Researchers from the University of Konstanz and other universities have sequenced the genome of the African (Protopterus sylvatica) and South American lungfish (Lepidosiren Paradox). The genome of the South American lungfish (about 91 gigabases, about 30 times the size of the human genome) is the largest animal genome sequenced to date and more than twice the size of the Australian (Neoceratodus forsteri) and African lungfish.

The South American lungfish (Lepidosiren Paradox). Photo credit: Katherine Seghers, Louisiana State University.

Lungfish – freshwater vertebrates from the order of lungfish – have survived for over 400 million years, from the Devonian period to the present day.

Some consider lungfish to be “living fossils” because their morphology has changed little over the eons.

What they have in common with land-dwelling vertebrates is that they have lungs that are similar to ours.

Of the only six living species of lungfish, four live in Africa, one in South America and one in Australia.

“It seems as if evolution has forgotten them, because these ancient ‘living fossils’ still look very similar to their ancestors,” say biologist Axel Meyer from the University of Konstanz and his colleagues.

“Since our genetic material, DNA, is made up of nucleobases and the sequence of these nucleobases contains the actual genetic information, a comparative analysis of lungfish genomes is only possible with knowledge of their complete sequences.”

“We already knew that lungfish genomes are huge, but how gigantic they really are and what can be learned from them was not clear until now,” they added.

“Therefore, sequencing the lungfish genome was very labor-intensive and complicated from both a technical and bioinformatics perspective.”

In their new research, the scientists sequenced the genomes of African and South American lungfish.

“At 91 gigabases (91 billion bases), the DNA of the South American species is the largest of all animal genomes and more than twice the size of the genome of the previous record holder, the Australian lungfish,” said Dr. Meyer.

“18 of the 19 chromosomes of the South American lungfish are each larger than the entire human genome with its almost 3 billion bases.”

The same team also sequenced the most comprehensive genome sequence of the Australian lungfish to date.

“Autonomous transposons are responsible for the lungfish genome growing to such enormous size over time,” the authors explained.

“These are DNA sequences that ‘replicate’ and then change their position in the genome, which in turn causes the genome to grow.”

“Although this also occurs in other organisms, the analyses showed that the rate of expansion of the genome of the South American lungfish is by far the fastest ever recorded: every 10 million years in the past, its genome has grown by the size of the entire human genome.”

“And it continues to grow. We have found evidence that the transposons responsible for it are still active.”

“We have identified the mechanism for this gigantic genome growth: the extreme expansion is at least partly due to a very low piRNA abundance.”

“This type of RNA is part of a molecular mechanism that normally silences transposons.”

The team’s article was published in the journal Nature.

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M. Schartl et alThe genomes of all lungfish provide information about genome expansion and the evolution of tetrapods. Naturepublished online on August 14, 2024; doi: 10.1038/s41586-024-07830-1