All of a living creature’s genetic information is stored in the genome, but this “blueprint” is different in length for each species. The South American lungfish has the largest genome in the animal kingdom, as scientists have now discovered during the first-ever sequencing of its genome. The DNA of the ancient fish is therefore over 90 billion base pairs long, making it 30 times longer than the human genome and twice as long as that of the previous record holder – the Australian lungfish.
All of today’s land vertebrates – amphibians, reptiles, birds and mammals, including humans – are descended from a fish-like creature that first ventured onto land 420 to 360 million years ago. This common ancestor had powerful front flippers with which it could drag its body across the muddy bottom of the river, and lungs for breathing. The closest living relatives of this ancient tetrapod from the group of lobe-finned fish (Sarcopterygii) are the coelacanths and the lungfish, the latter being the most closely related to land vertebrates. As “living fossils,” they and their genomes can tell us more about the time of the first landing on land.
South American lungfish has record genome
After the genome of the Australian lungfish (Neoceratodus forsteri) had already been decoded in 2021 and had revealed astonishing similarities to amphibians and other land vertebrates, researchers led by Manfred Schartl from the University of Würzburg now turned their attention to the genome of the South American and West African lungfish for the first time. However, the team was immediately surprised by a new world record, because as it turned out, the South American lungfish (Lepidosiren paradoxa) has over 90 billion base pairs, making it the largest genome ever sequenced in the entire animal kingdom.
The title remains in the family, however, because the previous record holder is the Australian lungfish with its 43 billion base pairs. However, it cannot keep up with the genome of its South American relative, which is around twice as large. Compared to the human genome, the latter is 30 times larger. “18 of the 19 chromosomes of the South American lungfish are each larger than the entire human genome with its almost three billion bases,” explains senior author Axel Meyer from the University of Konstanz.
Lots of repetitions and jumps
So-called autonomous transposons are responsible for the gigantic growth of the lungfish genome. These are DNA sections that “multiply” and insert copies at other positions in the genome – which in turn allows the genetic material to grow. These processes also occur in other living creatures, but not nearly as quickly as in the South American lungfish. As the researchers have discovered, in the past its genome has grown by the size of the entire human genome every ten million years.
“And it continues to grow,” reports Meyer. “We have found evidence that the responsible transposons are still active.” This could be because, unlike in human DNA, for example, there is hardly anything stopping them. The South American lungfish has very low levels of piRNA – a type of RNA that normally limits the spread of transposons. Despite the lively transposon activity in the lungfish genome, the arrangement of its genes has remained surprisingly stable and primitive over time, as Schartl and his team report. They now hope to find out more about the common ancestor of all land vertebrates by comparing the record genome with the genetic material of the other sequenced lungfish and even reconstruct the architecture of its chromosome set.
Source: University of Konstanz; Article: Nature, doi: 10.1038/s41586-024-07830-1