The largest genome on the planet is found in a small fern

Its extended DNA would have a length of about 300 feet… 50 times longer than that of humans

El genoma más grande del planeta lo tiene un pequeño helecho
This particular fern, 'Tmesipteris oblanceolata,' grows on fallen logs in the jungles of New Caledonia, Oceania.POL FERNANDEZ
Miguel Ángel Criado

The tree of life offers many things, but a fern — which may seem like a not-too-complex organism — has turned out to be the living being with the largest genome. It grows in New Caledonia, an island in Oceania that’s part of overseas France. And, if the 160,750,000,000 base pairs of its DNA could be placed on top of each other, they would reach a length of about 300 feet… 50 times longer than human DNA. The recent discovery raises new questions about how much genetic material can be stored in cells and about the lack of correlation between complexity and genetics.

Tmesipteris oblanceolata grows on the fallen trunks of the forests of New Caledonia. It’s a fern that belongs to a genus of vascular plants, of which there are barely 15 known species. At least two of their first cousins were known to have giant genomes. But until now, the organism that contained DNA with what was thought to be the greatest number of base pairs was another plant, known as Paris japonica. Now, however, several researchers — who determined the genetic length of Paris japonica — have discovered that the genome of Tmesipteris oblanceolata is 7% larger.

In a new study published in the scientific journal iScience, researchers from the Royal Botanic Gardens of Kew (United Kingdom) and the Botanical Institute of Barcelona present the results of their analysis of this fern. They demonstrate that it has the largest amount of DNA stored in the nucleus of its cells compared to any living eukaryotic organism on the planet. If it were a tangle to unravel, Tmesipteris oblanceolata would extend between 344 and 347 feet.

“It’s not an iconic plant; it doesn’t have flowers, nor is it striking. In fact, it’s a weed that, if you’re not looking for it, you would trample without realizing it,” admits Jaume Pellicer, a researcher at the Botanical Institute of Barcelona. “It doesn’t even look like a fern, [or at least] it doesn’t look like the traditional image we have of them. But it has something that makes it special: it has a giant genome.”

In 2023, Pellicer and his colleague Oriane Hidalgo traveled to New Caledonia to collect samples of Tmesipteris, which they subsequently analyzed to estimate the size of its genome. This process required isolating the nuclei of thousands of cells, staining them with a fluorescent dye and then measuring how much dye had bound to the DNA within each nucleus. The more dye, the larger the genome.

“To calculate the size, we use internal standards [and] cultivated plants such as peas, rice or tomatoes, which are very well known,” Pellicer explains. In this case, the standard they used was garlic, which is the cultivated plant with the highest number of base pairs: 34 gigabases (one Gbp is equivalent to one billion base pairs). By comparison, the human genome contains about 3.2 Gbp spread across 23 chromosomes. And, when stretched, the length of DNA in each cell of garlic barely exceeds six feet.

Tmesipteris is unique and fascinating… its ancestors evolved about 350 million years ago, long before dinosaurs walked the Earth. [The fern] is distinguished by its primarily epiphytic habit (growing primarily on tree trunks and branches),” Pellicer notes. In a video interview with EL PAÍS, he recalls that, when they encrypted the Paris japonica genome several years ago, they believed that they had reached the limit… that there could be no other organisms that were larger in genetic terms.

“The hypothesis that perhaps there was no greater diversity was based on the fact that there would be no possibility of biologically maintaining a genome beyond 150 gigabases. We were wrong,” he admits.

Jaume Pellicer
Researcher Jaume Pellicer is pictured in the foreground. Behind him is Pol Fernández. Both are photographed by Oriane Hidalgo, during the search for the fern. All three are scientists from the Botanical Institute of Barcelona.Oriane Hidalgo

Such a large genome has its costs. It requires more energy resources when replicating DNA, or dividing cells. In larger cells, the integrity of the physical structure requires greater energy input. It’s more costly at a metabolic level. “That’s why we think it makes them less advantageous when it comes to adapting to constant changes, both [related to] climate and pollution,” Pellicer explains. The ferns have reproductive cycles that are much slower, because the cell cycle is much longer than in a plant with a small genome, while the demand for nutrients required to build nucleic acids is much greater. “We believe that, throughout evolution, they’ve slowly been [worn down],” Pellicer adds. In fact, he concludes, “giant genomes are the exception; despite the extraordinary diversity of genomic sizes that exist, the vast majority of plants have small or very small genomes, which is why we’re so interested in [large ones].”

Biologists call this the C-value paradox: the size of the genome doesn’t correlate with the complexity of the organism. This fact has puzzled them for decades. “It was thought that the more complex an organism was, the larger the size of its genome must be. Now we know that this isn’t the case,” Pellicer says. “And this is mainly due to the fact that most of the genome is composed of repetitive DNA sequences, which has been called ‘junk DNA,’ because it was believed that it had no function.”

When looked at with human eyes, none of the 10 organisms with the largest genomes could be seen as particularly complex living beings. In addition to T. oblanceolata and P. japonica, another fern from the genus of the first mistletoe — a European species — appears on the list, with 100.84 Gbp. In the top 10, there are only four animals, such as the marbled lungfish (129.90 Gbp) or the Neuse River Water Dog (117.47 Gbp), related to salamanders.

Pol Fernández, also a researcher from the Botanical Institute of Barcelona and a co-author of the study, gives some reasons for the order of the list of the largest genomes: “The majority are plants and, at the genomic level, they’re capable of being viable by doing many hybridization processes. When there are such gigantic genomes, it’s because there have been many duplications of genome amplifications of repeated elements. We know that this is much more frequent in plant species than in animals.”

To date, scientists around the world have estimated the genome sizes of more than 20,000 eukaryotic organisms, in the process revealing a wide range of genome sizes. These, in turn, have been found to have a profound impact, not only on their anatomy — as larger genomes need larger cells to house them and take longer to replicate — but also on how they function, evolve and where and how they live.

“Who would have thought that this small, unassuming plant, which most people would probably walk past without noticing, could hold a world record for genome size?” says Ilia Leitch, from the Royal Botanic Gardens in Kew. She adds: “Compared to other organisms, plants are incredibly DNA diverse. This should lead us to think about their intrinsic value in the broader picture of global biodiversity.”

“This discovery,” she concludes, “also raises many new and exciting questions about the limits of what is biologically possible. We hope to solve these mysteries one day.”

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