The Australian lungfish (Neoceratodus forsteri), a species of lungfish native to the Mary and Burnett River systems in south-eastern Queensland, has a genome size about 14 times larger than that of humans.
Lungfish, which belong to lobe-fined fish (Sarcopterygii), appeared in the fossil record in the Devonian period about 400 million years ago.
Some consider lungfish as ‘living fossils’ since their morphology barely changed over eons.
Lungfish share with the land-dwelling vertebrates the ability to breathe air though lungs that are similar to ours.
Of the only six living lungfish species four live in Africa, one in South America, and Neoceratodus forsteri in Australia.
Recent genetic studies tended to support the hypothesis that lungfish are the closest living relatives of tetrapods. They hence are crucial for understanding the evolution and the preadaptations accompanying the transition of vertebrate life from water to land.
“What can we find in the lungfishes’ genome that explains how these fish could leave the water and move to land?” said lead author Professor Axel Meyer, a researcher in the Department of Biology at the University of Konstanz.
“The major evolutionary event of colonizing the land required a number of other evolutionary innovations in addition to air breathing through the development of the lung, such as the abilities to smell scents in the air and to move on land.”
To sequence and assemble the genome of Neoceratodus forsteri, Professor Meyer and colleagues used long and ultra-long read nanopore technology.
They found that the lungfish genome contains more than 43 billion base pairs, about 14 times larger than the human genome, making it the largest animal genome sequenced so far.
“Phylogenomic analyses ascertained that lungfish occupy an evolutionary key-position as closest living relatives to tetrapods, underscoring their importance for understanding innovations associated with terrestrialization,” the researchers said.
“Preadaptations to living on land include gaining of limb-like expression of developmental genes such as hoxc13 and sall1 in their lobed fins.”
“Increased rates of evolution and duplication of genes associated with obligate air-breathing such as lung surfactants and the expansion of odorant receptor gene-families that detect airborne odors contribute to their tetrapod-like biology.”
“These findings advance our understanding of this major transition during vertebrate evolution,” they concluded.
A paper on the findings was published this week in the journal Nature.
A. Meyer et al. Giant lungfish genome elucidates the conquest of land by vertebrates. Nature, published online January 18, 2021; doi: 10.1038/s41586-021-03198-8