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Endosymbiotic Theory
Were the cellular organelles mitochondria and chloroplasts of eukaryotic cells once free living prokaryotic organisms?
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In line with drestie's answer, the theory proposed by Margulis, et al is termed serial endosymbiosis theory (SET), which you probably already know. My understanding of the theory, (greatly simplified) is that possible evolution of eukaryotes from prokaryotes involved the symbiotic union of several previously independent ancestors. According to the theory, these ancestors included a host cell, an ancestor of mitochondria, an ancestor of chloroplasts, and, more controversially, a prokaryote that brought with it the structures that today provide cellular motion.(Source: The Serial Endosymbiosis Theory of Eukaryotic Evolution
by Jeremy Mohn)
An important work seeming to support Masrgulis definition of the SET theory occurred in the laboratory of Kwang W. Jeon, a biologist at the University of Tennessee. Jeon witnessed the establishment of an amoeba-bacteria symbiosis in which new bacterial symbionts became integrated in the host amoeba (Jeon 1991). In 1966, when the bacteria first infected the amoebas, they were lethal to their hosts. However, as time progressed, some of the infected amoebas survived and became dependent on their newly acquired endosymbionts within a few years. Jeon was able to prove this dependency by performing nuclei transplants between infected amoebas and amoebas lacking the bacteria. If left alone, the hybrid amoebas died in a matter of days. Yet if he reinfected these hybrids with the once-lethal bacteria, the amoebas recovered and once again began to grow (Margulis and Sagan 1987). So, nothing deffinintive but additional support for the theory relies on experiments in progress, in my opinion...
by Jeremy Mohn)
An important work seeming to support Masrgulis definition of the SET theory occurred in the laboratory of Kwang W. Jeon, a biologist at the University of Tennessee. Jeon witnessed the establishment of an amoeba-bacteria symbiosis in which new bacterial symbionts became integrated in the host amoeba (Jeon 1991). In 1966, when the bacteria first infected the amoebas, they were lethal to their hosts. However, as time progressed, some of the infected amoebas survived and became dependent on their newly acquired endosymbionts within a few years. Jeon was able to prove this dependency by performing nuclei transplants between infected amoebas and amoebas lacking the bacteria. If left alone, the hybrid amoebas died in a matter of days. Yet if he reinfected these hybrids with the once-lethal bacteria, the amoebas recovered and once again began to grow (Margulis and Sagan 1987). So, nothing deffinintive but additional support for the theory relies on experiments in progress, in my opinion...
.Not really Drestie but thanks for the vote of confidence. I thought that it was the prevailing theory due to the research stated by you and Clanad; and that they have their own DNA which is similar to many simple organisms; and they grow and split in a form of binary fission much like bacteria. So there is a lot of information about how they look and how they behave which supports the theory. But the Prof said something doubting it and it made me think. So I thought i'd ask the question.
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Drestie: He just said something about it not being the prevailing theory and It got me wondering what competing theories had to say.
Mr Pedant: I should have qualified my question. I am looking for information on why SET could be controversial (if indeed it is)? And, what are alternative scientific theories to SET.
Thanks
D
Mr Pedant: I should have qualified my question. I am looking for information on why SET could be controversial (if indeed it is)? And, what are alternative scientific theories to SET.
Thanks
D
Apologies for not contributing to this new thread earlier - it's been a bit hectic here at work. Please forgive me for not having discussed alternative theories to SET in the original post.
I've noticed growing support over the last few years for the theories of Michael Gray of Dalhousie University, highly regarded in the field of molecular evolutionary biology.
He has published papers proposing that mitochondria arose in a common ancestral extinct ancestor of all extant eukaryotes and raises the possibility that mitochondria arose at the same time as the nuclear component of the cell rather than in a separate subsequent event as proposed by the SET theory.
Much of the data about mitochondrial origins and evolution is based upon large scale organelle genome sequencing projects, to which Gray et al have contributed vast amounts of data.
Gray states that studies of mtDNA suggests that mitochondrial ancestry can be traced to a single ancestor related to Proteobacteria genera such as Rickettsia, Anaplasma and Ehrlichia which are known to be the closest relatives of mitochondria.
