The largest genome on the planet was discovered in a small fern | The science

The Tree of Life has it all. The fern, which may not seem like a very complex organism, turns out to be the living creature with the largest genome. It grows in New Caledonia (an island in Oceania under the sovereignty of France), and if the 160,750,000,000 base pairs of its DNA could be stacked on top of each other, its height would rise to 100 meters, which is 50 times higher than that of human DNA. The discovery raises new questions about how much genetic material can be stored in cells and the lack of correlation between complexity and genetics.

Grows on fallen tree trunks in New Caledonia Tmesipteris oblancolata, a fern belonging to the genus of vascular plants, numbering only fifteen species. At least two of their cousins ​​are known to have giant genomes. But until now, the organism that contained the DNA with the most base pairs was another plant, Paris Japan. Now several researchers who have characterized the genetic length P. japonicadiscovered that the genome T. olancolata This is 7% higher.

In a new study published in a scientific journal iScienceResearchers from the Royal Botanic Gardens, Kew (UK) and the Botanical Institute of Barcelona (IBB-CSIC) present their work on this fern, showing that the largest amount of DNA is stored in the nuclei of its cells. .any living eukaryotic organism on the planet. If it was a tangle that needed to be untangled, T. olancolata will extend from 105 to 106 meters. “It’s not an iconic plant, it doesn’t have flowers and it’s not conspicuous. In fact, it is a weed that, if you weren’t looking for it, you would trample without even realizing it,” says Jaume Pellicer, a researcher at the Institute of Botany. “They don’t even look like ferns, they don’t look like the traditional idea we have of them. But he has something special: he has a gigantic genome,” he recalls.

“It’s not an iconic plant, it doesn’t have flowers and it’s not conspicuous. It’s actually a weed that, if you weren’t looking for it, you would trample underfoot without even realizing it.”

Jaume Pellicer, researcher at the Botanical Institute of Barcelona (IBB-CSIC)

In 2023, Pellicer and his IBB colleague Oriana Hidalgo traveled to New Caledonia to collect samples Tmesipteris, which they then analyzed to estimate the size of their genomes. In its short version, this process requires isolating the nuclei of thousands of cells, staining them with a fluorescent dye, and then measuring the amount of dye that has bound to the DNA inside each nucleus: the more dye, the larger the genome. “To calculate the size, we use internal standards, cultivated plants such as peas, rice or tomatoes, which are very well known,” says Pellicer. In this case, the standard they used was garlic, the crop with the largest number of base pairs, in their case 34 gigabases (Gbp; each is 1,000 million base pairs). By comparison, the human genome contains about 3.2 Gb, spread across 23 chromosomes, and when stretched out, the length of DNA in each cell is barely more than two meters.

“Tmesipteris “This is a unique and fascinating small genus of ferns, with ancestors dating back about 350 million years ago, long before dinosaurs walked the Earth, and characterized by being predominantly epiphytic (growing primarily on tree trunks and branches),” says Pellicer. In a video interview, he admits that when the genome was encrypted P. japonica Many years ago they believed that they had reached the limit that there could not be another organism that was genetically larger. “The hypothesis that there might not be more diversity was based on the fact that it would not be possible to biologically maintain a genome beyond 150 gigabases. “We were wrong,” he adds.

Such a large genome comes at a price. More energy resources are required for DNA replication and cell division. In larger cells, the integrity of the physical structure requires more energy. It is more costly on a metabolic level. “That’s why we think this makes them less advantageous when it comes to adapting to constant changes in both climate and pollution,” explains Pellicer. They have much slower reproductive cycles because the cell cycle is much longer than plants with small genomes. And the need for nutrients necessary for the construction of nucleic acids is much higher. “So we think they were eliminated by evolution,” Pellicer adds. In fact, he concludes, “giant genomes are the exception; despite the extraordinary diversity of genome sizes that exist, the vast majority of plants have small or very small genomes, which is why we are so interested in them.”

Biologists know this as the C-value paradox: genome size does not correlate with organismal complexity, and it has puzzled them for decades. “It was believed that the more complex an organism is, the larger the size of its genome should be. We now know that this is not the case,” comments the IBB researcher. “And this is mainly due to the fact that most of the genome consists of repeated DNA sequences, which were called junk DNA because it was thought to have no function,” he adds.

None of the ten organisms with the largest genomes are, when viewed through human eyes, very complex living things. Except T. olancolata And P. japonica, the list includes another fern from the genus of the first ones and the last one is European mistletoe with 100.84 GB. There are only four animals in this top ten, such as the marbled lungfish (129.90 GB) or the Neuse River water dog (117.47 GB), which are related to salamanders.

Paul Fernandez, a co-author of the study and also a spokesman for the IBB, gives some reasons for the order of the list of the largest genomes: “Most of them are plants, and at the genomic level they are able to be viable by performing many hybridization processes. . When such giant genomes exist, it is because there has been a lot of duplication of genomes, amplifications of repetitive elements, and this in plants, as we know, occurs much more often and produces viable species more often than in animals.”

To date, scientists around the world have estimated the genome sizes of more than 20,000 eukaryotic organisms, revealing a wide range of genome sizes across the tree of life. This, in turn, has been found to have a profound impact not only on their anatomy (as larger genomes require larger cells to house them and take longer to replicate), but also on how they function, develop, 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 become the world record holder for genome size?” – concludes Elijah Leitch from the Royal Botanic Gardens, Kew. He adds: “Compared to other organisms, plants are incredibly diverse in terms of DNA, and this should make us think about their intrinsic value in the larger picture of global biodiversity. “This discovery also raises many new and interesting questions about the upper limits of what is biologically possible, and we hope to one day solve these mysteries.”

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