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Last year, Jaume Pellicer led a team of fellow scientists into the forests of Grande Terre, an island east of Australia, in search of a fern called Tmesipteris oblanceolata. At just a few inches tall, the fern was not easy to find on the forest floor.
“They’re inconspicuous,” said Dr Pellicer, of the Botanical Institute of Barcelona in Spain. “You might step on them and not even notice.”
Scientists were finally able to find this unremarkable fern. When Dr. Pellicer and his colleagues studied it in the lab, they discovered that it was hiding a surprising secret: Tmesipteris oblanceolata has the largest genome known on Earth. As the researchers note in a study published on Friday, the fern’s cells contain more than 50 times as much DNA as humans.
If you’re wondering how such an unassuming plant could have such a huge genome, scientists agree. The mystery emerged in the 1950s, when biologists discovered that the double helix of DNA codes for genes. Each gene is made up of a string of genetic letters, and our cells read those letters to make the corresponding protein.
Scientists had assumed that humans and other complex species had large genomes because they make many different proteins, but when they measured the DNA of a variety of animals, they found that this was a big mistake: Frogs, salamanders, and lungfish have genomes that are much larger than ours.
The genome turns out to be a lot stranger than scientists expected: for example, we have about 20,000 protein-coding genes, but they account for just 1.5 percent of the 3 billion letter pairs in our genome.
The remaining 9 percent or so is made up of regions of DNA that don’t code for proteins but have important roles — some of them act like switches that turn nearby genes on or off, for example.
The remaining 90 percent of the human genome has no known function — some scientists nickname this vast amount of mysterious DNA “junk.”
Some species have very little junk DNA, while others have a surprisingly large amount: the African lungfish, for example, has roughly the same number of protein-coding genes as humans, but they are scattered throughout a gigantic genome of 40 billion pairs of DNA letters, 13 times the amount of DNA contained in the human genome.
When Dr. Pellicer was training as a botanist in the early 2000s, he was intrigued to learn that some plant lineages also have giant genomes: the onion genome, for example, is five times the size of a human’s.
When Dr Pellicer began working at Kew Gardens in London in 2010, he had the opportunity to study a family of plants called bunchflowers, known for their large genomes. He spent months shredding leaves with a razor blade, isolating cells from dozens of different species and weighing their DNA.
When he measured the genome of a plant called Paris japonica, which grows in the mountains near Nagano Prefecture, Japan, he was stunned by the results: the genome of this common flower contained 148 billion letter pairs, a world record.
Over the next few years, his colleagues sent him fresh fern samples from Australia and New Zealand to chop up, and he found that those plants also had giant genomes, though not as large as those of Paris japonica.
Dr Pellicer knew that similar fern species live on several islands in the Pacific Ocean, so in 2016 he began planning an expedition to Grande Terre, part of the archipelago known as New Caledonia.
It wasn’t until 2023 that he finally reached the island, where he and a team including a colleague from Kew Gardens, graduate student Pol Fernandez, and local plant experts, collected a number of plants.
Returning to Barcelona, Fernández was surprised to discover that the genome of Tmesipteris oblanceolata contains approximately 160 billion pairs of DNA letters. Thirteen years after Dr. Pellicer discovered his record-breaking genome, his graduate students were also experiencing the excitement of breaking a record.
There are two main ways genomes expand during evolution: Many species have virus-like DNA regions. When making new copies of their genome, they may accidentally make additional copies of the viral regions. Over many generations, the species accumulates thousands of new copies, and its genome expands.
It’s also possible for a species to suddenly have two genomes instead of one. One way an extra genome can arise is when two closely related species mate. The offspring of that hybrid can inherit a complete set of DNA from both parents.
Dr. Pellicer and his colleagues believe that a combination of virus-like DNA and duplicated genomes is responsible for Tmesipteris oblanceolata’s vast amount of genetic material, but they don’t know why this humble fern ended up with a record-breaking genome when other species (like us humans) have much less DNA.
Most species may be accumulating DNA in their genomes gradually, without any harm. “Most questions in biology are not ‘why?’ but ‘why not?'” says Julie Blommaert, a genomics scientist at the New Zealand Plant Foods Institute, who was not involved in the study.
But eventually the genome may become too large and burdensome. Cells may have to expand to accommodate all the extra DNA, and it may take more time and nutrients to make new copies of the giant genome. Organisms with oversized genomes may be outcompeted by rivals with smaller genomes. So evolution may favor mutations that cut out unnecessary DNA.
It may be that only in special circumstances, such as a stable climate with little competition, can animals and plants evolve truly large genomes. “Maybe that’s why they’re so rare – they get cut off because they’re inefficient,” Dr Pellicer said.
Even in the most favorable environments, genomes cannot grow indefinitely. In fact, Pellicer thinks that the Tmesipteris oblanceolata genome may be close to its physical limits. “I think we’re pretty close,” he says.
Others aren’t so sure.
“We don’t know if we’ve hit the limit yet,” says Brittany Sutherland, a botanist at George Mason University who was not involved in the study. She points out that botanists have measured the genome sizes of only 12,000 plants, leaving another 400,000 species to study. “We’re only at the very tip of the iceberg,” she says.
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