Gray gave a very illuminating lecture on his theories at York University around eight years ago, which enthralled the audience, many of whom had been convinced to date that Margulis had been correct.. Regretfully, I was not invited to attend.
(Cont)
I've noticed growing support over the last few years for the theories of Michael Gray of Dalhousie University, highly regarded in the field of molecular evolutionary biology.
He has published papers proposing that mitochondria arose in a common ancestral extinct ancestor of all extant eukaryotes and raises the possibility that mitochondria arose at the same time as the nuclear component of the cell rather than in a separate subsequent event as proposed by the SET theory.
Much of the data about mitochondrial origins and evolution is based upon large scale organelle genome sequencing projects, to which Gray et al have contributed vast amounts of data.
Gray states that studies of mtDNA suggests that mitochondrial ancestry can be traced to a single ancestor related to Proteobacteria genera such as Rickettsia, Anaplasma and Ehrlichia which are known to be the closest relatives of mitochondria.
Gray gave a very illuminating lecture on his theories at York University around eight years ago, which enthralled the audience, many of whom had been convinced to date that Margulis had been correct.. Regretfully, I was not invited to attend.
(Cont)
I have not read extensively the theories of Gray et al myself and I cannot recall all the issues he raised. Nevertheless, I can recall that one of the points he made was that mitochondria and plastids cannot survive outside the cell wall due to the loss of the essential genes required for survival. However other workers have pointed out that due to the large time span mitochondria and plastids have co-existed with their host, they were no longer necessary and were effectively deleted. Evidence of transfer into the host genome has also been found.
Nevertheless other workers have investigated the theories of Margulis and her collaborators. These include the likes of D Lloyd, �The Mitochondria of Microorganisms�(1974). R.Raff, H.Mahler and Gray and Doolittle( �Has the Endosymbiant Hypothesis Been Proven?� Microbiological Review 1982 vol 30 p46). Invariably the SET theory has been invalidated in the view of these workers.
In addition Whitfield when reviewing a later text by Margulis - �Symbiosis in cell evolution� (�Book Review of Symbiosis in Cell Evolution, Bilogical Journal of Linnean Society, 1982, pp77-79) stated that::
�Prokaryotic endocytosis is the cellular mechanism on which the whole of SET presumably rests. If one prokaryote could not engulf another it is difficult to imagine how endosymbiosis could be set up. Unfortunately for Margulis and SET, no modern examples of prokaryotic endocytosis or endosymbiosis exist�.
The events in the laboratory of Kwang W Jeon have not resulted in Whitfield revising his statement as far as I can ascertain.
Nevertheless other workers have investigated the theories of Margulis and her collaborators. These include the likes of D Lloyd, �The Mitochondria of Microorganisms�(1974). R.Raff, H.Mahler and Gray and Doolittle( �Has the Endosymbiant Hypothesis Been Proven?� Microbiological Review 1982 vol 30 p46). Invariably the SET theory has been invalidated in the view of these workers.
In addition Whitfield when reviewing a later text by Margulis - �Symbiosis in cell evolution� (�Book Review of Symbiosis in Cell Evolution, Bilogical Journal of Linnean Society, 1982, pp77-79) stated that::
�Prokaryotic endocytosis is the cellular mechanism on which the whole of SET presumably rests. If one prokaryote could not engulf another it is difficult to imagine how endosymbiosis could be set up. Unfortunately for Margulis and SET, no modern examples of prokaryotic endocytosis or endosymbiosis exist�.
The events in the laboratory of Kwang W Jeon have not resulted in Whitfield revising his statement as far as I can ascertain.
The Turkish evolutionist Ali Demirsoy admits that one of the most difficult stages to be explained in evolution is to scientifically explain how organelles and complex cell developed from bacteria. He points out that no transitional form between prokaryotes and eukaryotes has ever been found. One-celled to multi-cellular creatures carry all this complicated structure, and no creature or group has been found with organelles of a simpler construction in any way, or which are more primitive.
Demirsoy has cited the following structural differences between bacteria and plant cells as evidence against the SET theory.
(Cont)
Demirsoy has cited the following structural differences between bacteria and plant cells as evidence against the SET theory.
