A bizarre and ugly fish that has been around for hundreds of millions of years has been discovered to have the most DNA of any animal ever found.
These South American lungfish (Lepidosiren paradoxa) have around 30 times as much DNA as humans, according to a new paper in the journal Nature, which may give us clues as to how life transitioned from water to land.
These lungfish, alongside two other species currently alive today—African lungfish (Protopterus annectens) and Australian lungfish (Neoceratodus forsteri)—are sometimes known as "living fossils" because they are nearly exactly the same as their ancestors from the Devonian period, some 420 to 360 million years ago.
"The Lepidosiren genome (about 91 Gb, roughly 30 times the human genome) is the largest animal genome sequenced so far and more than twice the size of the Australian (Neoceratodus forsteri) and African lungfishes," the researchers wrote in the paper.
Modern lungfish can breathe air using their lungs, in addition to extracting oxygen through gills like other fish. Lungfish are the closest living ancestor of early lobe-finned fish (Sarcopterygii), which were also our ancient ancestors, giving rise to the first four-limbed animals that eventually colonized land.
The researchers analyzed the genomes of modern lungfish to figure out how the ancient lobe-finned fish managed to make the swap from water to land all those millions of years ago.
The researchers already knew that lungfish had very large genomes, but in this paper, they found the exact number of bases contained in total across their DNA, discovering that the South American lungfish has a record-breaking 30 times larger genome than humans.
"With over 90 gigabases (in other words, 90 billion bases), the DNA of the South American species is the largest of all animal genomes and more than twice as large as the genome of the previous record holder, the Australian lungfish. 18 of the 19 chromosomes of the South American lungfish are each individually larger than the entire human genome with its almost 3 billion bases," study author Axel Meyer, a researcher at the University of Konstanz in Germany, said in a statement.
Only a small proportion of these DNA sequences actually coded for proteins, however.
These fish may have gained their enormous DNA thanks to a genetic phenomenon called autonomous transposons. These are a type "jumping gene" that can move within a genome on their own, cutting or copying themselves and inserting into different locations in the DNA.
If the autonomous transposon copies itself, it can increase the size of the genome when it pastes a copy back in at another location. The researchers think that this is how the South American lungfish grew so large over millions of years.
This process occurred much faster in the South American lungfish than in other organisms, the researchers suggest, increasing by a margin the size of the entire human genome once every 10 million years.
"In particular, Lepidosiren's genome grew extremely fast during the past 100 million years (Myr), adding the equivalent of one human genome every 10 Myr," the authors wrote in the paper.
This may be because the South American lungfish has very little of a substance called piRNA in its cells, which usually silences the effects of transposons in other species.
"And it continues to grow," said Meyer. "We have found evidence that the transposons responsible are still active."
By sequencing the entirety of the genomes of the South American lungfish, Australian lungfish and African lungfish, the researchers were able to figure out the genetic basis of any differences between the lineages. They found that the Australian lungfish still has fins more similar to those found in fossils from the ancient lobe-finned fish from the Devonian, while the African and South American lungfish evolved more filamentous fishlike fins over the past 100 million years.
The researchers hope to use these lungfish genomes to gain a better understanding about how the lobe-finned fish climbed onto land all those years ago, and how its descendants became our own land-dwelling ancestors.
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References
Schartl, M., Woltering, J. M., Irisarri, I., Du, K., Kneitz, S., Pippel, M., Brown, T., Franchini, P., Li, J., Li, M., Adolfi, M., Winkler, S., de Freitas Sousa, J., Chen, Z., Jacinto, S., Kvon, E. Z., Correa de Oliveira, L. R., Monteiro, E., Baia Amaral, D., ... Meyer, A. (2024). The genomes of all lungfish inform on genome expansion and tetrapod evolution. Nature, 1–8. https://doi.org/10.1038/s41586-024-07830-1
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