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 the-the-Open ↗
scientist.com/yr2004/may/research3_040510.html
Posted by Robert Karl Stonjek.