(Cont)
The walls of bacterial cells are formed of polysaccharide and protein, whilst the walls of plant cells are formed of unrelated cellulose.
The DNA of bacteria is a closed loop, whilst the DNA of plants is linear.
The DNA structures in plant and bacterial cells are different.
The DNA molecule in plant cells is protected by a double layered membrane, whereas the DNA in bacterial cells is free.
While plant cells possess many organelles covered in membranes and possessing very complex structures,
bacterial cells lack organelles. In bacterial cells there are merely freely moving ribosomes. Plant ribosomes are larger and are attached to the cell membrane. Furthermore, the mechanism for protein synthesis differs in the two types of ribosomes.
The DNA molecule in bacterial cells carries information belonging to just one cell, but in plant cells the DNA
molecule carries information about the entire plant.
The biochemistry of messenger RNA formation in bacterial cells is very different from that in eukaryotic cells. Darnell
postulated some years ago that the differences were so profound, they suggest that prokaryotic to eukaryotic cell
cell evolution seems unlikely. ( Darnell, �Implications of
RNA-RNA Splicing in Evolution of Eukaryotic Cells� Science vol.202, 1978, p1257)
Some species of bacteria are photosynthetic eg. Cyanobacteria. However, the bacteria do not contain
chloroplasts containing chlorophyll and other photosynthetic pigments. Instead, the molecules are buried throughout the bacterial cell.
Finally, I have to say that I do not advocate one theory over any other. I have outlined them here merely to provide a sense of balance to the original question.
The DNA of bacteria is a closed loop, whilst the DNA of plants is linear.
The DNA structures in plant and bacterial cells are different.
The DNA molecule in plant cells is protected by a double layered membrane, whereas the DNA in bacterial cells is free.
While plant cells possess many organelles covered in membranes and possessing very complex structures,
bacterial cells lack organelles. In bacterial cells there are merely freely moving ribosomes. Plant ribosomes are larger and are attached to the cell membrane. Furthermore, the mechanism for protein synthesis differs in the two types of ribosomes.
The DNA molecule in bacterial cells carries information belonging to just one cell, but in plant cells the DNA
molecule carries information about the entire plant.
The biochemistry of messenger RNA formation in bacterial cells is very different from that in eukaryotic cells. Darnell
postulated some years ago that the differences were so profound, they suggest that prokaryotic to eukaryotic cell
cell evolution seems unlikely. ( Darnell, �Implications of
RNA-RNA Splicing in Evolution of Eukaryotic Cells� Science vol.202, 1978, p1257)
Some species of bacteria are photosynthetic eg. Cyanobacteria. However, the bacteria do not contain
chloroplasts containing chlorophyll and other photosynthetic pigments. Instead, the molecules are buried throughout the bacterial cell.
Finally, I have to say that I do not advocate one theory over any other. I have outlined them here merely to provide a sense of balance to the original question.
Thanks dawkins.
On balance, I consider that the SET theory remains the most plausible explanation to date despite the work of Gray et al.
Gray and his colleagues leave too many unanswered questions for my mind. For example:
Plastids are present in very different groups of protists, some of which are closely related to forms that lack plastids. This suggests that if chloroplasts originated de novo, they did so multiple times, in which case their close similarity to each other defeats explanation. Many of these protists contain secondary plastids that have been acquired from other plastid-containing eukaryotes and not from cyanobacteria directly.
Much of the internal structure and biochemistry of plastids eg the presence of thylakoids and chlorophyll is very similar to that of cyanobacteria. Phylogenetic estimates constructed with bacteria, plastids and eukaryotic genomes also suggest that plastids are related to cyanobacteria.
DNA sequence analysis and phylogenetic estimates suggest that nuclear DNA contains genes that probably came from the plastid.
Mitochondrial and plastid DNA differs in structure from that of the DNA in the cell nucleus, but does bear a resemblance to the DNA contained within bacteria.
Mitochondria and plastids are surrounded by at least two membranes. The innermost membrane is unique in composition compared to the other membranes in the cell although it does closely resemble that of a prokaryote cell membrane.
(Cont)
On balance, I consider that the SET theory remains the most plausible explanation to date despite the work of Gray et al.
