E-G Models that propose the origin of mitochondria in a prokaryotic host, followed by the acquisition of eukaryotic-specific features. The relevant microbial players in each model are labelled. Archaebacterial and eubacterial lipid membranes are indicated in red and blue, respectively.
A methanogenic cell gets surrounded by several delta-proteobacteria. When the membranes of the delta-proteobacteria fuse, the original methanogenic cell forms the nucleus of the resulting eukaryotic cell. In panel c, an eocyte is represented by an oval outlined in red, and a gram-negative eubacterium is represented by an oval outlined in blue.
An arrow points to a star-shaped cell outlined in blue engulfing an oval shaped cell outlined in red. In panel d, an oval-shape with a blue outline represents a gram-positive eubacterium.
An arrow shows that the eubacterium evolves into a Neomuran, which is represented by a star-shaped cell with a blue outline. Arrows indicate that the Neomuran gives rise to both achaebacteria and eukaryotic lineages. Four arrows point upward from panels a through d to a star-shaped cell, which is outlined in blue and has a nucleus outlined in blue.
This illustration represents an amitochondriate eukaryote. Three arrows pointing to crosses indicate that some lineages from this amitochondriate eukaryote die off. A fourth arrow points to an oval shape outlined in blue, which represents an oxygen-consuming O 2 -consuming alpha-proteobacterium. Another arrow indicates that this alpha-proteobacterium gets engulfed by the amitochondriate eukaryote to produce a eukaryotic cell with mitochondria.
The three models illustrated in panels e through g show the acquisition of mitochondria before the acquisition of a nucleus.
The steps leading to a prokaryotic host with mitochondria are shown in the panels, and the acquisition of a nucleus is shown above. In panel e, an oval outlined in blue represents a hydrogen-producing H 2 -producing alpha-proteobacterium, and an oval outlined in red represents a hydrogen-consuming H 2 -consuming archaebacterium.
The result of this theoretical event is an archaebacterial host cell that contains the common ancestor of mitochondria and hydrogenosomes. In panel f, a blue oval represents an O 2 -consuming alpha proteobacterium, and a red oval represents an archaebacterium. An arrow represents an engulfment, after which the archaebacterial host contains a mitochondrial symbiont, which is represented by the oval outlined in blue inside the oval outlined in red. In panel g, an oval outlined in blue represents a hydrogen sulfide-consuming H 2 S-consuming alpha-proteobacterium, and a star shape outlined in red represents a hydrogen sulfide-producing H 2 S-producing archaebacterium.
An arrow points to a oval outlined in blue within a star-shaped cell outlined in red, which represents the archaebacterial host that contains a mitochondrial symbiont. Three arrows pointing from panels e through g to a eukaryotic cell with mitochondria indicate that the prokaryotic host cell with mitochondria acquires a nucleus to produce a mitochondriate eukaryote. Arrows pointing from this final eukaryotic cell indicate that eventually this progenitor cell produces progeny that lead to eukaryote diversification.
This view is linked to the ideas that the mitochondrial endosymbiont was an obligate aerobe, perhaps similar in physiology and lifestyle to modern Rickettsia species ; and that the initial benefit of the symbiosis might have been the endosymbiont's ability to detoxify oxygen for the anaerobe host. Because this theory presumes the host to have been a eukaryote already, it does not directly account for the ubiquity of mitochondria.
That is, it entails a corollary assumption an add—on to the theory that brings it into agreement with available observations that all descendants of the initial host lineage , except the one that acquired mitochondria, went extinct. The oxygen detoxification aspect is problematic, because the forms of oxygen that are toxic to anaerobes are reactive oxygen species ROS like the superoxide radical, O 2 -.
In eukaryotes, ROS are produced in mitochondria because of the interaction of O 2 with the mitochondrial electron transport chain. In that sense, mitochondria do not solve the ROS problem but rather create it; hence, protection from O 2 is an unlikely symbiotic benefit. This traditional view also does not directly account for anaerobic mitochondria or hydrogenosomes, and additional corollaries must be tacked on to explain why anaerobically functioning mitochondria are found in so many different lineages and how they arose from oxygen-dependent forebears.
