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Sunday, March 06, 2005 The Evolution of Powerhouses The endosymbiont theory, originally proposed by Lynn Margulis, says that mitochondria are the decaying remenants of an aerobic bacteria that lived inside one of our eukaryotic ancestors. Some evidence for this theory include the facts that mitochondria synthesizes its own proteins, contains its own genome, and replicates independently of the host cell. But surely, this sort of thing couldn't have just happened once. One would think that with all the symbiotic relationships in the natural world, eukaryotic cells would have a bunch of different organelles derived from long ago endosymbionts.  In 1973, two Trichromonas researchers--Donald Lindmark and Miklós Müller--were studying the cattle pathogen Trichromonas foetus (the human version, Trichromonas vaginalis, is an STD). Trichromonas species are parasitic flagellate anaerobes and at the time not much was known about this microbe's metabolism aside from the production of waste products, acetate and hydrogen. Lindmark and Müller were characterizing the enzymes in the hydrogen production pathway when they noticed that these enzymes were concentrated in a "subcellular particle" that did not resemble any of the other organelles in the cell. They named this new organelle the "hydrogenosome."The hydrogenosome is a special organelle found in eukaryotic microbes as well as some fungi that like to live in poor oxygen environments. In order to generate energy for the cell, it oxidizes pyruvate and releases hydrogen and produces ATP. Sound familiar? In fact, hydrogenosomes have an uncanny likeness to mitochondria--not only do both organelles produce ATP, but morphologically they are similar--they are both surrounded by a double membrane and they both compartmentalize their different metabolic functions. The only big difference between the two is that hydrogenosomes use the anaerobic (fermentative) pathway while mitochondria use the aerobic pathway (Krebs cycle). So is it possible that hydrogenosomes and mitochondria are related? Both of these organelles multiply independently of the host cell and anaylses of hydrogenosome associated proteins and mitochondria associated proteins show that both have similar functions. But while mitochondria have their own genomes, the hydrogenosomal genome has been quite elusive. Experiments aimed at Trichomonas or anaerobic fungi have failed to turn up significant amounts of hydrogenosomal DNA which, understandably, has made it hard to determine what sort of evolutionary relationship exists between the two organelles. A recent paper in Nature by Boxma et al. reports success in finally isolating a hydrogenosomal genome. The researchers turned to a different anaerobic microbe, Nyctotherus ovalis--a resident of the termite hindgut. Analysis of the hydrogenosomal genome of N. ovalis showed that it had significant homology to that of mitochondrial genes. Further examination of organelle associated proteins revealed that the hydrogenosome had many of the proteins that were associated with mitochondria except for the ones involved in aerobic respiration. From this data, perhaps the hydrogenosome is an evolutionary precursor to mitochondria. Right now, it's still too hard to really tell if mitochondria and hydrogenosomes are related or the descendants of two different endosymbionts. There are plenty of hypotheses floating about though. The Hydrogen hypothesis says that the host cell engulfed a hydrogen-producing bacterium because it needed hydrogen. The Syntrophy theory says that a hydrogen-producing bacterium and a second bacterium that was the precursor to the mitochondria were engulfed together. Another theory, the detoxification hypothesis, proposes that aerobic bacteria were engulfed by the anaerobic host cell when there was a sudden increase in oxygen levels, or "oxygen spike", around two billion years ago. However, the Boxma et al. paper lends credence to another possibility--that mitochondria and hydrogenosomes have a common ancestor. In this hypothesis, a hydrogen-producing bacteria would be associated as endosymbionts of a eukaryotic host before the oxygen spike. During the oxygen spike, the eukaryotes would be forced to associate with aerobic bacteria for detoxification and instead of forming a new organelle, the hydrogen-producing endosymbiont would fuse with the second aerobic endosymbiont to create a hybrid that would later give rise to mitochondria and hydrogenosomes. [posted by S. Y. Affolee on 3:50 AM : ]
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