- The largest known animal genome decoded
The genome is 30 times larger than that of humans and more than twice as large as that of the previous record holder: An international team led by evolutionary biologist Axel Meyer of the University of Konstanz and biochemist Manfred Schartl of the University of Würzburg has decoded the largest known animal genome: the genetic material of the South American lungfish (Lepidosiren paradoxa). "The data will help to understand how the ancestor of today's tetrapods conquered land," a statement accompanying the publication of the study results in the journal "Nature" said.
Lungfish are considered the closest living relatives of those animals that, around 400 million years ago, dragged themselves onto land with powerful fins and were able to survive there thanks to their lungs. All so-called tetrapods, including humans, other mammals, amphibians, reptiles, and birds, go back to them. There are said to be three lines of lungfish today: one in Africa, one in Australia, and one in South America, which is found, among other places, in the Amazon basin.
From the analysis of the genome, researchers hope to gain insights into the origins and further development of these unusual animals. "Evolution seems to have forgotten them," the researchers write. For these ancient "living fossils" still look largely like their ancestors from the late Devonian period, around 400 million years ago. By comparing genome sequences, for example, it is possible to demonstrate the connection between the ray fins of bony fish and the fingers of tetrapods.
Record genome 30 times larger than that of humans
That the genomes of lungfish are large was already known, but the study now shows just how gigantic they can be: The genome of the South American lungfish consists of more than 90 billion bases, the group writes. It is thus 30 times larger than the human genome and more than twice as large as that of the previous record holder, the Australian lungfish (Neoceratodus forsteri). "18 of the 19 chromosomes of the South American lungfish are each larger than the entire human genome," Schartl is quoted as saying in a statement about the study.
This is due to so-called autonomous transposons. These DNA segments "replicate" and then change their position in the genome. They make up more than 90 percent of the lungfish genome. The expansion rate of the South American representative is thus by far the fastest known: In the past, its genome has grown by the size of the entire human genome every ten million years.
Nevertheless, the lungfish genome is surprisingly stable. Therefore, the research team was able to reconstruct the original architecture of the chromosome set of the first tetrapod from the sequences of the still living lungfish species.
Comparative studies provide insights into evolution
Moreover, the genomes of the various lungfish can be compared, allowing conclusions to be drawn as to whether differences between the still living specimens are due to genetic causes. For example, the Australian lungfish still had limb-like fins with which the ancestors could move on land. In contrast, in the African and South American lungfish representatives, these fins, which resembled human arms and legs in bone structure, have been reduced to thread-like fins over the last 100 million years.
Based on the analysis of the genome of the Australian lungfish, the team led by Meyer and Schartl had already shown a few years ago that the same genes in humans and lungfish control the development of the lung. The lung of lungfish has a developmental origin similar to that of terrestrial vertebrates, including humans. Furthermore, fingers, ulna, and radius are present in the fin of the animal, for which the same genes as in humans are responsible. The current study also involved scientists from Dresden, Hamburg, Sweden, Austria, and France.
Despite their large size, being 30 times larger than the human genome and more than twice as large as the previous record holder, the South American lungfish genome remains surprisingly stable. This stability allows researchers to reconstruct the original architecture of the chromosome set of the first tetrapod from the sequences of the still living lungfish species.
The comparison of genome sequences between different lungfish species can provide valuable insights into evolution. For instance, the reduction of limb-like fins in the African and South American lungfish representatives compared to the Australian lungfish, which still possesses these fins, may be due to genetic changes revealed through such comparative studies.
