The acquisitions of mitochondria and plastids were important events in the evolution of the eukaryotic cell, supplying it with compartmentalized bioenergetic and biosynthetic factories. Ancient invasions by eubacteria through symbiosis more than a billion years ago initiated these processes. Advances in geochemistry, molecular phylogeny, and cell biology have offered insight into complex molecular events that drove the evolution of endosymbionts into contemporary organelles. In losing their autonomy, endosymbionts lost the bulk of their genomes, necessitating the evolution of elaborate mechanisms for organelle biogenesis and metabolite exchange.
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This page has been archived and is no longer updated. What variety is there in mitochondria? Mitochondria occur in various forms across various eukaryotic groups, yet considerations on the origin of mitochondria sometimes neglect this understanding.
Four main mitochondrial types can be distinguished on the basis of functional criteria concerning how or whether ATP is produced. These functional types do not correspond to natural groups, because they occur in an interleaved manner across the tree of eukaryotic life. Instead they correspond to ecological specializations. Trachipleistophora and Cryptosporidium mitosomes denotes that these organisms are not anaerobes in the sense that they do not inhabit O 2 -poor niches, but that their ATP supply is apparently O 2 -independent.
A Schematic summary of salient biochemical functions in mitochondria, including some anaerobic forms. B Schematic summary of salient biochemical functions in hydrogenosomes. C Schematic summary of available findings for mitosomes and 'remnant' mitochondria. The asterisk next to the Trachipleistophora and Cryptosporidium mitosomes denotes that these organisms are not anaerobes in the sense that they do not inhabit O 2 -poor niches, but that their ATP supply is apparently O 2 -independent.
Eukaryotic evolution, changes and challenges. Nature , — All rights reserved. The double membranes that enclose the organelles are depicted as rectangles with rounded corners. A green oval on the bottom membrane of each organelle indicates that there is protein import. Iron-sulfur Fe-S clusters, which are used in electron transport, are depicted as clusters of small black and yellow circles.
Proteins are either represented by their names in text or by circles with labels. The colors of the circles indicate closer homology to eubacteria or archaebacteria. Mitochondria are shown in panel A. The left side of this panel represents aerobic mitochondria, and the right side of this panel represents anaerobic mitochondria. Hsp60 and Hsp70 are written on the dashed black line between the two sides because these proteins are found in both types of mitochondria.
Aerobic mitochondria produce ATP in a biochemical pathway where oxygen is the terminal electron acceptor. STK is shown in blue to indicate that it has more similarity to eubacterial homologs, and MCF is shown in green because it lacks a clear prokaryotic homolog.
Text at the bottom of the aerobic mitochondria diagram indicates that the Krebs cycle and the mitochondrial complexes are found in most life forms. The right side of panel A represents anaerobic mitochondria, which do not use O 2 as the terminal electron acceptor.
They also contain the following proteins, which are represented by blue circles to indicate that they have more similarity to eubacteria: pyruvate:ferredoxin oxidoreductase PFO , ferredoxin Fd , iron-only hydrogenase HDR , pyruvate:formate lyase PFL , bi-functional alcohol acetaldehyde dehydrogenase ADHE , and acetate-succinate CoA transferase ASC.
Hydrogenosomes, which are anaerobic, are shown in panel B. ASC is represented by grey circles because its presence is based only on biochemical data. MCF is represented by a green circle because it lacks a clear prokaryotic homolog. Mitosomes, which are also anaerobic, and remnant mitochondria are shown in panel C. A general mitosome is shown at the top of panel C, and the bottom shows four smaller rectangles, which represent the organelles in specific organisms.
ACS ADP , which is represented by a red circle to show that it has greater similarity to the archaebacterial homolog, is shown at the top section of the panel. The Trachipleistophora rectangle contains Hsp70, and the Cryptosporidium rectangle contains Hsp60 and Hsp The mitochondria typical of mammalian cells respire O 2 during the process of pyruvate breakdown and ATP synthesis, generating water and carbon dioxide as end products.
The Krebs cycle and the electron transport chain in the inner mitochondrial membrane enable the cell to generate about 36 moles mol of ATP per mole of glucose, with the help of O 2 —respiring mitochondria.
Such typical mitochondria also occur in plants and various groups of unicellular eukaryotes protists that, like mammals, are dependent on oxygen and specialized to life in oxic environments. In contrast, the mitochondria of many invertebrates worms like Fasciola hepatica and mollusks like Mytilus edulis being well—studied cases do not use O 2 as the terminal acceptor during prolonged phases of the life cycle. These mitochondria allow the anaerobically growing cell to glean about 5 mol of ATP per mole of glucose, as opposed to about 36 with O 2.
The typical excreted end products are carbon dioxide, acetate, propionate, and succinate, which are generated mostly through the rearrangement of Krebs cycle reactions and the help of the mitochondrial electron transport chain. These organelles are commonly called anaerobic mitochondria.
Mitochondria of yet another kind yield even less ATP per molecule of glucose. These are mitochondria of several distantly related unicellular eukaryotes protists that lack an electron transport chain altogether.
They synthesize ATP from pyruvate breakdown via simple fermentations that typically involve the production of molecular hydrogen as a major metabolic end product. These mitochondria are called hydrogenosomes and allow the cell to gain about 4 mol of ATP per mole of glucose. Hydrogenosomes were discovered in in trichomonads, a group of unicellular eukaryotes. They were later found in chytridiomycete fungi that inhabit the rumen of cattle, as well as some ciliates, and they continue to be found in other groups.
The enzymes of hydrogenosomes are not unique to these anaerobes. They are found also in the mitochondria, the cytosol , or even the plastids of other eukaryotes Figure 1.
A fourth category of eukaryotes possesses small, inconspicuous mitochondria that are not involved in ATP synthesis at all. These eukaryotes synthesize their ATP in the cytosol with the help of enzymes that are otherwise typically found in hydrogenosomes.
They obtain mol of ATP per mole of glucose. Their typical end products are carbon dioxide, acetate, and ethanol, and their mitochondria are called mitosomes. Mitosomes were discovered in the human intestinal parasite Entamoeba histolytica in , and were subsequently found in many additional eukaryotes, including Giardia lamblia in There are currently two main, competing theories about the origin of mitochondria.
They differ with regard to their assumptions concerning the nature of the host, the physiological capabilities of the mitochondrial endosymbiont, and the kinds of ecological interactions that led to physical association of the two partners at the onset of symbiosis.
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.
Ancient invasions: from endosymbionts to organelles
This page has been archived and is no longer updated. What variety is there in mitochondria? Mitochondria occur in various forms across various eukaryotic groups, yet considerations on the origin of mitochondria sometimes neglect this understanding. Four main mitochondrial types can be distinguished on the basis of functional criteria concerning how or whether ATP is produced. These functional types do not correspond to natural groups, because they occur in an interleaved manner across the tree of eukaryotic life. Instead they correspond to ecological specializations.
Endosymbiosis, cell evolution, and speciation
Skip to search form Skip to main content You are currently offline. Some features of the site may not work correctly. DOI: Dyall and Mark T. Brown and Patricia J.
Ancient Invasions: From Endosymbionts to Organelles
Ancient invasions: from endosymbionts to organelles.