The World’s Largest Animal Genome Belongs to an Odd, Air-Breathing Fish

Scientists have sequenced the largest animal genome to date—and it belongs to an eel-like fish that breathes air. The genome of the South American lungfish (Lepidosiren paradoxa) takes the cake at a staggering 91 billion base pairs of DNA, according to a study published last week in the journal Nature.

“It’s a real enigma how these fish are able to tolerate such a large genome,” says study co-author Axel Meyer, a biologist at the University of Konstanz in Germany, to New Scientist’s Michael Le Page.

Lungfish interest researchers because they are the closest living relatives to the first four-legged creatures, which are the ancestors of all vertebrates. Those early tetrapods evolved to leave the water and live on land during the Devonian Period, between 419 million and 359 million years ago. Among fish, lungfish are the closest relatives to humans.

Previously, many of the same researchers deciphered the Australian lungfish’s genome in 2021. At the time, it was considered the largest genome of any animal ever sequenced, with 43 billion base pairs. The South American lungfish now beats the Australian lungfish, boasting a genome more than twice as large. And it even dwarfs the human genome, which has fewer than three billion base pairs. “18 of the 19 chromosomes of the South American lungfish are each individually larger than the entire human genome,” Meyer says in a statement.

Why is the lungfish genome so bulky? Researchers say it’s mostly “junk DNA” that doesn’t serve much of a purpose. It comes from DNA sequences called autonomous transposons—also known as “jumping genes”—which can copy themselves and change positions on the genome over time, causing it to expand, writes Imma Perfetto for Cosmos.

These genes are also common in humans, but our species has built-in mechanisms to control them. The South American lungfish, meanwhile, has much weaker controls. As such, the species’ genome expands faster than any known creature—and it’s clunky. In every ten million years of its evolution, the lungfish’s genome grew by the size of the entire human genome.

Despite such fast growth, the lungfish’s DNA is stable. And under the right circumstances, these jumping genes could help the fish adapt to changing environments and conditions, reports Elizabeth Pennisi for Science. A genome that keeps its messy data may also drive the evolution of new genes.

By comparing the genomes of lungfish species, the team also revealed clues to the evolution of four-legged vertebrates. For example, the Australian lungfish still has the limb-like fins that allowed its early relatives to move to the land from the water. The fins of African and South American lungfish, however, have regressed to slender, thread-like filaments in the last 100 million years. They found that a gene called Sonic hedgehog—which is named for the video game character and responsible for forming fingers in four-legged creatures—is not active in African and South American lungfish, reports Ars Technica’s John Timmer. Finding common traits among the living lungfish species could hint at what features might have been shared with the first ancestor of tetrapods.

“Only by studying the biology of the surviving lungfish lineages can we investigate the genomic basis and molecular-developmental mechanisms that facilitated the water–land transition of vertebrates,” says study co-author Igor Schneider, a biologist at Louisiana State University, in a statement.

While the South American lungfish reigns on top as having the largest genome in the animal kingdom, other organisms, such as plants, can have even larger genomes. A tiny fern called the New Caledonian fork fern (Tmesipteris oblanceolata) has 160 billion base pairs in its DNA, making it the owner of the largest known genome on Earth.

“But for animals,” Meyer tells Science, “I would be surprised if there were a larger genome.”

Elizabeth Gamillo | | READ MORE

Elizabeth Gamillo is a correspondent for Smithsonian and a science journalist based in Milwaukee, Wisconsin. She has written for Science magazine as their 2018 AAAS Diverse Voices in Science Journalism Intern.