The Largest Animal Genome in the World

The Australian lungfish replaces the Mexican cross-toothed newt Axolotl as the holder of the record "largest animal genome in the world". Its genetic makeup shows the evolutionary innovations that made life in the country possible. 

The bone arrangement in the fins of the Australian lungfish is very similar to that in the limbs of humans. 380 million years ago the first fish began to conquer the land. The Australian lungfish - a critically endangered aerial breather - is one of the few surviving relatives of these first "land-fish". It has hardly changed and is therefore referred to as a “living fossil”. 

An international research team has now for the first time decoded the huge genome of this fish using the latest DNA sequencing technology. The analysis is published in the journal Nature. It gives new insights into the evolutionary innovations that made it possible for fish to colonize the country. The study was the result of a collaboration between researchers from Hamburg, Konstanz, Vienna, Lyon and Würzburg. Senior professor Manfred Schartl from the Biozentrum of the University of Würzburg and Co-PI of the Xiphophorus Genetic Stock Center at the Texas State University, who is an expert on the biology and evolution of fish, with his postdoc Kang Du from Texas State University and the bioinformatician Susanne Kneitz from his Würzburg team were significantly involved  in the study. 

Genome is 14 times larger than that of humans 
According to the study, the lungfish genome is the largest animal genome that has ever been deciphered. At 43 billion base pairs, it is 14 times larger than that of humans. This surpasses the genome of the axolotl, the previous record holder in the animal kingdom, by an impressive 30 percent. 

Why is the genome so big? Amazingly, the lungfish does not have significantly more genes than other vertebrates. But it has significantly more mobile genetic elements, so-called transposons. "These elements can be seen as a kind of computer viruses. They multiply on their own, but have no function. As a scientist, one is surprised that the lungfish's 'genetic hard drive' has not long since crashed in view of the high number of transposons", says Manfred Schartl. 

Fins resemble human limbs 
The Australian lungfish (Neoceratodus forsteri) lives in slow-flowing rivers and stagnant waters. Because of its newt-like body, it was wrongly assigned to the amphibians in the 19th century. Today we know that as a lungfish it belongs to an archaic group of aquatic organisms from which all terrestrial vertebrates evolved. The "fleshy" fins of the lungfish have an anatomical bone arrangement that is similar to that in the human limbs. This allows the Australian lungfish to move around in the water and on land like salamanders. They also have lungs with which they have to breathe air on the surface of the water in order not to drown. 

Closer to amphibians than to fish 
The analyses of the genome revealed striking similarities between the Australian lungfish and terrestrial vertebrates. For example, the number and the spatial and temporal expression patterns of genes associated with the development of lungs, articulated limbs, and the detection of odors in the air are much more similar to amphibians and other terrestrial vertebrates than to their fish relatives. So far, science has controversially discussed whether the lungfish or the also archaic coelacanth are more closely related to the terrestrial vertebrates. The study in Nature now shows that the lungfish are genetically closer to land animals and humans: They split off from the coelacanths 420 million years ago and formed a line that leads to the land animals. 

Axel Meyer, Siegfried Schloissnig, Paolo Franchini, Kang Du, Joost Woltering, Iker Irisarri, Wai Yee Wong, Sergej Nowoshilow, Susanne Kneitz, Akane Kawaguchi, Andrej Fabrizius, Peiwen Xiong, Corentin Dechaud, Herman Spaink, Jean-Nicolas Volff, Oleg Simakov, Thorsten Burmester, Elly Tanaka, Manfred Schartl: “Giant Lungfish genome elucidates the conquest of land by vertebrates”. Nature, 18. Januar 2021. DOI: 10.1038/s41586-021-03198-8

With press release material from University of Würzburg, see: