Small fern species has a genome 50 times larger than that of humans

A printed version of the entire human genome would fill 220 large books. To do the same for a small, seemingly unremarkable fern found on a few Pacific islands would require nearly 11,000 books.

The plant, called Tmesipteris oblanceolata, has the largest known genome of any organism, Jaume Pellicer at the Botanical Institute of Barcelona in Spain and his colleagues have discovered.

Each cell in the fern has 321 billion letters – or base pairs – of DNA in its nucleus. If arranged in a line, this would stretch for around 105 metres. “From what we know, that’s the largest,” says Pellicer.

By comparison, the nucleus of a human cell contains just over 6 billion base pairs, or around 2 metres, of DNA – around 50 times less than the fern.

Before this discovery, the largest known genome was that of a Japanese flowering plant called Paris japonica, which has 298 billion base pairs in each nucleus, Pellicer reported in 2010. The largest known animal genome is that of the marbled lungfish, Protopterus aethiopicus, with 260 billion base pairs per nucleus.

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

To work out the size of the fern’s genome, they extracted the nuclei of cells from its stems, stained the DNA inside the nuclei with a fluorescent dye and then measured the light intensity as the nuclei passed under a light detector.

Pellicer says there are two reasons why some plants have massive genomes. Firstly, many plants have multiple sets of chromosomes, rather than the two sets that are usual in animals. T. oblanceolata has eight sets of chromosomes.

But most plants with multiple sets of chromosomes still have small genomes overall, says Pellicer. Rather, the key factor is a failure to control the growth of genetic parasites called transposons.

Transposons are bits of DNA that can copy and paste themselves, causing genomes to expand rapidly unless organisms evolve ways to suppress them or manage to get rid of the excess DNA. Many genomes, including that of humans, consist largely of repetitive sequences generated by transposons.

Having a massive genome is a disadvantage, says Pellicer. “Everything takes longer,” he says. “Every time a cell has to divide, it has to replicate all the DNA. So the more DNA there is, the longer it takes to be replicated.”

It also means cells have to be larger to accommodate all the DNA, and the pores in leaves and stems, called stomata, cannot respond as quickly to changes in the environment when they are made of larger cells, says Pellicer.

He thinks plants that fail to control transposons and limit the size of their genomes tend to go extinct. “That’s why we only see them in a very few lineages,” says Pellicer. T. oblanceolata may survive only because competition is less intense on the small islands where it grows, he says.

The researchers plan to sequence a small part of the fern’s genome rather than attempting to do so for the entire sequence. This is because they lack the computational power needed to assemble and analyse such a large and repetitive genome, says Pellicer.

“It is exciting to see that we are still finding new boundaries on how large nuclear DNA contents can get,” says Ryan Gregory at the University of Guelph in Canada. However, there is some debate about how to define genome size, he says. Some think it should be defined as the size of one set of chromosomes, rather than the total amount of DNA in a cell, which means the record for the largest genome would go to the marbled lungfish.

Many biologists define genome size as the amount of DNA in egg, pollen or sperm cells, which is half the amount in normal cells. According to this definition, the genome size of T. oblanceolata is just 160.45 billion base pairs.