Research Vision
Evolutionary TeamworkConstructing eukaryotes through endosymbiosis
By Frederic D. Bushman

The endosymbiotic theory, which posits that organelles such as chloroplasts
and mitochondria descended from formerly independent cells, has received
wide acceptance in the last third of the 20th century. But recent findings
suggest that endosymbiotic processes may have contributed still more
cellular components, chloroplasts and mitochondria being simply the most
easily identified examples.

Genomic analyses across a broad spectrum of organisms have solidified the
case for mitochondria and chloroplasts and suggested that less well known
organelles, the hydrogenosome and mitosome, are remnants of genome-depleted
mitochondrial descendants. These findings raise questions as to whether
still other structures in eukaryotic cells, now lacking their own DNA, might
also have originated through endosymbiosis.

THE FEELING IS MUTUAL According to the presently favored views on
endosymbiosis, an anaerobic cell engulfed a respiring a-proteobacterium,
allowing respiration in the resulting consortium. This may have taken place
during early evolution concomitant with Earth's planetary transition from a
reducing to an oxidizing atmosphere. Later, some descendents of this fused
cell captured a cyanobacterium capable of photosynthesis. The
a-proteobacterium evolved to become modern mitochondria, and the
cyanobacterium gave rise to chloroplasts.

The endosymbiotic theory became topical early in the 20th century, largely
because mitochondria looked like bacteria inside larger cells. But the
theory lost favor after many unsuccessful attempts to cultivate mitochondria
outside the host cell. In the 1960s and 1970s Lynn Margulis revitalized the
idea when she articulated a case for endosymbiosis that didn't rely on
independent cultivation but on biochemical and molecular data. More
recently, genome sequence comparisons have supported this idea:
Mitochondrial genes closely match a-proteobacteria such as Rickettsia, and
chloroplast genes match cyanobacteria such as Prochlorococcus marinus.

Mutualistic relationships today may illustrate some of the steps involved in
forming a eukaryote/ prokaryote endosymbiosis. Our guts, for example, are
thought to harbor some 500 bacterial species that aid in digestion and
obstruct colonization by pathogens. The giant tubeworm, Riftia pachyptila,
which crowds about hydrothermal vents, also associates with bacterial
mutualists that provide the sole source of nutrition to the animal by
chemolithotrophic energy generation from hydrogen sulfide. The giant vent
clam, Calyptogena magnifica, demonstrates an even closer relationship: The
chemolithotrophic bacteria are inherited by descent, rather than captured
from seawater as with Riftia. Similarly many insect species harbor
intracellular bacteria, including Wigglesworthia, Buchnera, and others, that
carry out reactions essential for host nutrition. Some of these bacteria are
unable to live outside the insect host; such obligate mutualists are close
to qualifying as new organelles, though they do apparently still move
between cells in some cases.

Read the rest at The Scientist.com
http://www.the-scientist.com/yr2004/may/research3_040510.html

Posted by
Robert Karl Stonjek.
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