An alternative theory posits that the host that acquired the mitochondrion was a prokaryote , an archaebacterium outright. This view is linked to the idea that the ancestral mitochondrion was a metabolically versatile, facultative anaerobe able to live with or without oxygen , perhaps similar in physiology and lifestyle to modern Rhodobacteriales. The initial benefit of the symbiosis could have been the production of H 2 by the endosymbiont as a source of energy and electrons for the archaebacterial host, which is posited to have been H 2 dependent.
This kind of physiological interaction H 2 transfer or anaerobic syntrophy is commonly observed in modern microbial communities. The mechanism by which the endosymbiont came to reside within the host is unspecified in this view, but in some known examples in nature prokaryotes live as endosymbionts within other prokaryotes. In this view, various aerobic and anaerobic forms of mitochondria are seen as independent, lineage-specific ecological specializations, all stemming from a facultatively anaerobic ancestral state.
Because it posits that eukaryotes evolved from the mitochondrial endosymbiosis in a prokaryotic host, this theory directly accounts for the ubiquity of mitochondria among all eukaryotic lineages. Eukaryotes are genetic chimeras. They possess genes that they inherited vertically from their archaebacterially related host.
Genes for cytosolic ribosomes in eukaryotes, for example, reflect that origin. But eukaryotes also possess genes that they inherited vertically from the endosymbiont - for example, mitochondrially encoded genes for mitochondrial ribosomes.
But even the largest mitochondrial genomes possess only about sixty protein-coding genes, while typical mitochondria harbor up to a thousand proteins or more that are encoded in the nucleus. During the course of mitochondrial genesis, many genes were transferred from the genome of the mitochondrial endosymbiont to the genome of the host.
This kind of endosymbiotic gene transfer is nothing unusual; endosymbiosis very often entails gene transfers from the endosymbiont to the host.
It happened during the origin of plastids too, and it is still ongoing in our own genome: Mitochondrial DNA constantly escapes from the organelle and becomes integrated as copies into nuclear DNA. The vast majority of mitochondrial proteins are encoded by nuclear genes, and many of these are endosymbiotic acquisitions from the mitochondrial ancestor. Figure 3 Figure Detail The oldest undisputedly eukaryotic microfossils go back 1.
Given the coincidence of mitochondria with the eukaryotic state, this can also be seen as a minimum age for mitochondria and a rough best-guess starting date for eukaryotic evolution. According to newer geochemical views, this date of origin corresponds to a protracted phase in Earth history when the oceans were mostly anoxic — from 1. Eukaryotes thus arose and diversified in an environment where anoxia was commonplace.
Accordingly it is hardly surprising that many independent eukaryotic lineages have preserved anaerobic energy-producing pathways in their mitochondria Figure 3. Like eukaryotes themselves, mitochondria appear to have arisen only once in all of evolution. The best evidence for the single origin of mitochondria comes from a conserved set of clearly homologous and commonly inherited genes preserved in the mitochondrial DNA across all known eukaryotic groups.
In the case of hydrogenosomes which usually lack DNA and mitosomes which so far always lack DNA , the strongest evidence for their common ancestry with mitochondria is twofold. First, aspects and components of the mitochondrial protein import process are conserved in hydrogenosomes and mitosomes, arguing strongly for common ancestry with mitochondria.
Second, all known lineages of eukaryotes that possess hydrogenosomes or mitosomes branch as sisters to mitochondrion-bearing lineages. Mitochondria arose once in evolution, and their origin entailed an endosymbiosis accompanied by gene transfers from the endosymbiont to the host. Anaerobic mitochondria pose a puzzle for traditional views on mitochondrial origins but fit nicely in newer theories on mitochondrial evolution that were formulated specifically to take the common ancestry of mitochondria and hydrogenosomes into account.
The presence of mitochondria in the eukaryote common ancestor continues to change the way we look at eukaryote origins, with endosymbiosis playing a more central role in considerations on the matter now than it did twenty years ago. The integral part that mitochondria play in many aspects of eukaryote biology might well reflect their role in the origin of eukaryotes themselves.
