Lungfish genome: insights into shore leave

The Australian lungfish are the closest living relative of humans in fish. (Image: Pixabay)

A living fossil has revealed its genome: the genome of the lungfish provides insights into the evolutionary innovations that made it possible for vertebrates to go ashore around 420 million years ago, researchers report. With many repeating DNA sequences, it is the largest animal genome sequenced to date. The analyzes confirmed that the quirky Australian is our closest living relative among the fish. The researchers were also able to identify genetic traits in the lungfish that are linked to limb and lungs development in terrestrial vertebrates.

They crawled out of the water and founded a development from which ultimately we also emerged: In the age of the Devonian the first vertebrates began to conquer the land. The initially fish-like creatures developed lungs for breathing air and powerful fins with which they could move on solid ground. These pioneers formed the ancestors of all land vertebrates today – from amphibians to reptiles to birds and mammals. These ancestors themselves have long since died out – but it is believed that today’s lungfish still resemble the fish that once left the water. Especially the Australian lungfish (Neoceratodus forsteri) In addition to its fish characteristics, it also has clear characteristics of land vertebrates.

On the trail of the code of the pioneers

Its “fleshy” fins have an anatomical bone structure that resembles that in limbs. This means that these animals “walk” on the bottom of water in a similar way to salamanders. As the name suggests, lung fish also have gills and lungs that allow them to breathe air on the surface of the water. To what extent these characteristics and the evolutionary history are reflected in the lungfish genome, biologists have long wondered. But so far there has been a problem: the lungfish’s genetic makeup is one of the largest in the animal world – it is almost 14 times more extensive than that of humans. But the researchers working with Axel Meyer from the University of Konstanz have now succeeded in deciphering this mega-code as well.

As they report, the genome is inflated by many repetitive sequences that apparently only emerged in the genetic make-up of these fish in the more recent evolutionary history. Despite this expansion, the genetic organization reflected the signature of the similarities with the terrestrial vertebrates, say the scientists. As they report, the genetic analyzes confirmed the key evolutionary position of the lungfish as the closest living relative of the terrestrial vertebrates. They are therefore more closely related to us than the famous coelaceans, which are also considered living fossils.

Signature of revolutionary innovations

“But what can be seen in the lungfish genome that sheds light on the evolutionary history of the terrestrial vertebrates?”, Meyer sums up the overarching question. According to the scientists, traces can actually be seen in the genome of these animals that give clues as to how their primeval relatives once conquered the country: They discovered so-called genomic pre-adaptations – genetic pre-adaptations to the new, previously non-existent demands of rural life. Accordingly, the lung fish have genes that are similar to those that control the embryonic development of the lungs in terrestrial vertebrates. “In terms of developmental history, the lung fish’s lungs can therefore be traced back to the same origin as those of terrestrial vertebrates – including humans,” explains Meyer.

The researchers also found evidence of an aspect that is linked to air respiration: the gene families that play a role in the olfactory abilities of terrestrial vertebrates are also increasingly present in lung fish. The scientists also found indications regarding the evolution of the limbs: The development of the fins of the lungfish is therefore similar at the genetic level to processes in the embryonic development of the human hands. The focus is on the hox-c13 and sal1 genes. As the researchers explain, the structure of the fingers in the hands and also of the ulna and radius is already laid out in the fin of the lungfish, for which the same genes and the same gene regulation as in humans are responsible.

Concerning the significance of the genetic findings on the lung fish, co-author Oleg Simakov from the University of Vienna concludes: “Our findings expand our understanding of decisive evolutionary advances and thus the conquest of the Devonian land 420 million years ago.”

Source: University of Konstanz, University of Vienna, specialist article: Nature, doi: 10.1038 / s41586-021-03198-8

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