An article published in the journal “Current Biology” describes a research of one genus of protists called Monocercomonoides. A team led by Anna Karnkowska, now at the University of British Columbia in Vancouver, Canada, sequenced the genome of a species that belong to that genus. The result was surprising because it’s a eukaryotic but has no mitochondria, the first case of this kind ever discovered.
Organisms belonging to the genus Monocercomonoides are unicellular flagellate protists related with other organisms that, like these ones, live in low oxygen environments such as gut parasites Giardia and Trichomonas that affect humans. They’ve been known for over 80 years but only now genetic analyzes were performed on a species that belongs to them, called Monocercomonoides sp. PA203.
The sequencing of the Monocercomonoides sp. PA203 DNA allowed to discover that it has no mitochondria. So far, these organelles were found in all eukaryotic organisms, which share certain cellular characteristics but also the presence of mitochondria, ancient symbionts that ended up being absorbed within a cell.
In some eukaryotic organisms mitochondria lost some functions but keep on performing others and it seemed that some of them were necessary for any organism belonging to that domain of life. This is the first time a eukaryotic organism entirely devoid of mitochondria was discovered.
According to the researchers, the strangeness of Monocercomonoides is the result of a unique type of evolution. These protists use a cytosolic sulfur mobilization system (SUF) used by bacteria. Put simply, energy generation is generally handled by the mitochondria is replaced by this other mechanism in Monocercomonoides.
A eukaryotic organism with some characteristics of the bacteria is an anomaly but the biogenetic research of recent years are proving an increasing biological diversity among living organisms. In the case of Monocercomonoides we have eukaryotes that acquired another energy production system and lost their mitochondria.
It’s a definitely out of the normal evolutionary path. Being a single-cell organism, the hypothesis proposed by the researchers is that there was a horizontal gene transfer, meaning that genetic material was transferred from a bacterium to an ancestor of today’s Monocercomonoides.
The sulfur mobilization system genes probably proved more efficient than mitochondria in the environment in which that organism lived and over time the evolution caused the loss of mitochondrial DNA. At this point it’s necessary to try to understand if Monocercomonoides are the only eukaryotes that don’t have mitochondria, which means that more genetic research will be needed.
According to Anna Karnkowska and her co-workers, it’s possible that all the organisms that belong to the order Oxymonadida, the one that includes Monocercomonoides, have no mitochondria. Someone’s already proposing to sequence the DNA of other eukaryotes belonging to little known phyla to figure out if that characteristic is widespread. In short, this surprising discovery will certainly have follow-ups in future biogenetic research.
