Re: Article: Researchers Find Smallest Cellular Genome

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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.

"It's the smallest genome -- not by a bit but by a long way," said
co-author
Nancy A. Moran, UA Regents' Professor of ecology and evolutionary biology
and a member of the National Academy of Sciences. "It's very surprising.
It's unbelievable, really. We would not have predicted such a small size.
It's believed that more genes are required for a cell to work."

Carsonella ruddii has only 159,662 base-pairs of DNA, which translates to
only 182 protein-coding genes, reports a team of scientists from The
University of Arizona in Tucson and from Japan.

The finding provides new insights into bacterial evolution, the scientists
write in the Oct. 13 issue of the journal Science.

Atsushi Nakabachi, a postdoctoral research associate in UA's department of
ecology and evolutionary biology and a visiting scientist at RIKEN in
Wako,
Japan, is the first author on the research report, "The 160-kilobase
genome
of the bacterial endosymbiont Carsonella." The research was conducted in
senior author Masahira Hattori's laboratory in Japan and in Moran's lab at
the UA.

A complete list of authors is at the bottom of this release. The Ministry
of
Education, Culture, Sports, Science and Technology of Japan funded the
work.

Many insects feed on plant sap, a nutrient-poor diet. To get a balanced
diet, some sap-feeders rely on resident bacteria. The bacteria manufacture
essential nutrients, particularly amino acids, and share the goodies with
their hosts.

In many such associations, the bacteria live within the insect's cells and
cannot survive on their own. Often the insect host cannot survive without
its bacteria, known as endosymbionts.

The relationship between some insects and their endosymbionts is so close
and so ancient that the insects house their resident bacteria in special
cells called bacteriocytes within specialized structures called
bacteriomes.

Studying the genomes of such endosymbionts can provide clues to how
microorganisms' metabolic capabilities contribute to both their hosts and
to
biological communities.

An organism's genome, its complete complement of DNA, provides the
operating
instructions for everything the organism needs to do to survive and
reproduce.

Endosymbiotic bacteria live in an extremely sheltered world and have a
pared-down lifestyle, so they need a simpler set of instructions. Many of
the metabolic pathways that free-living bacteria maintain have been lost
after so many generations of living within insects.

Nakabachi and Hattori were interested in sequencing the genome of the
bacteria Carsonella.

Moran had done some previous work on the Carsonella genome and found its
DNA
composition and evolution to be unusual. She suggested the team pursue the
Carsonella that lived inside an Arizona psyllid insect called Pachypsylla
venusta. The insect has only one species of endosymbiotic bacteria, which
would simplify the genomic analysis.

The researchers collected Pachypsylla venusta psyllids from hackberry
trees
(Celtis reticulata) on the UA campus and around Tucson. The team extracted
the Carsonella DNA and sequenced it.

Even though endosymbionts need fewer operating instructions to survive,
the
bacteria's itsy bitsy genome was a surprise.

"It lost genes that are considered absolutely necessary. Trying to explain
it will probably help reveal how cells can work," said Moran, who is a
member of UA's BIO5 Institute.

The scientists speculate that in the bacteria's evolutionary past, some of
its genes were transferred into the insect's genome, allowing the insect
to
make some of the metabolites the bacteria needed. Once the insect
shouldered
those responsibilities and provided the bacteria with those metabolites,
the
bacteria lost those genes.

Animal and plant cells have specialized structures inside them called
organelles that are derived from symbiotic bacteria that became
incorporated
into the cell over the course of evolution.

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

Posted by
Robert Karl Stonjek




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?


.

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