Which organism has the largest known chromosome?

Plant has the largest genome in the world

An inconspicuous herbaceous plant, the oneberry Paris japonica, has the largest known genome of all living things. With a good 152 picograms of mass per cell and a length that corresponds to that of Big Ben in London, it exceeds the previous record holder, a lungfish, by a whopping 15 percent. Why the small plant has developed such a gigantic genome is still unclear, because this actually has numerous disadvantages.

For several decades now, scientists have been researching the genome of more and more plants and animals. It turned out early on that the size of the genome, and thus also the number of base pairs and genes contained in the DNA, differs drastically from living thing to another. Why, however, is still unclear today. It only seems clear that these variations seem to have biological and ecological effects.

Lungfish replaced as record holder

The range of genome sizes extends from the smallest known genome of a eukaryotic cell, that of the endoparasitic fungus Encephalitozoon intestinalis with only 0.0023 picograms of mass per cell, to the human genome with at least 3.0 picograms, to the giant genomes of amphibians. The record holder in terms of genetic material among the animals is the lungfish Protopterus aethiopicus with a fantastic 132.8 picograms. Among the plants, a forest lily hybrid with 132.5 picograms was previously considered the plant with the largest genome.

Now, however, scientists at the Jodrell Research Center at the Botanical Garden in Kew near London have discovered a plant whose genome size overshadows anything known up to now: Paris japonica, which belongs to the berries, is another 15 percent larger than that of trillium and lungfish: 152 The DNA of a cell alone weighs 23 picograms, as the researchers now report in the “Botanical Journal of the Linnean Society”.

Big genome has drawbacks

"We were completely amazed when we discovered that this little plant has such a large genome - it is so large that if spread out it would rank higher than Big Ben in London," explains Ilia Leitch of the Jodrell Research Center. “Some may wonder what is special about such a large genome and whether it really makes a difference if one organism has more DNA than another. The answer to that is a resounding 'Yes, it does'. "

Interestingly, when it comes to the genome, larger does not mean better, on the contrary: "In the case of plants, it has been shown that those with a large genetic makeup have a greater risk of extinction," says Leitch. "They are less well adapted to thrive in contaminated soils and are less able to tolerate extreme environmental conditions - both of which are extremely important properties in today's rapidly changing world."

Smaller means faster

One reason for this is already known: a larger genome, for example, requires more time until all of the DNA is copied during cell division and thus the growth of the organism. Living things with a larger genome often have a significantly longer life cycle than species with a smaller one. It is therefore no coincidence that desert plants, for example, which have to grow very quickly after a rain in order to take advantage of the precious moisture, usually only have a very small genetic make-up.

It is still unclear why Paris japonica, which occurs mainly in Asia, developed such a gigantic genome. The berries of the plant contain large amounts of poisonous saponins and glycosides, but the roots were previously used in folk medicine to treat contagious diseases.

(Royal Botanic Gardens, Kew, 10/13/2010 - NPO)

October 13, 2010