Description
Hydrogenosomes are approximately 1 micrometre in diameter but under stress conditions can reach up to 2 micrometre and are so called because they produce molecular hydrogen. Like mitochondria, they are bound by distinct double membranes and one has an inner membrane with some cristae-like projections. Some hydrogenosomes may have evolved from mitochondria by the concomitant loss of classical mitochondrial features, most notably its genome. A hydrogenosomal genome could not be detected in Neocallimastix, Trichomonas vaginalis and Tritrichomonas foetus. However, a hydrogenosomal genome has been detected in the cockroach ciliate Nyctotherus ovalis, and the stramenopile Blastocystis. The similarity between Nyctotherus and Blastocystis, which are only distantly related, is believed to be the result of convergent evolution, and calls into question whether there is a clear-cut distinction between mitochondria, hydrogenosomes, and mitosomes (another kind of degenerate mitochondria).
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