October 8, 2010

Rare Japanese Plant Has Longest Known Genome

A rare and striking plant native to Japan may contain the longest genome ever discovered, according to researchers at Britain's Kew Botanical Gardens.

Dubbed the Paris japonica, the plant has a genetic code 50 times longer than that of a human being.  However, that means the plant is at high risk of extinction, the scientists said on Thursday.

Ilia Leitch of Kew's Jodrell Laboratory said she and her colleagues suspected the plant might have a lengthy genetic code since its relatives have rather large ones, but the magnitude of this flower's genome took them by surprise.

"We were astounded really," she told the Associated Press, referring to the length of the plant's genetic code, which easily exceeds its nearest competitor, the marbled lungfish.

If stretched end-to-end, the Paris japonica genome would be taller than Big Ben, she added.

"We certainly didn't expect to find it."

A genome is the full set of an organism's DNA -- the complex molecules that guide the formation and function of all life forms. 

A genome's size is measured by the number of DNA base pairs, or building blocks, it contains.  For instance, the human genome includes about 3 million bases, while the marbled lungfish has an astonishing 130 million bases. But the Paris japonica has 150 million.

Even outside experts were amazed.

"This is certainly an enormously large genome," said Nick Lane, a fellow at the Department of Genetics, Evolution and Environment at University College London.

"I don't know of any larger genomes among plants or animals," he told the AP.

However, some microorganisms such as amoebas might have longer codes, he added.

Leitch and Lane said the discovery demonstrates the enormous diversity of genome sizes.  Paris japonica and the marbled lungfish have vast genomes, while other genetic codes, such as those for the parasite Encephalitozoon intestinalis, are tiny, carrying just 2,300 bases.

It's not entirely clear why the range of DNA base pairs varies so much.  Larger genomes do not necessarily translate to a more complex organism.  Furthermore, while genes typically correspond to certain traits "” blonde hair, for instance, is genetically determined "” in organisms with large genomes, many genes do not seem to correspond to anything.

"Effectively, some cells carry massive amounts of 'junk,' or at least non-coding DNA, whereas others have very little," Lane told the AP.

Scientists are still debating why some organisms carry large amounts of non-coding DNA.  Leitch said the Paris japonica findings will likely add to the discussions.

"It's a question that's long intrigued scientists," she said.

The research is being published in the Botanical Journal of the Linnean Society.


Image Caption: Paris japonica. Credit: Karl Kristensen, Denmark


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