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Sunday, October 17, 2010

Largest eukaryotic genome

Biologists from the Royal Botanic Gardens, Kew, have discovered the world’s largest eukaryotic genome known to date. That distinction belongs to the Paris japonica, a small white flower native to Japan. At 152 billion base pairs in its haploid genome, this little plant has about 50 times more DNA than humans do.

Scientists have known for almost a century that genome size does not correlate with either number of chromosomes or with number of genes. For example, the haploid human genome has about three billion base pairs, which are divided into 23 chromosomes. In comparison, an aquatic rat called Anotomys leander has 46 chromosomes in its haploid genome. Granted, chromosomes are fairly arbitrary divisions of DNA. More importantly, those 3 billion base pairs in the human genome are divided into about 23,000 genes. That makes it seem rather odd that the nematode worm Caenorhabditis elegans has a genome containing over 20,000 genes. This seems to be implying that it takes nearly as many genes to make a 1 mm long worm as to make a human.

The previous eukaryotic record holder was the marbled lung fish Protopterus aethiopicus, weighing in at about 133 billion base pairs. Like all organisms with huge genomes, P. aethiopicus and P. japonica face a steep penalty for their massive amounts of DNA. Because it takes so long to replicate their DNA, they grow very slowly, making them unsuited for certain habitats. It’s known that creatures with very large genomes are less able to tolerate extreme environmental conditions.

As I stated, P. japonica is the award winner among the eukaryotes. You may be surprised to learn that the organism with the largest genome of all is an amoeba called Polychaod dubium. Its genome weighs in at whopping 670 billion base pairs. Why an amoeba would need that much genetic information is a mystery, at least to me.