Last year, Jaume Pellicer led a team of fellow scientists to a forest on Grande Terre, an island east of Australia. They were looking for a fern called Tmesipteris oblanceolata. Being only a few centimeters in size, it was not easy to find on the forest floor.
“It’s not noticeable,” says Dr. Pellicer, who works at the Botanical Institute of Barcelona in Spain. “You would probably step on it and not even realize it.”
The scientists eventually managed to discover the inconspicuous fern. When Dr. Pellicer and his colleagues studied it in the laboratory, they discovered that it contained an extraordinary secret. Tmesipteris oblanceolata has the largest known genome on Earth. As the researchers describe in a study published Friday, the fern’s cells contain more than 50 times as much DNA as ours.
If you think it’s strange that such a modest plant has such a gigantic genome, scientists think so too. The mystery arose in the 1950s, when biologists discovered that the double helix of DNA codes for genes. Each gene is made up of a series of genetic letters, and our cells read those letters to make corresponding proteins.
Scientists assumed that humans and other complex species need to make many different proteins and therefore have larger genomes. But when they weighed the DNA of several animals, they found they were completely wrong. Frogs, salamanders and lungfish had much larger genomes than humans.
It turns out that genomes are much stranger 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 consists of pieces of DNA that do not 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. For example, the African lungfish has about the same number of protein-coding genes as we do, but they are spread out in a huge genome that contains a total of 40 billion pairs of DNA letters – 13 times as much DNA as our own genome contains.
When Dr. Pellicer trained as a botanist in the early 2000s, he was intrigued to learn that a few plant lines also have enormous genomes. Onions, for example, have a genome five times larger than ours.
In 2010, when Dr. Pellicer started working at Kew Gardens in London, he had the opportunity to study a family of plants known as woodland flowers, which were known to have large genomes. He spent months chopping 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, he was shocked by the result. The common flower had a genome with 148 billion letter pairs – a world record.
In the years that followed, colleagues sent him fresh samples of ferns from Australia and New Zealand to chop up. He discovered that these plants also had enormous genomes, although not as large as that of Paris japonica.
Dr. Pellicer knew that related fern species grew on some islands in the Pacific Ocean. In 2016, he began making plans for an expedition to Grande Terre, part of the archipelago known as New Caledonia.
It wasn’t until 2023 that he finally reached 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.
Back in Barcelona, Mr Fernández was shocked to discover that the genome of Tmesipteris oblanceolata contained around 160 billion pairs of DNA letters. Thirteen years after Dr. Pellicer had discovered a record-breaking genome, his graduate student also experienced the thrill of breaking the record.
There are two main ways in which genomes expand over evolutionary time. Many species carry virus-like pieces of DNA. As they make new copies of their genomes, they sometimes accidentally make an extra copy of that viral piece. Over many generations, a species can accumulate thousands of new copies, expanding its genome.
It is 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. 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 enormous 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 so much less DNA.
It is possible that most species gradually accumulate DNA in their genomes without suffering any damage. “A lot of biology is ‘why not?’ instead of ‘why?’” says Julie Blommaert, a genomicist at the New Zealand Institute for Plant and Food Research, 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 all the extra DNA. They also need more time and more nutrients to make new copies of their giant genomes. An organism with a genome that is too large may lose out to a rival with a smaller genome. Thus, mutations that remove unnecessary DNA may be favored by evolution.
It is possible that animals and plants can develop truly gigantic genomes only in special environments, such as in stable climates where there is little competition. “Maybe that’s why they’re so rare: They’re being snatched away because they’re not efficient,” said Dr. Pellicer.
Even in the most hospitable home, genomes cannot grow infinitely large. Dr. Pellicer even suspects that Tmesipteris oblanceolata has almost reached the physical limit of a genome. “I believe we’re close,” he said.
Others aren’t so sure.
“I don’t know if we’ve reached 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 the genome sizes of only 12,000 plant species, leaving 400,000 more species to be studied. “What we have estimates for is a drop in the bucket,” she said.