In low-oxygen environments, eukaryotes often possess a reduced form of the mitochondrion, but it was believed that some of the mitochondrial functions are so essential that these organelles are indispensable for their life.
Flagellated protozoa of genus Monocercomonoides are known for more than 80 years. They are distant relatives of the human pathogens Giardia and Trichomonas, comprised in the phylum Metamonada, whose members live exclusively in low-oxygen environments. Another interesting fact about them is that they apparently lack mitochondria. Originally they were considered among the most primitive eukaryotes, members of a lineage that diverged from the others before mitochondria appeared. However, they are now known to have lost mitochondria secondarily, and it was thought they retained functions and nuclear genes derived from them in some relic structures.
In a new study, researchers from the Czech Republic and Canada sequenced the Monocercomonoides genome. They were surprised to find that this organism lacks all mitochondrial proteins. It seems to have gotten by without mitochondria thanks to a cytosolic sulfur mobilization system that they acquired from bacteria and that appears to be the substitute for essential mitochondrial functions. Through a unique combination of events including the loss of many mitochondrial functions and the acquisition of this essential machinery from prokaryotes this organism has found its own way of survival.
This amazing organism is a striking example of a cell which refuses to adhere to the standard cell biology text book, and we believe there may be many more similar examples in the so far hidden diversity in the world of microbial eukaryotes--the protists. It is very likely that the mitochondrion is absent in the whole group called oxymonads. We would like to know how long ago the mitochondria were lost.