Scientists at Kew's Jodrell Laboratory have discovered that Paris japonica, a striking rare native of Japan(1), has the largest genome(2) of them all -- bigger than the human genome and even larger than the previous record holder -- the marbled lungfish.

The results are published in the Botanical Journal of the Linnean Society.

The diversity of genome sizes (the amount of DNA) in plants and animals has fascinated but at the same time puzzled scientists since this variation was first detected in the early 20th century. How and why such diversity evolved are important unanswered questions because we know that it has biological and ecological consequences that affect the distribution and persistence of biodiversity.

There is a staggering diversity of genome sizes. The smallest genome (3) so far reported (0.0023 pg of DNA) is found in a parasite (Encephalitozoon intestinalis) of humans and other mammals. The human genome, at 3.0 pg, is 1300 times larger than this, but this pales into insignificance compared to those found in some animals and plants.

Among animals, some amphibians have enormous genomes, but the largest recorded so far is that of the marbled lung fish (Protopterus aethiopicus) with 132.83 pg(3) . Among plants, the record holder for 34 years was a species of fritillary(4) (Fritillaria assyriaca) until earlier this year when a Dutch group knocked the fritillary off the top spot when they found that a natural hybrid of trillium (Trillium × hagae), related to herb paris had a genome just 4% larger than the fritillary (132.50 pg).

This was widely thought to be approaching the maximum size that a genome could reach, until this summer when a team of Kew scientists discovered that the genome of another close relative of herb paris, Paris japonica from Japan, is a staggering 15% bigger than the genome of either the trillium or the fish at a whopping 152.23 pg

Ilia Leitch, Research Scientist in the Jodrell Laboratory, says "We were astounded when we discovered that this small stunning plant had such a large genome ­ -- it's so large that when stretched out it would be taller than Big Ben.

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"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. The answer to this is a resounding "yes, it does," and the consequences operate at all levels from the cell up to the whole organism and beyond. In plants, research has demonstrated 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 -- all highly relevant in today's changing world."

Another example of the significance and importance of genome size in both animals and plants, is the fact that the more DNA there is in a genome, the longer it takes for a cell to copy all 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. It is no coincidence that many plants living in deserts which must grow quickly after rains have small genomes enabling them to grow rapidly. In contrast, species with large genomes grow much more slowly and are excluded from such habitats.

Genome size is also positively correlated with nuclear size (the more DNA you have the more space you need for it), and, in many cases, also with cell size which can have knock-on consequences at the whole organism level.

Notes:

(1) The group investigated genome size in Paris japonica using flow cytometry, comparing it with a range of other plants known to have large genomes.

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(2) Genome size is the total amount of DNA in the nucleus of an organism and includes both the genes and the non-coding sequences of the DNA.

(3) Here we are referring to eukaryotes (organisms with membrane enclosed nuclei); viruses and bacteria have even smaller genomes.

(3) This is almost 58,000 times more than the smallest genome in the parasite

(4) The Fritillaria assyriaca has a genome with 127.4 pg of DNA This is 55,000 times more than the parasite.