Star Wars bacterium provides glimpse into the evolution of life

Fans of Star Wars would know of Midi-chlorians as helpful microbes that live inside cells granting the mystical power known as “The Force”, but a bacterium of the same name has provided new clues into the evolution of our own cells and how they came to possess the vital energy-producing units called mitochondria.

University of Sydney research on the bacterium Midichloria mitochondrii, which is named after the Force-generating microbes, has revealed that mitochondria may have entered our cells though a parasitic bacterium that swam around using a tail and could survive with almost no oxygen.

The work, published in the current issue of Molecular Biology and Evolution, challenges traditional explanations of how the ancestors of mitochondria first entered our cells 1.5–2 billion years ago and sheds new light on a process recognised as one of the major transitions in the history of life on earth.

“Our results challenge the paradigm, shown in every biology textbook, that mitochondria were passive bacteria that were gobbled up by a primordial cell. We have found instead that the mitochondrial ancestor most likely had a flagellum, so was able to move, and possibly acted as a parasite, rather than prey, on early eukaryotic cells”, says co-author Dr Nathan Lo, from the School of Biological Sciences, who collaborated with scientists from Italy and Spain.

The most conspicuous and complex organisms on earth are eukaryotes, which include all forms of life besides bacteria and archaea. Eukaryotes differ from bacteria and archaea – called prokaryotes – by a number of key features. One of these is the presence of mitochondria, which are like little batteries that generate energy to power the cell.

“How eukaryotic cells evolved some 1.5–2 billion years ago remains one of the most vexing problems in biology and part of the knowledge gap involves how mitochondria came to be in the cells”, said Dr Lo.

“Mitochondria are actually highly reduced bacteria, with their own set of DNA, that reside in our cells. It has long been thought that this relationship developed when an ancient eukaryotic cell engulfed the mitochondrial bacteria.

“But there is still mystery surrounding a number of issues. First, how exactly was the mitochondrial ancestor engulfed? Second, how did the mitochondrial ancestor, which apparently required oxygen, survive in the oxygen-poor atmosphere of early eukaryotic life?”

To search for clues, Dr Lo and collaborators studied the bacterium Midichloria mitochondrii from the family Rickettsiales, which are considered to be the closest living relatives of the ancestor of mitochondria.

“We chose to study M. mitochondrii because its genome has never been analysed and because it is the only bacterium known to be able to enter into the mitochondria of living cells”, said Dr Lo who discovered M. mitochondrii in 2004 and successfully obtained permission from Star Wars director, George Lucas, to name the bacterium after the Midi-chlorians.

After determining the DNA sequence of the entire genome of M. mitochondrii, Dr Lo and collaborators found that the bacterium contained 26 genes coding for an entire flagellum – including all the key components such as hook, filament, and basal body. He also found a second set of genes, which coded for enzymes that would allow the bacterium to survive in low-oxygen environments. These genes have never been seen before in bacterial relatives of mitochondria.

Dr Lo says: “Our analyses show that these two sets of genes were inherited from the common ancestor shared by M. mitochondrii and our own mitochondria. This confirms that the ancestor of mitochondria most likely possessed a flagellum, which is a key characteristic of many parasitic bacteria. Our results show that the ancestor of mitochondria probably played a much more active, even parasitic, role in the early interactions with its eukaryotic host than previously thought and also provide an explanation for how the relationship could have evolved in the low-oxygen environments 1–2 billion years ago.

“This should cause us to rethink how the symbiosis between mitochondria and eukaryotic cells originally developed, which is one of the most controversial topics in biology.”

Read the scientific publication here:
Phylogenomic evidence for the presence of a flagellum and cbb3 oxidase in the free-living mitochondrial ancestor
Sassera, Lo, Epis, D’Auria, Montagna, et al. (2011) Molecular Biology and Evolution, 28: 3285–3296