The tree of life has these things. A fern, which may seem like a not too complex organism, turns out to be the living being with the largest genome. It grows in New Caledonia (an island in Oceania under French sovereignty) and if the 160,750,000,000 base pairs of its DNA could be placed on top of each other, it would rise to 100 meters, 50 times higher than human DNA. The discovery raises new questions about how much genetic material can be stored in cells and about the lack of correlation between complexity and genetics.
On the fallen trunks of the forests of New Caledonia grows the Tmesipteris oblancolata, a fern that belongs to a genus of vascular plants of which there are barely fifteen species. At least two of her first cousins were known to have giant genomes. But until now, the organism that contained DNA with the greatest number of base pairs was another plant, the Paris Japonica. Now several researchers who characterized the genetic length of the P. japonicahave discovered that the genome of the T. oblancolata It is 7% higher.
In a new study published in the scientific journal iScience, researchers from the Royal Botanic Garden of Kew (United Kingdom) and the Institut Botànic de Barcelona (IBB-CSIC) present the results of their work with this fern, demonstrating that it has the largest amount of DNA stored in the nucleus of its cells. any living eukaryotic organism on the planet. If it were a ball to unravel, the T. oblancolata would extend between 105 and 106 meters. “It is not an iconic plant, it does not have flowers, nor is it striking. In fact, it is a weed that, if you are not looking for it, you would trample it without realizing it,” says Jaume Pellicer, researcher at the Institut Botànic. ”It doesn’t even look like a fern, 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,” he recalls.
“It is not an iconic plant, it does not have flowers, nor is it striking. In fact, it is a weed that, if you are not looking for it, you would trample it without realizing it”
Jaume Pellicer, researcher at the Botanical Institute of Barcelona (IBB-CSIC)
In 2023, Pellicer and his IBB colleague Oriane Hidalgo traveled to New Caledonia to collect samples of Tmesipteris, which they then analyzed to estimate the size of their genomes. In its short version, the process requires 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, cultivated plants such as pea, rice or tomato, which are very well known,” says Pellicer. In this case, the standard they used was garlic, which is the cultivated plant with the highest number of base pairs, in their case, 34 gigabases (Gbp; each one is 1,000 million base pairs). bases). By comparison, the human genome contains about 3.2 Gbp spread across 23 chromosomes, and when stretched, the length of DNA in each cell barely exceeds two meters.
“Tmesipteris is a unique and fascinating small genus of ferns, whose ancestors evolved about 350 million years ago, long before dinosaurs walked the Earth, and is distinguished by its primarily epiphytic habit [que crece principalmente en troncos y ramas de árboles]”says Pellicer. In an interview by video call, he acknowledges that when they encrypted the genome of the P. japonica Years ago, they believed they had reached the limit, that there could be no other organism 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 adds.
Such a large genome has its costs. It requires more energy resources when replicating DNA, dividing cells. In larger cells, the integrity of the physical structure requires a greater energy input. It is 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 climate and pollution,” explains Pellicer. They have reproductive cycles that are much slower because the cell cycle is much longer than in a plant with a small genome. And the demand for nutrients required to build nucleic acids is much greater. “So we believe that, throughout evolution, they have been eliminated,” adds Pellicer. 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 are so interested in them.”
Biologists know it as the C-value paradox: the size of the genome does not correlate with the complexity of the organism, and this 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 is not the case,” comments the IBB researcher. “And it is mainly due to the fact that most of the genome is made up of repetitive DNA sequences, which has been called junk DNA because it was believed that it had no function,” he adds.
None of the ten organisms with the largest genome could be, seen with human eyes, a very complex living being. Besides the T. oblancolata and the P. japonica, another fern of the genus of the first appears on the list and the European mistletoe, which closes the list, with 100.84 Gbp. In this top ten 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, co-author of the study and also from the IBB, gives some reasons for the order of the list of the largest genomes: “The majority are plants and at the genomic level they are capable of being viable by doing many hybridization processes. When there are such gigantic genomes it is because there have been many duplications of genomes, amplifications of repeated elements and this in plants we know is much more frequent and gives viable species more frequently 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 across the tree of life. 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?” concludes Ilia Leitch, of Royal Botanic Gardens, Kew, in a note. She adds: “Compared to other organisms, plants are incredibly DNA diverse, and that should lead us to think about their intrinsic value in the broader picture of global biodiversity. “This discovery also raises many new and exciting questions about the upper limits of what is biologically possibl
e, and we hope to solve these mysteries one day.”
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