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You wouldn’t know it by looking at them, but some plants have much more DNA than others. And perhaps even more surprising, some have much more than most animals, including humans. Now scientists are uncovering more and more extreme examples of this giant genome phenomenon, although their existence remains a mystery.
New research has identified the largest known genome of any living organism in a humble fern found in New Caledonia, an island chain in the South Pacific Ocean. According to the research, this rare plant contains 160 billion base pairs, which are the paired units that make up DNA strands. By comparison, the human genome consists of only 3 billion.
“Small modest things can hide one of the most surprising secrets within them,” said evolutionary biologist Jaume Pellicer, a researcher at Spain’s Barcelona Botany Institute and corresponding author of the study published May 31 in the journal Cell.
Large genomes are also rare. According to Pellicer, of the 12,000 or so documented plant genomes, all but a half-dozen are at least somewhat smaller than that of the forked fern, Tmesipteris oblanceolata, described in the paper.
“The boundaries of biology are there to be revealed,” Pellicer said. He needs to know; continues to reveal them.
Before this study, the largest known genome of any fungus, plant or animal belonged to the Paris japonica flower, which has 149 billion base pairs (gigabase pairs, or Gbp).
Pellicer was part of the team that uncovered the P. japonica finding in a September 2010 study. He also co-authored a paper in May 2017 that suggested why 150 Gbp might be an upper limit for genome sizes. Now he proved him wrong.
“To be honest, the main goal of this project is not to find a record of genome size in plants,” Pellicer said. “We want to investigate different cases of genomic gigantism.”
Ferns are known to have highly repetitive genomes. After analyzing relevant samples from New Zealand and Tasmania, Pellicer zeroed in on the New Caledonian fork fern as a potentially interesting target for study.
In May 2023, he and his team collected samples of this tiny plant in nature and brought them back to the laboratory; Here they compared the fern’s genome to species of known genome size.
The amount of DNA in the fork fern’s massive genome may be 50 times greater than that of a human, but that doesn’t make the fern more complex or mean it has more genes. Pellicer estimates that only 1% of its genome consists of genes that code for proteins. What remains are non-functional, repetitive sequences that have long been considered “junk DNA”; But scientists now know that it’s not actually garbage. A lot of this genetic nonsense can affect how genes work.
Exactly how the fern accumulates so many base pairs is still an unsolved puzzle. What’s at play, according to Pellicer, may be a combination of common polyploidy (or having more than two sets of chromosomes, which is common in plants but not animals) and the accumulation of non-coding DNA sequences that can replicate themselves. and move around the genome.
“The more interesting question, which the study inherently cannot solve, is why (the genome) has grown so large, and how ferns in particular are able to cope with the consequences of having such an unnecessarily large genome,” Fern said. expert Eric Schuettpelz, a research botanist and curator in the department of botany at the Smithsonian Institution. He did not cooperate with this research. “Ferns seem to be really bad at getting rid of repetitive DNA and extra chromosomes.”
It turns out that super-sized genomes are not advantageous. Experts explain that genome gigantism or genome obesity is extremely inefficient. Making this much DNA requires a lot of energy and nutrients. Most plants have much smaller genomes, allowing them to reproduce faster, produce more offspring, and adapt to the environment more quickly. Having to copy such a large amount of genetic material every time a cell divides “it’s just crazy,” Schuettpelz said.
David Baum, a professor of botany at the University of Wisconsin-Madison who was not involved in the research, said it was surprising how plants with such bulky genomes could survive.
Pellicer and his colleagues believe that genome size may affect a plant’s chances 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 vegetation, and analyzing whether genome size can help conservationists identify potentially threatened plants.
Could there be a plant with a larger genome than the fork fern? Maybe.
“I’m not going to play with biological boundaries anymore,” Pellicer said. “They need to be broken.”
Amanda Schupak is a science and health journalist in New York.
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