This Tiny Fern Has the World’s Largest Known Genome

Researchers have discovered the largest genome of any known organism. The new champion, a small, unassuming fern called Tmesipteris oblanceolata, has a whopping 160 billion base pairs in its DNA, the researchers reported last week in the journal iScience.

The fern’s genome is far longer than that of the previous record holder, Paris japonica, a flowering plant that grows in Japan and has 149 billion base pairs in its genetic code.

Unraveled, the DNA from a single T. oblanceolata cell would stretch longer than a football field, while human DNA would only cover about six feet.

Ferns are known for having large genomes, Kenneth Birnbaum, a developmental biologist at New York University who did not contribute to the findings, says to Science’s Ashley Stimpson. “These are classic chromosome hoarders,” he tells the publication.

A genome is the entire set of DNA found inside a cell’s nucleus. DNA takes the shape of a twisted ladder, with its rungs made up of nucleotide base pairs that link one side of the ladder to the other. A genome’s size is determined by the number of base pairs across the genome.

Counterintuitively, the total number of base pairs doesn’t seem to be connected to an organism’s complexity. Humans have about three billion base pairs, and T. oblanceolata has roughly 50 times more. The fern’s genome is 61,000 times the size of the smallest eukaryotic genome.

“A great mystery is the meaning of all of this variation—how do genomes grow and shrink, and what are the evolutionary causes and consequences of these phenomena?” Jonathan Wendel, a botanist at Iowa State University who was not involved in the work, tells the Agence France-Presse (AFP).

Scientists have studied the genomes of about 20,000 species. Among animals, lungfish and a group of aquatic salamanders have some of the largest genomes. The smallest known eukaryotic genome, found in a unicellular fungal parasite, is 2.6 million base pairs. But most of the longest genomes belong to plants.

Studying organisms with large genomes can help researchers better understand how genomic size relates to evolution and ecology. In the new study, the researchers focused on members of the genus Tmesipteris, which includes about 15 species—mainly ferns that grow in Oceania and several Pacific islands. Before this research, scientists had recorded the genome size of only two other species in the genus, which have 73 billion and 147 billion base pairs.

The researchers collected T. oblanceolata from the island nation of New Caledonia. It’s just a few inches tall. “It doesn’t catch the eye,” Jaume Pellicer, a co-author of the study and a botanist at the Botanical Institute of Barcelona in Spain, tells the New York Times’ Carl Zimmer. “You would probably step on it and not even realize it.”

Plant genomes can grow in size when they inherit extra copies of chromosomes or accumulate repeating DNA sequences, writes Science News’ Jake Buehler. Researchers aren’t sure why some plant genomes get so large. Every time a cell of this fern divides, for instance, it would have to copy some 330 feet of genetic material, which seems inefficient.

“It’s hard to imagine,” Birnbaum tells Science, that this repetitive DNA confers “some kind of selective advantage [to the fern]. It’s an incredible burden making DNA for every cell and then packaging it and then protecting it.”

It could be that having more base pairs doesn’t particularly harm or help the fern’s chances at surviving, so it’s just continued to accumulate them, Julie Blommaert, a genomicist at the New Zealand Institute for Plant and Food Research who did not contribute to the findings, tells Nature News’ Max Kozlov.

While scientists have not even come close to measuring the genomes of all species, the study authors suggest it’s unlikely that a much larger genome will be found. T. oblanceolata, they write, is probably very close to the upper limit.

But Brittany Sutherland, a botanist at George Mason University who was not involved in the work, tells the New York Times the opposite. “I don’t know if we have reached an upper boundary yet.”

Will Sullivan | | READ MORE

Will Sullivan is a science writer based in Washington, D.C. His work has appeared in Inside Science and NOVA Next.