Lung fins reveal how limbs have evolved

The evolution of limbs with functional fingers of fish fins occurred about 400 million years ago in the Devonian. This morphological transition allowed vertebrates to leave water to conquer land and gave birth to all four-legged animals or tetrapods – the evolutionary lineage that includes all amphibians, reptiles, birds and mammals (including humans) . Since the 19th century, several theories based on both fossils and embryos have been advanced in an attempt to explain how this transformation took place. Yet how the hands with numbers came from the fins of a fish remained unknown.

An international team of biologists based at the University of Constance (Germany), Macquarie University in Sydney (Australia) and the Stazione Zoologica Anton Dohrn in Naples (Italy) determined how the members evolved from the fins using Australian lung embryos (Neoceratodus forsteri)) for their study. The Australian lungfish are the closest living fish to tetrapods and are often considered a “living fossil” because they still resemble the fish that existed when the first four-limbed vertebrates began to walk on land. For these reasons, the fins of the lungfish provide a better reference for studying the evolutionary transition from fins to limbs than any other species of fish in existence.

The team’s research, which is reported in the latest issue of Scientific advances, shows that a primitive hand is present in the fins of lungfish, but at the same time suggests that the unique anatomy of the toed limbs evolved only during the rise of tetrapods through changes in embryonic development.

Prospects for embryonic development: genes that form the limbs

To solve the puzzle of the emergence of fin limbs during evolution, the researchers focused on embryonic development. “During embryogenesis, a series of ‘architect’ genes transform an amorphous group of precursor cells into fully developed members,” says Dr. Joost Woltering, first author of the study and assistant professor in the evolutionary biology group of the University of Constance. by Professor Axel Meyer. The same “architect” genes also determine the development of the ailerons. However, as evolutionary changes have occurred in the activity of these genes, the developmental process produces fins in fish and limbs in tetrapods.

To compare this process in fins and limbs, the team studied such “architect” genes in Australian lungfish embryos. “Surprisingly, what we found is that the gene specifying the hand in the limbs (hoxa13) is activated in a similar skeletal region in fish-lung fins,” Woltering explains. Importantly, this area has never been observed in the fins of other fish that are more distant from the tetrapods. “This finding clearly indicates that a primitive hand was already present in the ancestors of land animals.”

Development models: differences and similarities

The “hand” of the lungfish, despite this modern genetic signature, only partially resembles the anatomy of tetrapod hands because it has neither fingers nor toes. To understand the genetic basis for this difference, the team then analyzed additional genes known to be associated with digit formation, finding that a gene important for finger and toe formation (hoxd13 – a “sister gene” from the above – mentioned hoxa13) seemed to be lit differently in the ailerons.

During the development of the tetrapod member, the hoxd13 gene is dynamically activated. It activates first in the developing little finger and then extends throughout the future hand towards the thumb. This process coordinates the correct formation of the five fingers. While Joost Woltering’s team observed a similar pattern of activation of this gene in lungfish fins, it did not show this expansion but only remained activated in exactly half of the fin. Additional differences were found for genes that are normally turned off in digits. In fish-lung fins, these genes remain active, but on the opposite side of the domain where hoxd13 is activated.

Old assumptions – future directions

“All of this shows that while the fins of lungfish unexpectedly have a primitive hand in common with tetrapods, the fins of our ancestors also needed an evolutionary ‘finishing touch’ to produce limbs. In that sense, it seems the hand was there first. , only to be supplemented by numbers later in the evolution, ”Woltering explains. An influential hypothesis regarding limb evolution first proposed by early 20th century paleontologists Thomas Westoll and William Gregory, and in the 1980s, developed by Neil Shubin, postulates that fingers and toes arose by l expansion of the skeletal elements on one side. of the fins of the tetrapod ancestor. This inferred expansion of fin elements matches exactly the differences the team found in the expansion of digital genes between fish-lung fins and tetrapod limbs. The team’s observations on the activation and deactivation of limb “architect” genes in fish-lung fins thus provide evidence to support this classical transformational model.

In the future, to fully understand the causes of the expansion of this area, making our limbs so different from fish fins, researchers plan to conduct further analyzes on fin and limb development, using lungs. but also more modern fish species such as cichlids their embryos are easier to study using techniques like CRISPR. “To complete the picture of what happened to our fish ancestors who crawled the earth hundreds of millions of years ago, we really rely on the species currently living to see how their embryos grow so differently. fins and their limbs, ”Woltering concludes.