So this is my first post (and quite a long one too at that!) Words in pink can be found in the glossary– let me know what you think!
Bio love, TBY x
Recently, a study of a microorganism from the gut of a chinchilla has taken the scientific world by storm. This discovery has shown that Monocercomonoides, a small eukaryotic protist, has no discernable mitochondria. It is already known that several species have small, reduced mitochondria, e.g. Giardia, but this is the first to be found with none at all. How can this organism survive without mitochondria, and what does this mean for our understanding of cellular biology?
Mitochondria- what do they do, and why are they important?
As learnt in secondary school biology lessons, mitochondria are the powerhouse of the cell. They produce energy in the form of adenosine triphosphate (ATP) that the cell uses to power reactions and cellular mechanisms. They also have a pivotal role in creating iron and sulfur clusters that are used in the cell’s proteins and enzymes. Mitochondria, along with a nucleus and membrane-bound organelles, are a feature of eukaryotes- A.K.A all animals, plants, fungi and some microorganisms. Bacteria and archaea are prokaryotic, and do not possess any of these cellular features.
There is a lot of dispute over how and when mitochondria arose (something which I may write about in the future, as it’s a fascinating topic of debate!). Mitochondria were originally prokaryotic alpha-proteobacteria, which were taken up (symbiosed) by early eukaryotic cells around 2 billion years ago. They adapted to a symbiotic life within the host cell, and over time underwent substantial horizontal gene transfer. This is where genes from the mitochondria genome transfer to the host eukaryotic genome- a very rare occurrence, but one that means mitochondria can no longer live outside the host cell, and rely on the host to synthesise many proteins.
So, if this protist does not have any mitochondria, how can it be classed as a eukaryote? Is it a missing link between our mitochondria-less ancestors and modern eukaryotes, or does it show an extreme reversal that occurred in this species alone? And most importantly, how can Monocercomonoides survive without a classical energy-producing mechanism?
How did they find this out?
This paper, published in Current Biology by Karnkowska et al (2016), outlines how the authors went about identifying this extraordinary organism. They used 454 pyrosequencing to sequence the entire genome of Monocercomonoides, and looked for genetic evidence of mitochondria. There are several tell-tale signs that a cell has mitochondria:
- They have mitochondrial genes in the genome (from the previously-mentioned horizontal gene transfer)
- Therefore, they produce proteins that are used by the mitochondria, for example energy metabolism proteins found in the TCA cycle and electron transport chain.
- There are also detectable TOM and TIM channels, which facilitate protein import into the mitochondria
Monocercomonoides has no evidence of horizontal gene transfer from mitochondria to the host genome. They do not produce any proteins that are needed by mitochondria, and there are no detectable proteins that are involved in import into the mitochondria. There is also no sign of the mitochondrial step of the iron-sulfur-complex- creating pathway. All this evidence put together suggests that this protist does not have any mitochondria. However, Monocercomonoides is definitely a eukaryote; it has a nucleus, and clearly other membrane-bound organelles such as a Golgi body.
Why does Monocercomonoides have no mitochondria?
Current hypotheses suggest that Monocercomonoides lost its mitochondria early in evolution, before the transfer of any genes to the host nucleus. It is speculated that this species may have lost its mitochondria due to the fact it lives in an energy- and nutrient-rich environment, and does not have a reliance on oxygen for respiration. The anaerobic respiration of glucose provides enough ATP for cellular mechanisms to occur, without mitochondrial input. There is also evidence that Monocercomonoides has obtained iron-sulfur cluster enzymes through horizontal gene transfer from bacteria, and that these enzymes function in the cytosol of the cell, not the mitochondria.
Why is this important?
Although this discovery might not yet have an obvious medical use, it’s still pretty neat to know that there are species out that challenge the fundamental principles of biology! It’s very unlikely that Monocercomonoides is a missing link between prokaryotes and eukaryotes, but rather an adaptation to an extreme environment. Therefore, there could be many other eukaryotic microorganisms living in similar environments that also lack mitochondria, we just haven’t found them yet…
(TBY feels the chinchilla deserves an honourable mention for its generous contribution towards science- we hear that fortunately no chinchillas were hurt in the making of this paper)
Karnkowska et al, A Eurkaryote without a Mitochondrial Organelle, 2016. Current Biology. 26 (10), 1274-1284
Image credit: Monocercomonoides, by Dr Naoji Yubuki