A printed model of your complete human genome would fill 220 large books. To do the identical for a small, seemingly unremarkable fern discovered on a couple of Pacific islands would require practically 11,000 books.

The plant, known as Tmesipteris oblanceolata, has the most important identified genome of any organism, Jaume Pellicer on the Botanical Institute of Barcelona in Spain and his colleagues have found.

Every cell within the fern has 321 billion letters – or base pairs – of DNA in its nucleus. If organized in a line, this is able to stretch for round 105 metres. “From what we all know, that’s the most important,” says Pellicer.

By comparability, the nucleus of a human cell accommodates simply over 6 billion base pairs, or around 2 metres, of DNA – round 50 occasions lower than the fern.

Earlier than this discovery, the most important identified genome was that of a Japanese flowering plant known as Paris japonica, which has 298 billion base pairs in every nucleus, Pellicer reported in 2010. The most important identified animal genome is that of the marbled lungfish, Protopterus aethiopicus, with 260 billion base pairs per nucleus.

T. oblanceolata is a uncommon plant that grows solely on some islands of New Caledonia and Vanuatu within the south-  west Pacific. In 2023, Pellicer and his colleagues collected samples from New Caledonia.

To work out the dimensions of the fern’s genome, they extracted the nuclei of cells from its stems, stained the DNA contained in the nuclei with a fluorescent dye after which measured the sunshine depth because the nuclei handed beneath a lightweight detector.

Pellicer says there are two explanation why some vegetation have large genomes. Firstly, many vegetation have a number of units of chromosomes, fairly than the 2 units which can be regular in animals. T. oblanceolata has eight units of chromosomes.

However most vegetation with a number of units of chromosomes nonetheless have small genomes total, says Pellicer. Slightly, the important thing issue is a failure to regulate the expansion of genetic parasites known as transposons.

Transposons are bits of DNA that may copy and paste themselves, inflicting genomes to develop quickly until organisms evolve methods to suppress them or handle to eliminate the surplus DNA. Many genomes, together with that of people, consist largely of repetitive sequences generated by transposons.

Having an enormous genome is an obstacle, says Pellicer. “Every little thing takes longer,” he says. “Each time a cell has to divide, it has to copy all of the DNA. So the extra DNA there may be, the longer it takes to be replicated.”

It additionally means cells should be bigger to accommodate all of the DNA, and the pores in leaves and stems, known as stomata, can’t reply as rapidly to modifications within the setting when they’re made from bigger cells, says Pellicer.

He thinks vegetation that fail to regulate transposons and restrict the dimensions of their genomes are inclined to go extinct. “That’s why we solely see them in a only a few lineages,” says Pellicer. T. oblanceolata could survive solely as a result of competitors is much less intense on the small islands the place it grows, he says.

The researchers plan to sequence a small a part of the fern’s genome fairly than trying to take action for your complete sequence. It’s because they lack the computational energy wanted to assemble and analyse such a big and repetitive genome, says Pellicer.

“It’s thrilling to see that we’re nonetheless discovering new boundaries on how giant nuclear DNA contents can get,” says Ryan Gregory on the College of Guelph in Canada. Nonetheless, there may be some debate about outline genome measurement, he says. Some assume it needs to be outlined as the dimensions of 1 set of chromosomes, fairly than the full quantity of DNA in a cell, which implies the file for the most important genome would go to the marbled lungfish.

Many biologists outline genome measurement as the quantity of DNA in egg, pollen or sperm cells, which is half the quantity in regular cells. In keeping with this definition, the genome measurement of T. oblanceolata is simply 160.45 billion base pairs.