Scientists have sequenced the genome of the South American lungfish and discovered that it is approximately 30 times larger than the human genetic blueprint.
The South American lungfish is an extraordinary species, in some ways, it is a living fossil. Residing in slow-moving and stagnant waters in Brazil, Argentina, Peru, Colombia, Venezuela, French Guiana, and Paraguay, it is the closest living relative of the first vertebrate land animals, resembling their primitive ancestors that date back over 400 million years.
A South American lungfish in a laboratory at Louisiana State University (USA). (Source: Reuters).
According to a study published in the journal Nature this week, this freshwater fish also has another distinction: it possesses the largest genome of any animal on Earth. Scientists have sequenced the genome of this species and found that it is roughly 30 times larger than the human genetic blueprint.
The measurement unit for genome size is base pairs, the fundamental units of DNA, within the nucleus of a cell. If stretched out like a ball of yarn, the length of DNA in each cell of this lungfish species would reach nearly 60 meters, while the human genome is only about 2 meters long.
“Our analyses indicate that the genome of the South American lungfish has been significantly expanding over the past 100 million years, adding the equivalent of one human genome every 10 million years,” said evolutionary biologist Igor Schneider from Louisiana State University (USA), one of the study’s authors.
According to Schneider, in fact, 18 out of the 19 chromosomes of the South American lungfish—structures resembling fibers that carry the genetic information of an organism—are larger than the entire human genome.
Despite the South American lungfish having a very large genome, there are plant species with even larger genomes. The current record holder is the triplefork fern, scientifically known as Tmesipteris oblanceolata, found in France’s New Caledonia overseas territory in the Pacific. This species’ genome is over 50 times larger than the human genome.
To date, the largest known animal genome belongs to another lungfish species, the Australian lungfish, scientifically named Neoceratodus forsteri. The genome of the South American lungfish is more than double that of this species. Four other lungfish species across the world, residing in Africa, also possess large genomes.
The lungfish genome primarily consists of repetitive elements—about 90% of the genome. Researchers state that the significant expansion of the genome observed in lungfish appears to be related to a reduction in a mechanism that typically prevents such genomic repetitions in these species.
“Animal genome sizes vary widely, but the significance and reasons behind the changes in genome size remain unclear. Our research advances the understanding of biology and genome structure by identifying the mechanisms that control genome size while maintaining chromosomal stability,” Schneider noted.
The South American lungfish can grow up to about 1.25 meters long. While other fish species rely on gills for breathing, lungfish also possess a pair of lung-like organs. They inhabit the oxygen-poor swamp environments of the Amazon and Paraná-Paraguay river basins, supplementing oxygen obtained from water by breathing air.
Lungfish first appeared during the Devonian period. During the Devonian period, one of the most significant moments in Earth’s history occurred—when fish with lungs and fleshy fins evolved into the first four-legged animals—vertebrates that include amphibians, reptiles, birds, and mammals.
Since the ancestors of today’s lungfish are the forebears of four-legged animals, their genome may provide insights into how vertebrates have long evolved features such as limbs that enable life on land.
“The ancestors of four-legged animals conquered land using limbs that evolved from fins and breathed air through lungs. These traits likely existed before the invasion of land. Only by studying the biology of surviving lungfish lineages can we understand the genomic basis and molecular development mechanisms that facilitated the transition of vertebrates from water to land,” Schneider explained.
