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What is the most genetically simple organism (except viruses) on this planet?

By simple I mean the least number of genes.

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I did a google search and got this –  biogirl Dec 10 '13 at 17:06
    
I would guess it is a symbiotic bacteria! The mitochondria maybe (if we accept it to be an organism). Here is an article that you will appreciate. If you search for "tiniest genome" on google you will find some stuff –  Remi.b Dec 10 '13 at 17:26
    
@biogirl So, the smallest organism (except viruses and mits) has 159,662 TACG pairs? No smallest number found so far? Any knowledge about this thing? –  Derfder Dec 10 '13 at 18:09
    
In the link I gave you, there is an insect that has only 112k base (112,000 ATCG pairs) –  Remi.b Dec 10 '13 at 19:19
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@Derfder I dunno much about this. So many organisms's dna has not been looked into. So, in the future one may find smaller genomes –  biogirl Dec 11 '13 at 10:48

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up vote 6 down vote accepted

Mycoplasma genitalium was one of the first full bacterium genomes sequenced and since its a symbiotic organism that lives on the moist and warm genital skin surface it doesn't need as many genes as many bacteria. It has a 582 kbp genome sequence with only 521 genes.

But that is so 1995.

The 159 kbp genome of Candidatus carsonella was published in 2006. It is thought to only contain 182 genes. Its an endosymbiont of some sap eating insects.

But the current record holder, published this year, is Nasuia deltocephalinicola, discovered survey of insect endosymbionts, has only 112 kbp in its genome. Smaller than some viruses. It's thought to contain 137 protein coding genes with 29 tRNA genes, so its not a lot smaller than Carsonella in terms of gene count.

There's no reason to expect that this is the minimal viable genome by any means. I'm sure this response will need revision in a year or two.

M. genitalium has become the model system for minimal genome work. Many of these other bacteria might be difficult to culture, molecular biology lab techniques might have to be developed, so hopping onto newly discovered bacteria isn't always the best idea from a standpoint of producing science. It does suggest that M gentalium doesn't need all of its genes! A possible cause for these extra genes might be a more complex immune response in human beings where its found. It might also be because of the symbiotic requirements of insects is simpler.

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Genome of 'Candidatus Tremblaya princeps' is a bit larger (138931 bp), but contains 116 protein-coding genes: kegg, original paper. It's an insect (P) symbiont and has in turn its own bacterial (S) symbiont. –  har-wradim Dec 12 '13 at 16:27
    
that's definitely worth a mention - thanks! –  shigeta Dec 12 '13 at 17:42

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