Re: What's smallest known "self-sufficient" genome?

IRR wrote:
"kj" <socyl@xxxxxxxxxxxxxxxxx> wrote in message
news:e1m9ah$17ia$1@xxxxxxxxxxxxxxxxxxxxxx
I'd like to know which, among those organisms whose genomes have
been analyzed and that are *not* intracellular parasites (such as
viruses, mycoplasmas, etc.), has the smallest genome? By "smallest"
I mean "fewest genes" (not fewest bases, or smallest chromosomes).

Actually I think it might be more useful to know what you mean by
"self-sufficient" rather than "smallest". PiP mentions Pelagibacter/SAR11,
which is the logical choice and IIRC was touted as such in a Science
magazine article not too long ago.
Alternatively, I'd nominate any of several Prochlorococcus species
(cyanobacteria), which are attractive for their "just add water" simplicity
(strictly speaking, water, light, and CO2).

Nature Heredity has an interesting News-and-Views article that
discusses why Pelagibacter and Prochlorococcus have such small
genomes.

http://www.nature.com/hdy/journal/v96/n5/full/6800766a.html

First paragraph:
The recent sequencing of the genomes of two of the most abundant
(comprising up to 25% of their local populations) pelagic bacteria,
Pelagibacter ubique and Prochlorococcus marinus, have revealed
that they have two of the smallest genomes of any known free-living
bacteria. In general, bacterial genomes are under strong positive
selection for economy of size: typified by the almost total absence
of nonfunctional DNA, around 85-90% of their genomes being
dedicated to encoding proteins. These two newly described genomes
appear to have undergone strong purifying selection for a small
genomic size. This discovery demonstrates that the drive towards
reduction, as with other complex biological processes, can be caused
by entirely opposing environmental circumstances. It can derive from
the relaxation of selection in small populations of subsidised
host-dependent bacteria, or from intense competition among the
most abundant marine bacteria. They also provide some valuable
insights into what conditions are necessary to allow genome
reduction.


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