Last year, Jaume Pellicer led a team of fellow scientists into a forest on Grande Terre, an island east of Australia. They were looking for a fern called Tmesipteris obblancolata. Measuring only a few centimeters tall, it was not easy to find on the forest floor.
“It doesn’t attract attention,” 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 the nondescript fern. When Dr. Pellicer and his colleagues studied it in the laboratory, they discovered that it contained an extraordinary secret. Tmesipteris obblancolata has the largest known genome on Earth. As researchers described in a study published Friday, the fern’s cells contain more than 50 times more DNA than ours.
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 codes for genes. Each gene is made up of a series of genetic letters, and our cells read these letters to make the corresponding proteins.
Scientists assumed 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 completely wrong. Frogs, salamanders and lungfish had genomes much larger than those of humans.
It turns out that genomes are much stranger than scientists had imagined. For example, we have about 20,000 protein-coding genes, but they make up only 1.5% of the 3 billion letter pairs in our genome.
About nine percent are made up of segments of DNA that don’t code for proteins but still perform important tasks. Some of them, for example, act as switches to turn neighboring genes on and off.
The remaining 90 percent of the human genome has no known function. Some scientists give this large amount of mysterious DNA an affectionate nickname: junk.
Some species have little junk DNA, while others have staggering amounts. The African lung, for example, has roughly the same number of protein-coding genes as we do, but they are scattered throughout a giant genome that totals 40 billion DNA letter pairs, or 13 times more DNA than our own genome.
In the early 2000s, when Dr. Pellicer trained as a botanist, he was intrigued to learn that a few lineages of plants also had massive 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 chance to study a family of plants known as bouquet flowers, known for having 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 ordinary flower had a genome containing 148 billion letter pairs – a world record.
In the years that followed, colleagues sent him new samples of ferns from Australia and New Zealand to chop. He found that these plants also had massive genomes, but not as large as that of Paris japonica.
Dr. Pellicer knew that related species of ferns grew on a few Pacific islands. In 2016, he began planning an expedition to Grande Terre, part of the archipelago known as New Caledonia.
It was not until 2023 that he finally arrived on the island. He collected a number of species with a team including colleagues from Kew, his graduate student Pol Fernández and local plant experts.
Back in Barcelona, Fernández was surprised to discover that the genome of Tmesipteris obblancolata contained around 160 billion pairs of DNA letters. Thirteen years after Dr. Pellicer discovered a record genome, his graduate student also experienced the thrill of breaking the record.
There are two main ways in which genomes develop during evolution. Many species carry virus-like DNA sequences. When they create new copies of their genome, they sometimes accidentally create an extra copy of that viral sequence. Over several generations, a species can accumulate thousands of new copies, causing its genome to bloat.
It’s also possible that a species could suddenly find itself with two genomes instead of one. One way to create an additional genome could be by mating 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 viral DNA and duplicated genomes is responsible for the enormous amount of genetic material in Tmesipteris obblancolata. But they don’t know why this humble fern ended up with a record genome when other species – like us – have much less DNA.
It is possible that most species gradually accumulate DNA in their genome without suffering any damage. “Much of biology is based on the question ‘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.
However, eventually genomes could become so large that they become a burden. The cells may need to grow to accommodate all the extra DNA. They also need more time and more nutrients to create new copies of their giant genome. An organism with an oversized genome may lose to a rival with a smaller genome. Thus, mutations that remove unnecessary DNA could be favored by evolution.
It is possible that animals and plants can develop truly giant 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 snatched up because they’re not effective,” Dr. Pellicer said.
Even in the most welcoming home, genomes cannot grow to infinite size. In fact, Dr. Pellicer suspects that Tmesipteris obblancolata may have almost reached the physical limit of a genome. “I think 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 another 400,000 species to study. “What we have estimates for is just a drop in the ocean,” she said.