Gray and his colleagues leave too many unanswered questions for my mind. For example:
Plastids are present in very different groups of protists, some of which are closely related to forms that lack plastids. This suggests that if chloroplasts originated de novo, they did so multiple times, in which case their close similarity to each other defeats explanation. Many of these protists contain secondary plastids that have been acquired from other plastid-containing eukaryotes and not from cyanobacteria directly.
Much of the internal structure and biochemistry of plastids eg the presence of thylakoids and chlorophyll is very similar to that of cyanobacteria. Phylogenetic estimates constructed with bacteria, plastids and eukaryotic genomes also suggest that plastids are related to cyanobacteria.
DNA sequence analysis and phylogenetic estimates suggest that nuclear DNA contains genes that probably came from the plastid.
Mitochondrial and plastid DNA differs in structure from that of the DNA in the cell nucleus, but does bear a resemblance to the DNA contained within bacteria.
Mitochondria and plastids are surrounded by at least two membranes. The innermost membrane is unique in composition compared to the other membranes in the cell although it does closely resemble that of a prokaryote cell membrane.
(Cont)
Mitochondrial and plastid ribosomes are similar to those found in bacteria.
New mitochondria and plastids are formed through a process similar to binary fission. In some algae spp such as Euglena, the plastids can be destroyed by chemicals or prolonged absence of light without affecting the cell. If this occurs, the plastids will not regenerate.
Some of the proteins encoded in the nucleus are transported to the organelle and both mitochondria and plastids have small genomes compared to bacteria. This is consistent with an increased dependance on the eukarotic host after forming an endosymbiosis. Most genes on the organelle genomes will have been lost or moved to the nucleus. Most genes needed for mitochondrial and plastid function are located in the nucleus and many originate from the bacterial endosymbiont.
Okamoato and Inouaye in 2005 reported that a heterotrophic protist ingested a green algae, which lost its flagella and cytoskeleton, while the host switched to photosynthetic nutrition and gained the ability to become phototropic. This was regarded as a possible secondary endosymbiosis ( ie involving eukaryotic plastids).
Anyhow that�s enough for tonight � I can see you yawning from here. Do you know dawkins, I find it very odd being able to sit here and speak for and against the SET theory. Talk about sitting on the fence!
...nope... an 'Intelligent Designer' would not stupidly go and make a new design for every living creature, instead he would use blueprints that can be used in other creatures and work in them as well, ie: circular DNA in mitochondria as well as in prokaryotes. The same goes with the genetic information in chimpanzees, rats and other organisms that have a lot of similarity and in some instances are identical.
Would the manufacturer of a car redesign every part separately for the sake of some misguided uniqueness to prove that each model was different?, or would he use the same parts that could be interchanged here and there in various models and keep a common thread of consistency with his designs that would save time and money, and more readily be exchanged/obtained? Then, how would that 'car designer/manufacturor' regard those who then philosophised and theorised that one of his cars morphed and rearranged itself into another car because some of the internal parts were similar and interchangeable? Would he regard that as an 'intelligent assumption' ?? LOL
Would the 'Intelligent Designer' have had common genetic blueprints with this in mind as a form of 'universal economy' and to maintain some kind of universal homoeostasis in all life forms ?!!!!
Because the humble 'walnut' kernel looks like 2 hemispheres of the human brain, does that mean that once upon a time, the brains of 'evolutionists' evolved at some point from a walnut kernel?? :-)
Would the manufacturer of a car redesign every part separately for the sake of some misguided uniqueness to prove that each model was different?, or would he use the same parts that could be interchanged here and there in various models and keep a common thread of consistency with his designs that would save time and money, and more readily be exchanged/obtained? Then, how would that 'car designer/manufacturor' regard those who then philosophised and theorised that one of his cars morphed and rearranged itself into another car because some of the internal parts were similar and interchangeable? Would he regard that as an 'intelligent assumption' ?? LOL
Would the 'Intelligent Designer' have had common genetic blueprints with this in mind as a form of 'universal economy' and to maintain some kind of universal homoeostasis in all life forms ?!!!!
Because the humble 'walnut' kernel looks like 2 hemispheres of the human brain, does that mean that once upon a time, the brains of 'evolutionists' evolved at some point from a walnut kernel?? :-)
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