Tmesipteris oblanceolata is an obscure species of fork fern found in New Caledonia, a French territory in the South Pacific. At only 4 to 6 inches tall, the humble plant is in one respect the most remarkable living thing in the world.
“You would walk over it. You could even step on it without knowing it,” says Ilia Leitch, plant evolutionary biologist and senior research leader at Britain's Royal Botanic Gardens, Kew. “But it holds this great secret.”
Recently, T. oblanceolata came the Guinness Book of World Records after a team of scientists determined that the wispy fern has the largest known genome of any living organism. Packed into the nucleus of each of its cells are 160.45 billion base pairs – 160.45 billion rungs on the rotating double-helix ladder that makes up the plant's DNA.
T. oblanceolata has more genes than the mighty California redwood (Sequoia sempervirens) or the enormous blue whale (Balaenoptera muscle). It has 50 times more DNA than Homo sapiens, the species that discovered what DNA is in the first place. The findings were published in the journal iScience.
“We were absolutely amazed when we discovered how big this genome was,” says botanist Jaume Pellicer of the Institut Botànic de Barcelona in Spain, co-author of the study with Leitch. “We already knew about the existence of giant genomes in the genus, but didn't expect them to be in Tmesipteris oblanceolata would beat all previous records.”
A genome contains all the information that cells need to control the growth and development of the organism. But life doesn't provide instructions in the neat, more-steps-is-more-complexity way of Ikea or Lego assembly manuals – hence little ferns with gigantic genetic codes.
You might be able to move on T. oblanceolata “Without knowing it,” said a plant evolutionary biologist. (Photos by Pol Fernandez and Oriane Hidalgo)
Measuring genome size is “not a way to measure the complexity or coding capacity of the genome,” says Elliot Meyerowitz, a Caltech biologist who was not involved in the study.
Only a tiny piece of genetic material that most plant and animal cells carry contains actual instructions for making the building blocks that make up living things. Less than 2% of the human genome actually codes for proteins. For the fork fern, the research team estimates that less than 1% of its genome does so.
The rest is known as non-coding DNA. Understanding what that non-coding genetic material does and why cells carry it around are among the biggest questions in evolutionary biology.
Half a century ago, scientists rejected this non-coding stuff as “junk DNA,” a term now considered “a reflection of our own ignorance,” Leitch said.
It's not that it doesn't amount to anything, she said. We just don't understand everything it does yet.
T. oblanceolata ferns grow among tangled branches and fallen leaves.
(Jaume Pellicer)
In recent years, researchers have discovered that manipulating or removing some of these non-coding sequences affects gene expression. This suggests that at least some of this material plays a role in the processes that turn genes “on and off,” “like the conductor of an orchestra, who tells who comes in here and who should be quiet here,” Leitch said.
This intricate choreography of gene expression is how we achieve the incredible diversity within our own species and across the kingdoms of living things.
“Understanding how these genomes function and are structured is the ultimate milestone in this field of research,” Pellicer wrote in an email. “But for now, it's like trying to read a book of instructions without even knowing where page one is!”
T. oblanceolata displaces the previous genome record holdera flowering plant of modest size called Paris japonica which has 149 billion base pairs. While there may be something else that has a greater genetic impact, botanists believe these plants are at the high end of the amount of DNA a living thing can have.
A researcher searches for fork ferns in New Caledonia.
(Oriane Hidalgo)
“If it's not the largest, it's very close,” Leitch said of the fork fern's genome. “There are so many consequences of having so much DNA that I think we are at the limit of what biology can handle.”
An organism must divide its cells in order to grow, and before it can do that, it must make a copy of all the DNA in its cells. Copying a colossal genome is a major investment in time, energy and nutrients, Leitch points out. For plants, larger genomes are associated with slower growth and less efficient photosynthesis.
As a result, organisms with huge genomes are often found in stable environments without much competition, Leitch said. That's true T. oblanceolatagrowing slowly Paris japonica and the marbled lungfishholder of the largest genome in the animal kingdom (nearly 130 billion base pairs).
Unfortunately for that T. oblanceolataIn a rapidly changing climate, stable conditions are increasingly difficult to achieve.
“As long as they're stable, as long as things don't change, selection isn't going to wipe them out, so to speak,” Leitch said. “I would predict that if the environment changed, they wouldn't be in a good position.”