Boxma, B. An anaerobic mitochondrion that produces hydrogen. Nature , 74—79 doi Cox, C. The archaebacterial origin of eukaryotes. PNAS , — doi Dolezal, P. Evolution of the molecular machines for protein import into mitochondria. Science , — doi Dyall, S. Ancient invasions: From endosymbionts to organelles.
Embley, T. Nature , — doi But mitochondria serve other purposes, too. Mitochondria are also involved in the vital formation of iron-sulfur proteins. So the team hunted for evidence of a mitochondrial iron-sulfur cluster assembly pathway. This mechanism would be indispensable in a eukaryotic cell. Or so the scientists thought, until they found no evidence of such a mechanism in this eukaryote.
That doesn't mean the organism is surviving without iron-sulfur proteins. However, that piece of the puzzle is a bit more complex. Prokaryotes can assemble iron-sulfur clusters without mitochondria. So Karnkowska and her colleagues suspect that this eukaryote picked up a prokaryote's system through a process called horizontal gene transfer, also known as lateral gene transfer.
Horizontal gene transfer HGT happens when an organism receives genes directly from another organism rather than from parent to offspring. Once Monocercomonoides acquired that system, a cytosolic sulfur mobilization system SUF , from a bacteria via horizontal gene transfer, Karnkowska suggests, the mitochondria likely became unnecessary for the eukaryote to function, so the organelle disappeared over generations. Among scientists studying eukaryotic evolution, this finding may not be shocking, says Gertraud Burger, a researcher at the Robert-Cedergren Center for Bioinformatics and Genomics at the University of Montreal who was not part of the study.
It fits in a "series of strange eukaryotes that we see. Other organisms, including many in the same family with Monocercomonoides , have been found with few mitochondria. So this one, Dr. Burger says, "is just the endpoint of the other forms that we have seen before. Scientists have been hunting for an organism with no mitochondria for decades, says Karnkowska. Monocercomonoides was a good candidate in this search because it sits among organisms with remnant mitochondria on the eukaryote family tree.
But why look for such a strange cell? One reason could be to help explain early organismal evolution. How did organisms go from prokaryotes, simple single-celled organisms that are basically just bubbles of DNA and protein, in other words, bacteria and archaea, to the much more complex eukaryotes, which today range from single-celled organisms up to large animals such as elephants, giraffes, bears, and humans?
One explanation scientists have proposed is the archezoa hypothesis, says Burger. In that scenario, one single-celled organism, the ancestor to all eukaryotes, absorbed a prokaryote. That little prokaryote began working for the ancestral eukaryote, eventually becoming the mitochondria. In theory, finding an organism that lacks mitochondria could mean discovering this ancestral cell. But this organism provides no support for that hypothesis, Burger says.
It's clear that Monocercomonoides is not ancestral for a few reasons. The cell is quite complex and still has a nucleus and other organelles. It is also well situated on the Eukaryote family tree. But perhaps most importantly, Monocercomonoides resides within the gut of vertebrates, which themselves are multicellular eukaryotes.
So it's unlikely that this organism could have evolved before that environment existed. When the search began, the team wasn't sure if they would find any trace of a mitochondrion in this organism. Some scientists believe that the chloroplast, a similar organelle, descended from blue-green algae that eventually lost their ability to live outside cells, much like mitochondria.
Chloroplasts allow some eukaryotes, like plants and algae, to use sunlight to produce energy and oxygen for their cells, which is then used by their mitochondria.
Additionally, the hydrogenosome plays a similar role to the mitochondria, but function in oxygen-poor environments. These were originally known as fungi and one-celled eukaryotes, but have recently been found in very small, simple animals that live in oxygen-poor seafloors. TL;DR Too Long; Didn't Read The mitochondrion, sometimes called the "powerhouse of the cell," is common among complex organisms, which use the organelle to convert oxygen into energy.
The Characteristics of the Mitochondria. Can Eukaryotes Survive Without Mitochondria? What Are the Two Prokaryotic Kingdoms? Discovery of the Mitochondria. What Are the Benefits of Prokaryotes? What is a Unicellular Eukaryote? Organisms in the Kingdom Monera. Structural Characteristics of Blue-Green Algae. Extensions of the Cytoplasm.
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