An international team of biologists headed by Axel Meyer of the University of Konstanz and Manfred Schartl of the University of Würzburg recently completed sequencing the genome of the South American lungfish, which turned out to be the largest animal genome to date. For instance, the South American lungfish (Lepidosiren paradoxa) has an estimated genome that is around 91 base pairs big; this figure is 30 times the size of the human genome. This makes it the largest of its kind ever to be sequenced for any animal species.

Lungfish are known as living fossils because they have little morphological changes from their ancestors, the Devonian lungfish that lived 420 to 360 million years ago. These fish are nearest to the lobe-finned fish that evolved into the first true four-limbed vertebrates known as the tetrapods who were the first to live on the land.

South American Lungfish
Image Source: Wikipedia

How the South American Lungfish Genome Outpaces Other Animals

To understand the development of these fish and identify genetic changes that enabled them to move to land, the researchers looked into the DNA sequences of all three extant lungfish species, the South American, African, and Australian ones. The South American lungfish has a genome that is more than twice as large as its Australian and African counterparts, it contains 18 of 19 chromosomes larger than humans’ whole genome.

Subsequently, the researchers revealed that a whopping 90% of their South American lungfish genome is made up of repetitive sequences known as transposable elements. These ‘jumping genes’ are capable of multiplying themselves, and then inserting those multiple copies into the DNA and therefore involve a process that leads to giant leaps in the size of the genome over millions of years.

In this manner, the team identified that the South American lungfish’s genome is among the fastest known to be growing, with the addition of a human’s amount of DNA every 10 million years over the last 100 million years. Such a high rate of cell division might be due to low levels of piRNA, a substance that is generally known to suppress the impact of transposons in other species.

"In particular, Lepidosiren's genome has expanded remarkably over the past 100 million years, adding an equivalent of one human genome every 10 million years," the authors noted in their paper published in Nature.

Yet, the whole genome of the South American lungfish contains approximately 20,000 genes for protein-coding functions – roughly as many as in humans. The researchers think that the other 90% of the non-coding DNA might also shed light on how lobe-finned fish came to live on land.

Through the analysis of the genomes of the three lungfish species, the team noted that the Australian lungfish possesses limbs that have fin-bony structures similar to those of fossil lobe-finned fish from the Devonian period. On the other hand, the African and South American lungfish-related species have developed even broader and more filamentous, fish-like fins during the past 100 million of evolution.

"The genomes of living lungfishes can inform on the molecular-developmental basis of the Devonian sarcopterygian fish–tetrapod transition," the researchers wrote in their Nature paper.

Though, it is relatively large to be the largest genome among the animals, but is not the largest genome in general. The New Caledonian fork fern (Tmesipteris oblanceolate) holds that crown, with a genome size of 160 billion base pairs. However, the researchers seem to agree that no other animal species will come anywhere near the South American Lungfish in terms of the absolute size of its genome.

"For animals, I would be surprised if there were a larger genome," said Axel Meyer.

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