Last year, Jaume Pellicer led a team of other scientists into a forest on Grande Terre, an island east of Australia. They were looking for a fern called Tmesipteris oblanceolata. Standing only a few inches tall, it wasn’t easy to find on the forest floor.
“It’s not noticeable,” said Dr. Pellicer, who works at the Botanical Institute of Barcelona in Spain. “You’d probably step on it and not even know it.”
Scientists eventually managed to spot the indescribable fern. When Dr. Pellicer and his colleagues studied it in the laboratory, they discovered that it held an extraordinary secret. Tmesipteris oblanceolata has the largest known genome on Earth. As researchers describe in a study published Friday, fern cells contain more than 50 times more DNA than our own.
If you find it strange that such a humble plant has such a gigantic genome, scientists do too. The puzzle emerged in the 1950s, when biologists discovered that the double helix of DNA encodes genes. Each gene is made up of a series of genetic letters, and our cells read those letters to make the corresponding proteins.
Scientists hypothesized that humans and other complex species must make many different proteins and therefore have larger genomes. But when they weighed the DNA of different animals, they found they were terribly wrong. Frogs, salamanders and lungfish had much larger genomes than humans.
It turns out that genomes are a lot weirder than scientists expected. We carry about 20,000 protein-coding genes, for example, but they make up only 1.5 percent of the 3 billion letter pairs in our genome.
Another about nine percent consists of stretches of DNA that don’t code for proteins but still perform important jobs. Some of them, for example, act as switches to turn neighboring genes on and off.
The other 90 percent of the human genome has no known function. Some scientists have a fond nickname for this large 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 spread across a giant genome that has a total of 40 billion pairs of DNA letters—13 times more DNA than our own genome contains .
In the early 2000s, when Dr. Trained as a botanist, Pellicer was intrigued to learn that some plant lineages also have massive genomes. The onion, for example, has a genome five times larger than ours.
In 2010, when Dr. Pellicer began working at Kew Gardens in London, where he had the opportunity to study a family of plants known as the bunch flowers, which were known to have large genomes. He spent months shredding leaves with razor blades, isolating cells from dozens of species and weighing their DNA.
When he measured the genome of a plant called Paris japonica, which grows in the mountains near Nagano, Japan, he was shocked by the result. The common flower had a genome containing 148 billion pairs of letters – a world record.
In the years that followed, colleagues sent him fresh samples of ferns from Australia and New Zealand to dissect. He found that those plants, too, had massive genomes, though not as large as that of Paris japonica.
Dr. Pellicer knew that related fern species grew on some Pacific islands. In 2016, he began making plans for an expedition to Grande Terre, part of the archipelago known as New Caledonia.
Only in 2023 did he finally reach the island. 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 contained about 160 billion pairs of DNA letters. Thirteen years after Dr. Pellicer had discovered a record-breaking genome, his graduate student was also experiencing the thrill of breaking the record.
There are two main ways in which genomes expand over evolutionary time. Many species carry virus-like stretches of DNA. As they make new copies of their genomes, they sometimes accidentally make an extra 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 an extra genome can arise is when two closely related species mate. 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 large amount of genetic material in Tmesipteris oblanceolata. But they don’t know why this humble fern ended up with a record-breaking genome, while other species – like us – have far less DNA.
It is possible for most species to gradually accumulate DNA in their genome without suffering any damage. “A lot of biology is ‘why not?’ rather than ‘why?'” said Julie Blommaert, a genomicist at the New Zealand Institute of Plant and Food Research, who was not involved in the new study.
Eventually, however, genomes can become so large that they become a burden. The cells may have to expand to accommodate all the extra DNA. They also need more time and more nutrients to make new copies of their giant genomes. An organism with a large genome may lose to a rival with a smaller genome. So mutations that remove unnecessary DNA may be favored by evolution.
It is possible that animals and plants can evolve 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 removed because they’re not effective,” said Dr. Pellicer.
Even in the most hospitable home, genomes cannot grow to infinite sizes. In fact, Dr. Pellicer suspects that Tmesipteris oblanceolata may have almost reached the physical limit of a genome. “I believe we are close,” he said.
Others are not so sure.
“I don’t know if we’ve hit an 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 genome sizes in only 12,000 plant species, leaving another 400,000 species to study. “What we have estimates for is a drop in the bucket,” she said.