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Tiny genome may be melting away, study
Researchers have identified the smallest known cellular genome, and say it may suffer a strange fate.
Oct. 12, 2006
Special to World
Biologists have long
wondered what is the smallest number of genes required for an organism
to survive. Identifying this “minimal genome,” some researchers
think, could reveal the most basic requirements of life, and thus
provide a more fundamental understanding of it.
Now, scientists say they’ve found the tiniest
known genome of a living thing, along with two huge surprises.
a type of modified fat cell in certain insects that contains
symbiotic bacteria. Bacteriocytes of Pachypsylla venusta,
shown above, contain Carsonella bacteria.
The microbes appear as dark blue, tubular structures. The
lighter circles are cell nuclei of the host organism. (© Science)
First, the genome is just one third the size of the “smallest”
known before. But possibly stranger, they claim, it appears too puny to
allow its owner, a bacterium, to live on as a species much
Like a cash-strapped company that has to merge with a richer firm to
keep going, they say, the microbe and its genes seem to be literally
fusing into a larger creature, becoming cogs in its cellular
“The race to find the smallest microbial genome has taken an
amazing turn,” wrote Siv Anderssen of Uppsala University,
Sweden, in a commentary in the Oct. 13 issue of the research
Researchers studying small genomes have faced surprises before.
Previous work, for instance, found that a bacterial genome
could be artificially pared down to minuscule sizes; but then
the bug could survive only on the microbial equivalent of
life support, bathed in a carefully selected nutrient mix.
That the creature could live only in artificial settings suggested
to some that the “minimal genome” concept was a bit fuzzy.
The new findings may further muddle things.
The announced new record-holder for tiniest genome is the bacterium
Carsonella ruddii, which lives in an insect, and has a sequenced
genome—or complete set of DNA—with about 182 functional genes.
These correspond to 160,000 “letters” of genetic code; previous
estimates had placed the minimal genome at about 400,000.
“It’s unbelievable, really,” said Nancy A. Moran of the
University of Arizona in Tucson, Ariz., one of the scientists
who conducted the new research. “It’s believed that more genes
are required for a cell to work.” The finding provides new insights
into bacterial evolution, Moran and colleagues wrote, also
in the Oct. 13 Science.
The microbe lives within an Arizona insect, Pachypsylla
venusta, that like many insects feeds on plant sap. This isn’t
very nutritious. To live, such insects often rely on resident
bacteria that make and share key nutrients with them. The
host and microbes depend on each other to live, a relationship
called endosymbiosis. The bond is so close and ancient that the
microbes live within special insect cells that have evolved to
house them, called bacteriocytes.
The bacteria thus live in a sheltered world with a simple, predictable
diet and lifestyle. So they get by with simple genetic instructions.
If they or their ancestors had any extra, unneeded genes, these
would generally have been lost over the course of evolution.
The researchers collected Pachypsylla venusta
bugs from hackberry trees on their university campus and
around town. They extracted the Carsonella DNA and sequenced
it, and got a jolt. “It lost genes that are considered absolutely
necessary. Trying to explain it will probably help reveal
how cells can work,” said Moran.
The scientists speculate that in the bacterium’s evolutionary
past, some of its genes moved into the insect’s own genome, beginning
a process of gene takeover.
Animal and plant cells have specialized internal structures
called organelles, tiny sacs of machinery used for various purposes.
Strong evidence suggests many of these organelles are descendants
of symbiotic bacteria that once lived free, but gradually
became incorporated into the cell. A transfer of genes from
the bacterium to the host is often part of the process.
Carsonella may likewise be turning into an organelle, the researchers
In a second study published in the same issue of the journal, researchers
with València University in València, Spain, described
finding a similar endosymbiotic bacterium, Buchnera
aphidicola, whose genome is about 5 percent larger than
that of Carsonella.
Buchnera may be on its way to total extinction, the authors
argued, because it has lost many functions that would let it help
its host. Other bacteria cohabiting in the same insect
seem to have have taken over the functions, they wrote.
* * *
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