The South American lungfish is an extraordinary creature – in some sense, a living fossil. Inhabiting slow-moving and stagnant waters in Brazil, Argentina, Peru, Colombia, Venezuela, French Guiana and Paraguay, it is the nearest living relative to the first land vertebrates and closely resembles its primordial ancestors dating back more than 400 million years.
This freshwater species (Lepidosiren paradoxa) also has another distinction: the largest genome – all the genetic information of an organism – of any animal on the earth. Scientists have now sequenced its genome, finding it to be about 30-times the size of the human genetic blueprint.
The metric for genome size was the number of base pairs, the fundamental units of DNA, in an organism’s cellular nuclei. If stretched out like from a ball of yarn, the length of the DNA in each cell of this lungfish would extend almost 60 metres. The human genome would extend a mere 2 metres.
“Our analyses revealed that the South American lungfish genome grew massively during the past 100 million years, adding the equivalent of one human genome every 10 million years,” said evolutionary biologist Igor Schneider of Louisiana State University, one of the authors of the study published this week in the journal Nature.
In fact, 18 of the 19 South American lungfish chromosomes – the threadlike structures that carry an organism’s genomic information – are each individually larger than the entire human genome, Schneider said.
While huge, there are plants whose genome is larger. The current record holder is a fork fern species, called Tmesipteris oblanceolata, in the French overseas territory of New Caledonia in the Pacific. Its genome is more than 50 times the human genome’s size.
Until now, the largest-known animal genome was that of another lungfish, the Australian lungfish (Neoceratodus forsteri). The South American lungfish’s genome was more than twice as big. The world’s four other lungfish species live in Africa, also with large genomes.
Lungfish genomes are largely composed of repetitive elements – about 90% of the genome. The researchers said the massive genome expansion documented in lungfish genomes seems to be related to a reduction in these species of a mechanism that ordinarily suppresses such genomic repetition.
“Animal genome sizes vary greatly, but the significance and causes of genome size variation remain unclear. Our study advances our understanding of genome biology and structure by identifying mechanisms that control genome size while maintaining chromosome stability,” Schneider said.
The South American lungfish reaches up to about 1.25 metres long. While other fish rely upon gills to breathe, lungfish also possess a pair of lung-like organs. It lives in oxygen-starved, swampy environs of the Amazon and Parana-Paraguay River basins, and supplements the oxygen gotten from the water by breathing in oxygen from the air.
Lungfish first appeared during the Devonian Period. It was during the Devonian that one of the most important moments in the history of life on the earth occurred: when fish possessing lungs and muscular fins evolved into the first tetrapods, the four-limbed land vertebrates that now include amphibians, reptiles, birds and mammals.
Because the forerunners of today’s lungfish were ancestral to the tetrapods, their genomes can provide insight into how vertebrates long ago evolved features such as limbs that enabled life on land.
For instance, the researchers showed that the genetic machinery controlling the activity of the so-called Sonic Hedgehog Shh gene, which regulates important events during embryonic development, likely governed the formation of the bony equivalent of digits in the lungfish fin. Those digits present in the lungfish fin eventually evolved into fingers and toes in tetrapods.
“Tetrapod ancestors conquered land with limbs that evolved from fins, and were breathing air through lungs. These features probably predated the colonisation of land. Only by studying the biology of the surviving lungfish lineages can we investigate the genomic basis and molecular-developmental mechanisms that facilitated the water-land transition of vertebrates,” Schneider said.
