A giant genome shows size doesn’t matter

The writer is a science commentator

Small things can sometimes come in very big packages. That is true of a little fern that grows on a handful of islands in the south Pacific.

The unremarkable-looking plant turns out to have the biggest genome ever discovered, around 50 times the size of the human one. It now holds three Guinness World Records: the largest genome for a living organism, largest plant genome and largest fern genome.

Conservation is a concern. Organisms with large genomes grow more slowly and adapt more sluggishly to changing conditions. Botanists fret that these plants, which predate the dinosaurs, might not survive climate change.

But the discovery also illustrates the mystery surrounding the code of life, just when we might be tempted to congratulate ourselves on our mastery of it. Even as scientists tweak human genomes to cure some genetic diseases and edit plant DNA to create climate-resistant crops, nature can still spring giant surprises.

Ilia Leitch, a plant evolutionary biologist at the Royal Botanic Gardens in Kew, England, and a member of the international team that made the discovery, tells me most people wouldn’t look twice at the new triple record-holder. The fork fern, Tmesipteris oblanceolate, is only about 10-15cm tall. With thin forked leaves hugging the stem rather than extravagant fronds, it looks from afar rather like a rosemary sprig.

It was plucked last year from its native New Caledonia, an overseas French territory in the south-west Pacific, by a team led by Jaume Pellicer from the Botanical Institute of Barcelona and including the Herbarium of New Caledonia. Then came the genome analysis: this involves isolating plant cell nuclei and staining them with a dye that binds to genetic material. The bigger the dye stain, the bigger the genome.

This one stood out. The size of a genome is measured in base pairs, loosely equivalent to “rungs” on the twisting double helix ladder. The human genome is more than 3bn base pairs: if unspooled, that DNA would stretch to two metres. In comparison, the fork fern genome ran to more than 160bn base pairs which, if unfurled, would be about 106m, taller than Big Ben. “I was blown away,” Leitch says.

That beats the largest animal genome, the marbled lungfish, by 30bn base pairs and the previous plant record-holder, the flowering Paris japonica, by about 11bn (also found by the same team). The findings were published last week in the journal iScience.

It confirms that, counter-intuitively, an organism’s genome size is unrelated to its complexity. But much remains unexplained: how does the fork fern fold up so much DNA in cells that are just a fraction of a millimetre across? Given that less than 1 per cent of that genetic material is thought to be functionally useful, how do cells comb through the vast genome to find it? “It’s like searching for a few books which contain instructions to live and survive, among a library of millions of books,” Leitch says. “It is just staggering to me.”

Such organisms, unlike those with slimmer genomes, seem unable to get rid of redundant DNA that has randomly copied itself and built up over time. It is almost as if, Leitch says, the genomes are becoming obese.

Given the servicing costs of a huge genome, it is unclear how they survive in nature. The mechanics of replicating 106m of DNA with every cell division mean that the plant grows slowly and needs lots of nitrogen and phosphorus. That requires stable, nutrient-rich habitats with little competition for resources. Large genomes also make photosynthesis less efficient. Biologists do not know whether there is a fundamental limit to how big a genome can be.

Further investigation will now aid conservation, something Leitch insists is critical, even for obscure ferns, given that plants underpin life on Earth. The next step will be to sequence the fork fern genome (it is expected to have about 100,000 genes; humans have 20,000). Leitch would also like to bring a sample back to plant at Kew, an onerous administrative task additionally complicated by Brexit.

Both goals require another expedition but, for now, genomics must bow to geopolitics. Since May, New Caledonia has seen ongoing political unrest linked to an attempt to extend voting rights on the island. Non-essential travel is not advised; visits are currently on hold.

As such, the tale of the fork fern is more than just the story of an overgrown genome. It also reveals something of the botanist’s burden: plants, politics and paperwork.