gene expression.
For their study, the scientists now examined the activity of these well-known developmental genes in young starfish of the species Patiria miniata. They used the technique known as “RNA tomography” and carried out in situ hybridization on the animals’ tissues to reveal gene expression patterns. From the results, they were then able to create a three-dimensional map of the activity relevant to the formation of body parts in the different tissue areas of the starfish.
The hull is missing
The results showed that starfish have areas with head-typical gene expression in the middle of each arm. In contrast, the scientists did not find any activity in any part of the animals of developmental genetic structural programs that are typical for the trunk in the representatives of the Bilateria. Ultimately, this means: Starfish don’t actually have a special head area, but rather they consist entirely of this body section. “You appear to be completely missing your torso. “They can best be described as heads that wander across the seabed,” says lead author Laurent Formery from Stanford University.
This result suggests that starfish and some other echinoderms developed their body structure by losing the trunk region of their bilaterally symmetrical ancestors, the scientists explain. “This opens up a lot of new questions that we can now explore,” says Formery. There is also an interesting clue: fossil finds provide evidence that the ancestors of these animals still had a torso. In further studies, Formery and his colleagues now want to investigate, among other things, the question of the extent to which the genetic structuring in starfish can also be found in other echinoderms such as sea urchins and sea cucumbers.
Source: Stanford University, Chan Zuckerberg Biohub, University of Southampton, specialist article: Nature, doi: 10.1038/s41586-023-06669-2