Massive Lungfish Genome Sheds Light On How Fish “Conquered” Land

'These fins were made for walkin', and that's what they'll (someday) do!' - Australian lungfish. Image courtesy of Dr Solomon R David

Once upon a time, a fish decided enough was enough and dragged itself on land to see what was happening up top. The move took place back in the Devonian about 430 million years ago and was executed by one of the lobe-fined fishes known to science as Sarcopterygii. It would become a defining moment in Earth’s history, as these fish began to adapt to life on land and eventually became tetrapods, four-footed animals that encompasses all vertebrates higher than fishes.

But the story doesn’t begin with the first fish to conquer land, as a series of pivotal innovations associated with terrestrialization had to unfold before they were ready to march their scales up-bank. A new study published in the journal Nature has taken a dive into these genetic adaptations by sequencing the entire genome of the Australian lungfish (Neoceratodus forsteri) and it’s a whopper.

The sequence was achieved using high-powered computer sequencers that took around 100,000 hours to piece together the entire genome, the researchers estimated. At 43 billion base pairs long, it’s 14 times larger than that of humans and the longest known genome to date. Its huge size is explained in part by massive intergenic regions (long stretches of DNA that sit between genes) as well as repeating introns, which are sections of a DNA or RNA molecule which don’t code for proteins.

Their genetic material actually resembles that of tetrapods more so than ray-finned fishes, some of which are also known to “walk” (like these sharks). “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 authors wrote in their study.

The move to a terrestrial habitat was foreshadowed by a series of pivotal changes in the DNA of lungfish, including the development of limb-like structures, a single dorsal lung (the clue’s in the title for that one), and even the ability to detect scents in the air. It was once argued that coelacanths were the closest relative to land-based vertebrates such as amphibians and reptiles, but the sequencing of the lungfish genome has confirmed their position as the mid-way point between fish and tetrapods. 

“In order to get out of the water, you need to adapt towards a terrestrial lifestyle,” said co-author Siegfried Schloissnig from the Research Institute of Molecular Pathology, Austria, to New Scientist. “You have to be able to breathe air, you have to be able to smell… When you look at it from a genomic perspective, it is genomically halfway between a fish and a land-based vertebrate.”

As well as being our closest fish relatives, Australian Lungfish are an impressive species for their longevity. However, even a long-lived species can creep towards extinction if we aren't careful. "Australian lungifish are not only "living fossils", extant members of an ancient lineage, but they can also live for a long time themselves!" wrote Dr Solomon David, fish ecologist and assistant professor at Nicholls State University in Louisiana who studies ancient fish, in an email to IFLScience. "Australian Lungfish in public aquariums have lived for over 80 years, and they are estimated to live for over 100 years! They've helped us humans learn more about the evolution of vertebrates, but they need our help from a conservation standpoint, with much of their native range disappearing, and threatened by habitat loss."

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