The South American lungfish has a whopping 180 gigabases of DNA in each cell, compared with 6 gigabases in human cells.
There is a new record for the largest genome to be sequenced, set at a massive 90 billion letters of DNA. It belongs to the South American lungfish.
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| The South American lungfish probably has a lot of “junk” DNA Katherine Seghers/Louisiana State University |
“It was a technical challenge, of course, to do this,” says Axel Meyer at the University of Konstanz in Germany. “It is the largest of all animal genomes.”
Each cell in the South American lungfish (Lepidosiren paradoxa) has two copies of the genome, comprising 180 gigabases (Gb) of DNA in total that would stretch for 55 metres if arranged in a line. That is 30 times more than the 6 Gb of DNA in each human cell.
The South American lungfish has 19 chromosomes, 18 of which are each larger than a single copy of the human genome, says Meyer.
His team also sequenced the 40 Gb of a single copy of the genome of the African lungfish (Protopterus annectens), meaning the researchers have now sequenced all six lungfish species found globally, which all have abnormally large genomes.
“It’s a real enigma how these fish are able to tolerate such a large genome,” says Meyer. The nucleus inside each cell has to be very large to fit in so much DNA, he says, which means each cell is larger than normal. Replicating so much DNA also takes a lot of energy each time cells divide.
There is no evidence that all this extra DNA does anything useful. Rather, it appears to be a result of “genetic parasites” making endless copies of themselves. It is probably mostly junk, says Meyer.
His team found that mechanisms that other organisms use to curtail the spread of genetic parasites seem to be damaged or missing in all the lungfish species. As a result, the genome of the South American lungfish has been growing by 3.7 Gb – more than a single copy of the human genome – every 10 million years.
The reason for sequencing all the lungfish species is to get a better idea of what their shared ancestor was like. That ancestor was a close relative of the lungfish that evolved into the first four-legged land animal.
“They are our closest relative among the fishes,” says Meyer. As their name implies, lungfish breathe air and drown without it.
They can also live for more than 100 years and regenerate their fins and tails, he says. Meyer’s team hopes to find out how they achieve this.
Some plant genomes are even larger than that of the South American lungfish. A small fern found on a few Pacific islands is thought to have 321 Gb of DNA in each cell, but there are no plans to try to sequence it.
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