The largest sequenced genome is 30 times larger than the human genome

The largest sequenced genome on record, containing 90 billion DNA letters, belongs to a South American lungfish.

“Obviously, making this happen was a technical challenge,” he said. Axel Meyer “This is the largest animal genome ever found,” said researchers from the University of Konstanz in Germany.

South American lungfish (Lepidosiren paradoxaThere are two copies of the human genome, with a total of 180 gigabases (Gb) of DNA, which would stretch to 55 metres if laid out in a line – 30 times the amount of DNA found in a single human cell (6 Gb).

The South American lungfish has 19 chromosomes, 18 of which are larger than a single copy of the human genome, Meyer said.

His team also sequenced a single copy of the 40 Gb African lungfish genome (Protopterus annectens), the researchers have now sequenced all six species of lungfish found around the world, all of which have unusually large genomes.

“It's really puzzling how these fish can tolerate such large genomes,” Meyer says. The nucleus in each cell must be very large to accommodate so much DNA, meaning each cell is larger than normal, he says. Replicating that much DNA also requires a lot of energy with each cell division.

There's no evidence that this extra DNA does anything useful. Rather, it appears to be the result of a “genetic parasite” replicating itself endlessly. It's probably mostly junk, Meyer says.

His team found that mechanisms that other organisms use to limit the spread of genetic parasites appear to be damaged or missing in all lungfish species, resulting in the South American lungfish genome growing by 3.7 Gb every 10 million years – more than one copy of the human genome.

The reason for sequencing all lungfish species is to get a better understanding of what their common ancestor was like, a close relative of the lungfish that evolved into the first tetrapod land animal.

“Of all fish, lungfish are our closest relatives,” Meyer says. As their name suggests, lungfish breathe air and would drown without it.

They can also live for more than 100 years and regrow fins and tails, Meyer said, and his team hopes to figure out how they do this.

Some plant genomes are even larger than that of the South American lungfish: a small fern found on several Pacific islands is thought to have 321 Gb of DNA per cell, but there are no plans to sequence it.