“Life is like a box of chocolates, you never know what you’re gonna get”. The iconic statement of Forrest Gump’s movie can easily be used for Science. And you must be prepared for whatever you get, because it can change concepts you might think you know.
For years, scientists believed that all eukaryotic cells contained mitochondria. Mitochondria is an organelle responsible for cell respiration in aerobic organisms, which means that it is responsible for energy generation for the individual’s life, through a metabolic process that requires oxygen. For that reason, mitochondria has been described as the “powerhouse of the cell”.
Mitochondria is thought to have originated from a symbiosis of an ancient aerobic prokaryotic cell (a primitive bacteria), that was engulfed by a nucleated host cell. Although details of this symbiosis process are still unclear, researches indicate that only after that eukaryotic organisms’ radiation has taken place. During the course of evolution, mitochondria has maintained its own DNA, which went through diverse modifications, so even non-aerobic eukaryotic cells present reduced forms of this organelle. Well… at least that’s what we knew so far.
In a recent research, published this month on Current Biology, Anna Karnkowska and colleagues have described an unicellular eukaryotic organism (Monocercomonoides sp) that contains not even a signal of mitochondria’s presence. The genus Monocercomonoides is originally part of a group that contains some anaerobic parasites and symbionts organisms, such as Giardia and Trichomona. But even in these organisms, the presence of mitochondria homologues has been reported, although we don’t know their function yet.
Karnkowska and her group set out a range of experiments. First of all, they looked at the complete genome of Monocercomonoides and found no mitochondrial genome at all. That was not a big surprise, because some other organisms are also known for lacking mitochondrial DNA. These organisms have some parts of nuclear DNA that encode proteins responsible for sustain mitochondrial metabolism, which is considered a strong evidence for the common origin of all mitochondria. Nevertheless, Karnkowska group has found out that Monocercomonoides also lack homologs of any component of mitochondrial machinery – they contain not even one known mitochondrial protein sequence.
So, next question we want to know the answer is: how can a eukaryotic organism obtain energy without a mitochondria?
As many other anaerobic organisms, Monocercomonoides doesn’t produce enzymes for aerobic energy generation. But Monocercomonoides seems to be capable of perform a complete glycolitic pathway, including anaerobic glycolisis, counting on the activity of alternative enzymes in the cytosol. In other word, these organisms might have an extended pathway that allows them to obtain energy from glucose in the intracellular region, but not in an organelle such as mitochondria. Apparently, mitochondrial-specific pathways were replaced by a bacterial system that functions in the cytosol. It suggests that Monocercomonoides might have acquired genome sequences from a bacteria and that allowed it to obtain energy and finally dispense mitochondria processes completely.
This discover shows us how scientific knowledge is always being rebuilt and it is also a good example of how research effort on barely known living groups can surprise us and help to elucidate questions in evolution. Of course, we need more studies on Monocercomonoides to confirm the total absence of mitochondria in this genus. Nevertheless, the lack of positive evidence for its presence, even after looking for it in different manners, strongly indicates that mitochondria is not as indispensable as it was thought to be for eukaryotic life.
By Bruna de Oliveira Cassettari.