Imagine meeting another human who lacks a heart, lungs or some other crucial organ, and yet seems to be functioning completely normally. An international team of scientists has discovered the single-celled version of this conundrum: a eukaryotic microbe that has lost its mitochondrion, which scientists long thought was essential for these complex cellular organisms.
The microbe Monocercomonoides sp., described in the journal Current Biology, upends the long-held assumption that mitochondria are essential to eukaryotic cells as we know them. The findings also shed light on the evolutionary pressures that have made the partnership between cells and their mitochondria so profoundly successful – and reveal the situations when that relationship may fall apart.
The discovery showcases "an example of the amazing evolutionary plasticity of eukaryotic cells," said first author Anna Karnkowska, a protistologist at the University of British Columbia in Vancouver who performed the research while she was a post-doctoral fellow at Charles University in Prague.
Eons ago, when the world was ruled by primitive single-celled creatures called prokaryotes, two of these microbes joined forces, forming an unlikely partnership that would fundamentally alter the direction of life on Earth.
In a microscopic mix-up, scientists think one microbe ended up inside the other. Such encounters usually spell doom for at least one of the involved parties. But instead of dying, that trapped microbe thrived – and eventually gave rise to mitochondria, providing the extra energy that the larger cell needed to grow in ways that had simply been impossible before.
Eukaryotes are fundamentally different from prokaryotes because they contain membrane-bound structures called organelles – including a nucleus, which houses the cell's DNA. In prokaryotes, the DNA floats freely in the cell. Mitochondria, one of those organelles, are thought to be a fundamental feature of eukaryotes.
Mitochondria are known as the powerhouses of the eukaryotic cell, providing it energy in the form of a molecule known as adenosine triphosphate, or ATP. They carry their own DNA, separate from the genetic information housed in the cell's nucleus. This genetic heritage is passed from mother to daughter in humans and has been used to trace matrilineal ancestry.
The power boost that mitochondria gave to eukaryotic cells seems to have made multicellular life possible. Pretty much all life that you can see with your eyes (and much of the life you can't) is eukaryotic – including plants, animals, algae and fungi.
Researchers have long thought that mitochondria were essential for eukaryotes, whether multi- or single-celled. Even in anaerobic microbes, which operate in environments poor in oxygen (which the mitochondria need in order to do their job), scientists have still found mitochondria – although often in a 'reduced' form, shrunken or lacking the characteristic wrinkles on their surface.
There were a few false alarms; for a brief time, scientists thought that the microbe Giardia intestinalis, which causes diarrhea in humans, had no mitochondria – but the missing organelles were always eventually found, even if they were a shadow of their former selves.
Perhaps that's because mitochondria turn out to be essential in other ways. As it turns out, there's a lesser-known function that the mitochondria play even in these low-oxygen environments: They build a molecular complex made of iron and sulfur that turns out to be an essential component of several crucial proteins in the cell.
But for this study, the researchers decided to take a look at another weird anaerobic microbe that they thought just might be the critter they were looking for: Monocercomonoides, a protozoa with whip-like flagella that hangs out in the guts of small mammals. They sifted though the DNA in the cell, looking for key fragments of mitochondrial DNA – but came up short. This microbe, somehow, seemed to be doing just fine without that organelle.
"This is exciting for me and for many of us searching for these very divergent eukaryotes, which of course help us to understand better the whole evolution of eukaryotes," Karnkowska said.
Just as the marriage between two powerful characters in our evolutionary history may fascinate us, so too does the divorce. What happened to precipitate the dissolution of such a fruitful relationship?
In this case, as happens so often in human romantic dramas, there was a third party involved. Monocercomonoides actually managed to replace the mitochondria's genes for building iron-sulfur complexes with genes from another bacteria for a very different system, called cytosolic sulfur mobilization. With those new genes available, the mitochondria may have become truly obsolete, and was eventually abandoned entirely.
Karnkowska said the researchers will continue to study this strange microbe's genome.
"We want to know how divergent it is ... why it's so weird," she said.
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