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 almost 11,000 books.

The plant, referred to as Tmesipteris oblanceolata, has the biggest 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 biggest,” says Pellicer.

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

Earlier than this discovery, the biggest identified genome was that of a Japanese flowering plant referred to as Paris japonica, which has 298 billion base pairs in every nucleus, Pellicer reported in 2010. The biggest 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 scale 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 underneath a light-weight detector.

Pellicer says there are two the explanation why some crops have large genomes. Firstly, many crops have a number of units of chromosomes, slightly than the 2 units which might be traditional in animals. T. oblanceolata has eight units of chromosomes.

However most crops with a number of units of chromosomes nonetheless have small genomes general, says Pellicer. Slightly, the important thing issue is a failure to manage the expansion of genetic parasites referred to as transposons.

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

Having a large genome is an obstacle, says Pellicer. “The whole lot takes longer,” he says. “Each time a cell has to divide, it has to duplicate 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, referred to as stomata, can not reply as rapidly to adjustments within the setting when they’re made from bigger cells, says Pellicer.

He thinks crops that fail to manage transposons and restrict the scale 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 might 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 slightly than making an attempt to take action for your complete sequence. It is 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 methods to outline genome measurement, he says. Some assume it needs to be outlined as the scale of 1 set of chromosomes, slightly than the entire quantity of DNA in a cell, which implies the report for the biggest 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.