The Genome Olympics

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Being big and complex doesn’t necessarily translate to having the largest amount of genetic material.

Case in point: A new study has found that a small, humble fern growing in the forests of a South Pacific island has a genome that is 50 times larger than that of humans.

“Who would have thought this tiny, unassuming plant that most people would likely walk past without notice, could bear a world-beating record in genome size,” said Ilia Leitch, a researcher with the Royal Botanical Gardens, Kew, which conducted the study along with the Institut Botànic de Barcelona.

The study began after researchers Jaume Pellicer and Oriane Hidalgo traveled to New Caledonia to collect samples of the fork fern, or Tmesipteris, a type of plant mainly found in the region, which were then analyzed to estimate the size of their genomes.

They surveyed six fork fern species in New Caledonia and analyzed their cells using a technique called flow cytometry: The researchers extracted cells from the fern leaves and isolated their nuclei, where the genome is housed. They stained the DNA in these nuclei with a fluorescent dye and measured the fluorescence. By comparing the fluorescence levels with those from several plants with smaller genomes, they were able to determine the size of each fork fern’s genome.

Their findings showed that one of the species, T. oblanceolate, has the largest genome ever recorded. Growing to about six inches long, the fern’s genome had a length of 160 billion base pairs – in comparison, humans have around three billion.

The amount exceeds the previous record holder, the Japanese flower dubbed Paris japonica, which has around 150 billion base pairs.

“For a long time, we thought that breaking the previous size record of Paris japonica was going to be an impossible mission,” said Pellicer in a statement, “but once again, the limits of biology have surpassed our most optimistic predictions.”

The team said the findings challenge our understanding of how big genomes can actually be, while also showing that large genomes are not necessarily related to the complexity of the organism.

The study will prompt more investigation into how such large genomes evolve and function, as well as the costs and benefits of maintaining such a large amount of DNA.

“You have to replicate over 100 meters of DNA every time a cell divides,” Pellicer told Science News. “To me, it’s very puzzling.”

Meanwhile, the unassuming plant is now a Guinness World Record holder.

“To think this innocuous-looking fern boasts 50 times more DNA than humans is a humbling reminder that there’s still so much about the plant kingdom we don’t know,” said Adam Millward, managing editor of Guinness World Records, adding, “and that record holders aren’t always the showiest on the outside.”

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