Pol Fernandez and Mato
The New Caledonian fork fern, Tmesipteris oblanceolata, has the largest known genome of any organism.
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Though it may not be obvious, some plants have more DNA than others. And perhaps more mysteriously, some plants have more DNA than most animals, including humans. Now, scientists have uncovered an even more extreme example of this mega-genomic phenomenon, but its existence remains a mystery.
A new study finds that an inconspicuous fern found on the South Pacific island of New Caledonia contains the largest genome of any living organism known to man. The unusual plant contains 160 billion base pairs, the linking units that make up a strand of DNA, according to the study. By comparison, the human genome has just 3 billion.
“Small, humble objects can hide some of the most amazing secrets,” said evolutionary biologist Jaume Pellicer, a researcher at the Barcelona Botanical Institute in Spain and corresponding author of the study published May 31 in the journal Cell.
Large genomes are also rare: Of the roughly 12,000 documented plant genomes, all but half a dozen are at least an order of magnitude smaller than the genome of the fern described in the paper, called Tmesipteris oblanceolata, Pellicer says.
“The limits of biology are what we don't yet understand,” Pellicer says. And he should know that: he's continuing to figure it out.
Prior to this study, the largest genome among fungi, plants, or animals was the floral genome of A. paris japonica, with 149 billion base pairs (gigabase pairs, Gbp).
Pellicer was part of the team that published the discovery of P. japonica in a September 2010 study, and co-authored a paper in May 2017 that hypothesized why the genome size limit is 150 Gbp. Now, he's proven himself wrong.
“To be honest, our main interest in this project is not to find a record of plant genome size,” Pellicer says. “We want to explore different instances of genome gigantism.”
Orian Hidalgo
Evolutionary biologist Jaume Pellicer, corresponding author of the new study, will be studying fern species that grow on trees on Grande Terre, the main island of the French archipelago of New Caledonia, in 2023.
Ferns are known for having highly repetitive genomes, and after analyzing related samples from New Zealand and Tasmania, Pellicer thought the New Caledonian fork fern might be an interesting subject to study.
In May 2023, he and his team will collect samples of the tiny plant in the wild and bring them back to the lab to compare the fern's genome with species of known genome size.
The fork fern's enormous genome may contain more than 50 times as much DNA as a human's, but that doesn't make the fern more complex or have more genes. Pellicer estimates that only about 1 percent of the genome is made up of protein-coding genes. The rest is nonfunctional, repetitive sequences that have long been labeled “junk DNA,” but scientists now know that it's not actually junk. Much of this meaningless information in genes can affect how genes function.
It's still unclear how ferns accumulated so many base pairs, but Pellicer says it could be due to a combination of the prevalence of more than one set of chromosomes – polyploidy – which is common in plants but not animals, and the accumulation of non-coding DNA sequences that can self-replicate and move around in the genome.
“The more interesting question that this study doesn't essentially answer is why[genomes]get so large, and how ferns in particular are able to deal with the consequences of having such unnecessarily large genomes,” says fern expert Eric Schuettpeltz, a research botanist and curator at the Smithsonian Institution's Department of Botany, who did not collaborate on the study. “Ferns seem to be really bad at getting rid of duplicated DNA and extra chromosomes.”
Pol Fernandez and Mato
The fork fern's enormous genome may contain more than 50 times the amount of DNA found in humans: the plant contains 160 billion base pairs, the linking units that make up a DNA strand.
Ultimately, having a large genome is not an advantage. Genome gigantism, or genome obesity, is extremely inefficient, experts explain. It takes a lot of energy and nutrients to make that much DNA. Most plants have much smaller genomes, which allows them to reproduce faster, produce more offspring, and adapt to their environments more quickly. Having to copy so much genetic material every time a cell divides is “just insane,” says Schuettpertz.
It's puzzling how a plant with such a tricky genome can survive, says David Baum, a professor of botany at the University of Wisconsin-Madison, who was not involved in the study.
Pol Fernandez and Mato
In 2023, researchers will travel to Grande Terre to study one of the populations of Tmesipteris oblanceolata.
Pellicer and his colleagues believe that genome size may influence a plant's likelihood of extinction. The team is currently focusing on research suggesting that plant species with large genomes are over-represented on the International Union for Conservation of Nature's Red List of Threatened Plants, and analyzing whether genome size can help conservationists identify plants that are potentially under threat.
Are there plants with genomes even larger than the fork fern? Probably.
“I'm not going to play with biological limits anymore,” Pellicer says. “Biological limits are meant to be broken.”
Amanda Shpack is a science and health journalist in New York City.