Last year, Jaume Pellicer led a team of fellow scientists into the forest on Grande Terre, an island east of Australia. They were looking for a fern called Tmesipteris oblanceolata. He stood only a few centimeters tall and was not easy to find on the forest floor.
“It won’t catch your eye,” said Dr. Pellicer, who works at the Botanical Institute in Barcelona, ​​Spain. “You’d probably step on it and not even realize it.”
Scientists finally managed to spot an indescribable fern. When Dr. Pellicer and his colleagues studied in the laboratory, they discovered that it was hiding an extraordinary secret. Tmesipteris oblanceolata has the largest known genome on Earth. Fern cells contain more than 50 times more DNA than ours, researchers described in a study published Friday.
If you find it strange that such a humble plant has such a gigantic genome, so do scientists. The mystery emerged in the 1950s, when biologists discovered that the double helix of DNA encodes genes. Each gene consists of a series of genetic letters, and our cells read these letters to make the corresponding proteins.
Scientists hypothesized that humans and other complex species must make lots of different proteins and therefore have larger genomes. But when they weighed the DNA in different animals, they found they were very wrong. Frogs, salamanders, and lungfish had much larger genomes than humans.
It turns out that genomes are a lot weirder than scientists expected. For example, we carry about 20,000 protein-coding genes, but they make up only 1.5 percent of the 3 billion letter pairs in our genome.
Another nine percent or so are stretches of DNA that don’t code for proteins but still perform important tasks. For example, some of them act as switches to turn neighboring genes on and off.
The remaining 90 percent of the human genome has no known function. Some scientists have an affectionate nickname for this vast amount of mysterious DNA: junk.
Some species have little junk DNA, while others have staggering amounts. The African lungfish, for example, has about the same number of protein-coding genes as we do, but they are scattered across a giant genome that has a total of 40 billion DNA letter pairs—13 times more DNA than our own genome contains.
At the beginning of the 21st century, when Dr. A trained botanist, Pellicer was intrigued by the discovery that several plant lineages also had massive genomes. For example, onions have a genome five times larger than ours.
In 2010, when Dr. Pellicer began working at Kew Gardens in London, given the opportunity to study a family of plants known as bunchflowers, which were known to have large genomes. He spent months shredding leaves with a razor blade, isolating cells from dozens of species and weighing their DNA.
When he weighed the genome of a plant called Paris japonica, which grows in the mountains near Nagano, Japan, the result shocked him. A common flower had a genome containing 148 billion letter pairs – a world record.
In the following years, colleagues sent him fresh samples of ferns from Australia and New Zealand to chop. He found that these plants also had massive genomes, although not as large as those of the Paris Japonica.
Dr. Pellicer knew that related species of ferns grew on several Pacific islands. In 2016, he began making plans for an expedition to Grande Terre, part of the archipelago known as New Caledonia.
He finally reached the island in 2023. He collected a number of species together with a team that included colleagues from Kew, his graduate student Pol Fernández and local plant experts.
In Barcelona, ​​Mr. Fernández was surprised to discover that the genome of Tmesipteris oblanceolata contains about 160 billion pairs of DNA letters. Thirteen years after Dr. Pellicer discovered the record-breaking genome, his graduate student was also experiencing the thrill of breaking the record.
There are two main ways that genomes expand during evolution. Many species carry stretches of DNA similar to viruses. When they make new copies of their genomes, they sometimes accidentally make another copy of that viral stretch. Over many generations, a species can accumulate thousands of new copies, causing its genome to swell.
It is also possible for a species to suddenly end up with two genomes instead of one. One way in which an extra genome can arise is through the mating of two closely related species. Their hybrid offspring can inherit complete sets of DNA from both parents.
Dr. Pellicer and his colleagues suspect that a combination of virus-like DNA and duplicated genomes is responsible for the vast amount of genetic material in Tmesipteris oblanceolata. But they don’t know why this humble fern ended up with the record-breaking genome, while other species – like us – have much less DNA.
It is possible that most species gradually accumulate DNA in their genomes without suffering any harm. “A big part of biology is ‘why not?’ rather than ‘why?'” said Julie Blommaert, a genomicist at the New Zealand Plant and Food Research Institute who was not involved in the new study.
Eventually, however, genomes can grow so large that they become a burden. Cells may need to expand to accommodate any excess DNA. They also need more time and more nutrients to make new copies of their giant genomes. An organism with an oversized genome can lose to an opponent with a smaller one. So mutations that sever unneeded DNA can be favored by evolution.
It is possible that animals and plants can develop truly giant genomes only in special environments, such as stable climates, where there is little competition. “Maybe that’s why they’re so rare — they’re being harvested because they’re not effective,” said Dr. Pellicer.
Even in the friendliest home, genomes cannot grow to infinite sizes. In fact, Dr. Pellicer suspected that Tmesipteris oblanceolata may have almost reached the physical limit of the genome. “I believe we are close,” he said.
Others aren’t so sure.
“I don’t know if we’ve hit the upper limit yet,” said Brittany Sutherland, a botanist at George Mason University who was not involved in the study. She noted that botanists have measured the genome sizes of only 12,000 plant species, leaving 400,000 more to study. “We have estimates that it’s a drop,” she said.