Lungfish fins discover how limbs evolve

The evolution of limbs with functional figures of fish fins occurred about 400 million years ago in the Devonian. Through this morphological transition, vertebrates left the water to conquer land and gave rise to all four-legged animals or tetrapods – the evolutionary strain that includes all amphibians, reptiles, birds and mammals (including humans). Since the nineteenth century, various theories based on both fossils and embryos have been attempted to explain how this transformation evolved. However, exactly how hands with figures from fish fins remained was unknown.

An international team of biologists based at the University of Konstanz (Germany), Macquarie University in Sydney (Australia) and the Anton Dohrn Stazione Zoologica in Naples (Italy) determined how the limbs evolved from fins with embryos of the Australian lungfish (Neoceratodus forsteri)) for their study. The Australian lungfish is the closest living fish ratio of tetrapods and is often considered a “living fossil”, as it still resembles the fish that roamed at the time of the first four-limbed landings. began to run. For these reasons, the fish of lungfish provide a better reference to study the evolutionary transition from fins to limbs than any other existing fish species.

The research of the team, which is reported in the latest issue of Science Advances, shows that a primitive hand is present in lungfish fins, but at the same time suggests that the unique anatomy of limbs with figures only evolved during the emergence of tetrapods due to changes in embryonic development.

Insight of embryonic development: genes of members of architect

To solve the puzzle of how limbs originated from fins during evolution, researchers have focused on embryonic development. “During embryogenesis, a suite of ‘architect’ genes forms an amorphous group of progenitor cells in fully grown limbs,” explains Dr. Joost Woltering, first author of the study and an assistant professor in the group Evolutionary Biology at the University of Konstanz. by Professor Axel Meyer. The same “architect” genes also drive fine-tuning. However, because evolutionary changes have taken place in the activity of these genes, the evolutionary process produces fins in fish and limbs in tetrapods.

To compare this process in fins and limbs, the team studied such “architect” genes in the embryos of the Australian lungfish. “Amazingly what we discovered is that the gene that specifies the hand in limbs (hoxa13) is activated in a similar skeletal region in lungfish fins,” explains Woltering. Importantly, this domain has never been observed in the fins of other fish that are further related to tetrapods. “This finding clearly indicates that a primitive hand was already present in the ancestors of land animals.”

Development patterns: differences and similarities

The lungfish “hand”, despite this modern genetic signature, is somewhat similar to the anatomy of tetrapod hands because it lacks no fingers or toes. To understand the genetic basis for this difference, the team went on to analyze additional genes known to be associated with the formation of numbers, and found that one gene is important for the formation of fingers and toes (hoxd13 – a “sister gene”). to above- named hoxa13) appeared otherwise to switch into fins.

During tetrapod limb development, the hoxd13 gene is activated in a dynamic manner. It is first activated in the developing pinky finger and then extends all the way through the future hand to the thumb. This process coordinates the exact formation of all five fingers. While the team of Joost Woltering observed a similar activation pattern of this gene in lungfish fins, it did not show this extension, but remained activated only in exactly half of the fin. Additional differences were found for genes that are normally excluded in numbers. In lungfish fins, these genes remain active, but on the opposite side of the domain where hoxd13 is activated.

Old hypotheses – future directions

“All of this shows that although lungfish fins unexpectedly have a primitive hand in common with tetrapods, the fins of our ancestors also needed an evolutionary ‘finish’ to produce limbs. In this sense, it seems like the hand was there first, only later. complement with figures later in evolution, “says Woltering. One influential hypothesis regarding the evolution of limbs first proposed by the early 20th century paleontologists Thomas Westoll and William Gregory, and famously further developed in the 1980s by Neil Shubin, postulates that fingers and toes were formed by a expansion of the skeletal elements on one side of the fins of the tetrapod ancestor. This derived extension of fin elements corresponds exactly to the differences the team found in the extension of the numerical genes between lungfish fins and tetrapod limbs. The team’s observations on the activation and deactivation of “architect” genes in lungfish fins thus provide evidence in support of this classical transformation model.

In the future, in order to fully understand what extends this domain, making our limbs so different from fish fins, the researchers plan to conduct further analyzes on the development of fins and limbs, use of lungfish, but also modern fish species such as cichlids and their embryos are easier to study with techniques such as CRISPR. “To complete the picture of what happened in our fish ancestors that crawled ashore hundreds of millions of years ago, we really rely on modern living species to see how their embryos’ fins and limbs grow so differently,” concludes Woltering.