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Rare plant has biggest genome yet found
When it comes to genomes, size matters - and British scientists say a rare and striking plant native to Japan is in a perilous position.

Researchers at Britain's Kew Botanical Gardens say the plant, Paris japonica, has the largest genome yet recorded, putting it at high risk of extinction.

"Some people may wonder what the consequences are of such a large genome and whether it really matters if one organism has more DNA than another," says Ilia Leitch, a researcher at Kew's Jodrell Laboratory. "The answer to this is a resounding 'yes'"

"Having a large genome increases the risk of extinction. The larger it is, the more at risk you are."

The vast range of genome size - the amount of DNA - in plants and animals has long fascinated and puzzled scientists.

With 152.23 picograms (pg) of DNA, the Paris japonica has around 15% more than the previous record holder, the marbled lungfish or Protopterus aethiopicus, with 132.83 pg.

It is also more than 50 times bigger than the human genome, which is 3.0 picograms. A picogram is one trillionth of a gram.
Long division

Leitch says the importance of size lies in the fact that the more DNA there is in a genome, the longer it takes for a cell to copy its DNA and divide.

"The knock-on effect of this is that it can take longer for an organism with a larger genome to complete its life cycle than one with a small genome," she says.

This explains why many plants living in deserts, which must grow quickly after rains have small genomes enabling them to grow rapidly, while species with large genomes grow much more slowly and are not found in such harsh habitats.

Leitch says that in plants, research has shown that those with large genomes are at greater risk of extinction, are less adapted to living in polluted soils and are less able to tolerate extreme environmental conditions, factors which she says were "all highly relevant in today's changing world."

The smallest genome so far reported is in a parasite of humans and other mammals called Encephalitozoon intestinalis, which has just 0.0023 picograms of DNA.

The record holder among plants for 34 years was a species called Fritillaria assyriaca, until earlier this year when a group of Dutch scientists found that a natural hybrid of trillium and hagae, related to the herb paris, had a genome 4% larger than the fritillary at 132.50 pg.

According to Kew's scientists, this had been widely thought to be around maximum size a genome could reach until the recent discovery of the 152.23 pg Paris japonica genome.

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