Re: Article: Researchers Find Smallest Cellular Genome


"DK" <dk@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message news:eh5otd$1t07$1@xxxxxxxxxxxxxxxxxxxxxx
In article <eh3fs2$pjn$1@xxxxxxxxxxxxxxxxxxx>, "Perplexed in Peoria" <jimmenegay@xxxxxxxxxxxxx> wrote:
"IRR" <iotarhorho@xxxxxxxxxxxxxxxxx> wrote in message
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"Robert Karl Stonjek" <rstonjek@xxxxxxxxxxxxxx> wrote in message
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Researchers Find Smallest Cellular Genome

The smallest collection of genes ever found for a cellular organism comes
from tiny symbiotic bacteria that live inside special cells inside a small
insect.

The bacteria Carsonella ruddii has the fewest genes of any cell. The
bacteria's newly sequenced genome, the complete set of DNA for the
organism,
is only one-third the size of the previously reported "smallest" cellular
genome.

[snip]
Carsonella's stripped-down genome may indicate that it is on its way to
becoming an organelle, the researchers write in their article.

Source: University of Arizona
http://www.sciencedaily.com/releases/2006/10/061012184647.htm


Maybe similar to the "why Pluto is not a planet" debates, the last sentence
made me wonder how these sorts of obligate endosymbionts (can't live w/o
host, host can't live without them) differ from organelles like the
mitochondrion?

These are all just conventions. All such boundaries are fuzzy anyway.

Several criteria come to mind.
- Intracellular or not. Termite gut endosymbionts are obligate, but not
intracellular.

Many mycoplasma species are obligate intracellular parasites.

- Whether transmitted in gametes. The fact that these seem to exist in
special cells called bacteriocytes makes me wonder in this case.

Not applicable to asexually reproducing species.

- Degree of universality. Organelles like mitochondria and chloroplasts
must be ancient since their distribution is at the kingdom level or
higher. These bacteria must have become endosymbionts much more
recently.

So if a revolutionary adaptation of, say, photosynthesizing mammal
happens two weeks from now, you would not consider it important
enough because it is so recent?

I didn't say that. What I came close to saying is that I would call the
new photosynthetic symbiont a symbiont, rather than an organelle.

Incidentally, you may be interested in reading about the three-toed-sloth.


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