Endosymbiotic Theory

[SilentSilo]

I am new to this forum, but wanted some thoughts on endosymbiosis. Does anyone here have any real objection to the theory? Honestly, I find it to be a very great idea.

[CharonY]

Not that I know of. There are just too many hints from various directions that point at the bacterial origin of mitchondria.

[lucaspa]

Nearly all biologists accept endosymbiosis as the explanation for mitochondria and chloroplasts in eukaryotic cells. The objections come when some people try to extend the theory

1) to other organelles (such as the Golgi or lysosomes)

2) to say that most of evolution resulted from endosymbiosis.

[thedarkshade]

Well it is much much better then the other "Procaryots somehow in some moment became ekucaryots" view!

[lucaspa]

Well it is much much better then the other "Procaryots somehow in some moment became ekucaryots" view!

 

Endosymbiosis does not explain the existence of a nucleus -- eukaryotes have and prokaryotes do not -- nor the existence if introns in eukaryotes and their absence in prokaryotes. It only explains the origin of 2 organelles and the ability to move.

http://www.geocities.com/jjmohn/endosymbiosis.htm

 

And the theory is not the way you stated. Instead, there is an acknowledged lack of data on the origin of eukaryotes if prokaryotes are thought to have evolved first. However, people are working on it: http://www.ncbi.nlm.nih.gov/pubmed/16615090

W. Martin and M. Müller, Nature 392, 37 (1998).

 

A rival theory is that cells started out as eukaryotes with nuclei and prokaryotes are derived from them. This has support for protocells - the only experimentally demonstrated way to get cells from non-living chemicals. The internal structure of protocells is analogous to an onion -- there are a series of membranes of decreasing size. The later evolution of the DNA/RNA and directed protein synthesis would then have the DNA within one of the layers, which later became the nucleus. Prokaryotes would then be a simplified version of eukaryotes.

[PhDP]

I am new to this forum, but wanted some thoughts on endosymbiosis. Does anyone here have any real objection to the theory? Honestly, I find it to be a very great idea.

 

It depends. When it comes to plastids and mitochondria, endosymbiosis is a solid hypothesis.

 

But about the evolution of other structures (i.e.: the flagellum), there's little positive evidences. Overall, I think it's fair to say that endosymbiosis was a great idea, but fails to live up to some expectations. It's probably a minor mechanism.

[foodchain]

Another competitive hypothesis from such is that endosymbiosis primarily operated on horizontal gene transfer and when you reached having vertical gene transfer is when eukaryotes and sexual reproduction came about I think. In its that the endosymbiosis allowed for selection in the form of vertical gene transfer.

 

My take follows.

 

Simply put eukaryotic life all posses mitochondria as far as I know, if such major divisions of life existed how did mitochondria for instance come to exist at large in eukaryotes then? Did all eukaryotic life simply die out without such, how did they even make it to endosymbiosis in the first place then? Also as a somewhat obvious question, bacteria would have to develop mitochondria and chloroplasts after evolving past being like a eukaryote cell, which seems to be an incredibly fixed trait, not saying it cant happen, then they would have to engage in endosymbiosis with eukaryotes and pass such off. I just don’t see how so much structure as found in a common eukaryotic cell would simply die out in the form of something like becoming bacteria, I don’t see how selection could favor this. I could see however selection favoring vertical gene transfer in a form that would allow say for a cell to maintain fitness, such as its own enzymes or even genetic pathways for such along with regulatory mechanisms.

[jimmydasaint]

I think compelling evidence for the existence of prokaryotes before eukaryotes in endosymbiosis comes from the sheer similarity of mitochondrial and rickettsial DNA as well as other bacterial features. Moreover, this is going on today in real life -

 

" In two groups, the eukaryotic nature of the endosymbiont can be seen by its retention of a vestige of a nucleus (called its nucleomorph).

 

A group of unicellular, motile algae called cryptomonads appear to be the evolutionary outcome of a nonphotosynthetic eukaryotic flagellate (i.e., a protozoan) engulfing a red alga by endocytosis.

 

Another tiny group of unicellular algae, called chlorarachniophytes, appear to be the outcome of a flagellated protozoan having engulfed a green alga."

 

These are functional living organisms.

 

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/E/Endosymbiosis.html

[lucaspa]

Another competitive hypothesis from such is that endosymbiosis primarily operated on horizontal gene transfer

 

This is different. Horizontal or lateral gene transfer is taking parts of DNA and sharing them across species lines. Endosymbiosis is when an entire organism was incorporated into another organism.

 

Simply put eukaryotic life all posses mitochondria as far as I know, if such major divisions of life existed how did mitochondria for instance come to exist at large in eukaryotes then? Did all eukaryotic life simply die out without such, how did they even make it to endosymbiosis in the first place then? Also as a somewhat obvious question, bacteria would have to develop mitochondria and chloroplasts after evolving past being like a eukaryote cell,

 

Imagine 1-2 billion years ago. There are lots of species of bacteria and Archaea and lots of species of eukaryotes. Some of the eukaryotes may even have been multicellular. The eukaryotes have a nucleus but they are not necessarily engaging in sexual reproduction: lots of unicellular eukaryotes reproduce by fission just like bacteria. Bacteria, archaea, and eukaryotes are all doing some form of oxidative phosphorylation (ox-phos) in the cytoplasm. Some bacteria and some eukaryotes are photosynthesizing in the cytoplasm.

 

Then one species of unicellular eukaryote is parasitized by a bacteria that is very good at ox-phos. This parasitism becomes mutualism over the course of generations as both bacteria and eukaryote adapts. Both gain. The bacteria gains if it is less pathological because it now doesn't kill the host and the eukaryote gains a more efficient source of energy in the ox-phos of the bacteria. Then each specializes: the eukaryote no longer makes the ox-phos enzymes and the bacteria no longer makes most of the metabolic enzymes. Both save in terms of energy cost in not having to make all the proteins and duplicate them. But the eukaryote-bacteria endosymbiosis is benefitting because it produces energy more efficiently and abundantly than eukaryotes without the bacteria. Thus this particular eukaryote symbiosis replaces all eukaryotes.

 

After that happens the eukaryotes with mitochondria diversify again and have many species. One of those species then undergoes another symbiotic event with a photosynthesizing bacteria to have chloroplasts. Again, this symbiosis is beneficial because the bacteria is more efficient at photosynthesizing than all the competitors and benefits in the eukaryote in not having to make all the proteins for metabolism -- it uses the eukaryotes anabolic pathways and, of course, the energy from ox-phos in the mitochondria from the earlier endosymbiosis.

 

I think compelling evidence for the existence of prokaryotes before eukaryotes

 

That still doesn't provide compelling evidence of prokaryotes before eukaryotes. Prokaryotes could still have been a simplification of eukaryotes as a parasitical/disease form. It was only after that simplification to prokaryotes that endosymbiosis took place. Remember, the characteristics of eukaryotes are the nucleus and introns in their DNA. The web page never suggests that the nucleus came from endosymbiosis. Rather, endosymbiosis gives some organelles within eukaryotes. So, life could still have started as eukaryotes and prokaryotes represent evolutionary simplification and loss of traits.

 

Thank you for the other examples of contemporary endosymbiosis in progress. They are not as well-accepted as mitochondria and chloroplasts. That endosymbiosis still apparently happens between contemporary eukaryotes and prokaryotes provides strong evidence that it happened in the past, but it doesn't do anything for "prokaryotes first" rather than "eukaryotes first".

[foodchain]

This is different. Horizontal or lateral gene transfer is taking parts of DNA and sharing them across species lines. Endosymbiosis is when an entire organism was incorporated into another organism.

 

 

I simply look at the evolution of life from the principle of least action really. Prokaryotes are by far more simple in structure then eukaryotes. Not to say what came first or not but I could think in a form of endosymbiosis from bacteria for instance using HGT that VGT became a reality via natural selection, and as soon as they could self sustain you had protista really. I think this is evident in the fact you can find chunks of say bacterial genome in very early eukaryotic life unless this is a constant act of HGT on a VGT organism? Plus the earliest recorded life is not eukaryotic in base, so I think you would suggest that eukaryotes developed independently of prokaryotes or at least not in enough of a population concentration to show evidence in the timeline anywhere close to prokaryotes, also that eukaryotic life would have had to sustain with some other mechanism besides mitochondria for instance.

[hilary:-)]

okay so the first 2 parts of the endosymbiotic theory are:

*mitochondria are the result of endocytosis of aerobic bacteria.

*chloroplasts are the result of endocytosis of photosynthetic bacteria.

 

i can't seem to find the other 2 parts. :-C

can anyone help me?

thanks

-hilary.

 

I simply look at the evolution of life from the principle of least action really. Prokaryotes are by far more simple in structure then eukaryotes. Not to say what came first or not but I could think in a form of endosymbiosis from bacteria for instance using HGT that VGT became a reality via natural selection, and as soon as they could self sustain you had protista really. I think this is evident in the fact you can find chunks of say bacterial genome in very early eukaryotic life unless this is a constant act of HGT on a VGT organism? Plus the earliest recorded life is not eukaryotic in base, so I think you would suggest that eukaryotes developed independently of prokaryotes or at least not in enough of a population concentration to show evidence in the timeline anywhere close to prokaryotes, also that eukaryotic life would have had to sustain with some other mechanism besides mitochondria for instance.

 

soo..i pretty much think you're a dumb butt for saying this..it sounds like u copied and pasted it off of google. >

[CharonY]

Actually I am pretty sure that the endosymbiotic theory primarily explains the history of mitochondria and plastids. Some other organelles (like the above mentioned flagella and cilia or peroxisomes) where at one or another point also discussed to be the result of endosymbiosis, however to my knowledge there is no real evidence for it.

[lucaspa]

I simply look at the evolution of life from the principle of least action really.

 

Foodchain, my point was to use the appropriate terms so that communication.

 

I could think in a form of endosymbiosis from bacteria for instance using HGT that VGT became a reality via natural selection, and as soon as they could self sustain you had protista really.

 

VGT (vertical gene transfer) happens in prokaryotes. That's how the chromosomal genes are transferred: the chromosome is copied and a copy goes to the daughter cell when the prokaryote divides. So VGT is not confined to multicellular organisms; unicellular organisms do it too. It's just that unicellular organisms have an additional mechanism to transfer genetic material: lateral gene transfer by plasmids.

 

I think this is evident in the fact you can find chunks of say bacterial genome in very early eukaryotic life unless this is a constant act of HGT on a VGT organism?

 

1. Lateral gene transfer takes place between ANY unicellular organism -- whether bacteria, Achaea, or eukaryote. So you can have LGT between bacteria and eukaryote. The plasmid can then become incorporated into the chromosome.

2. After endosymbiosis, some of the bacterial genes were moved to the nucleus of the eukaryotic organism.

 

Either or both of these will account for prokayrotic genes in the eukaryotic genome.

 

Plus the earliest recorded life is not eukaryotic in base,

 

As I noted in earlier posts, the earliest forms of life are so poorly preserved in the fossil record that we are not sure whether they have a nucleus or not. And it is the nucleus that distinguishes eukaryotes from prokaryotes. Based on the genomes, some biologists have argued that eukaryotes (having a nucleus but no organelles) were first and the prokaryotes and Archaea are simplifications -- doing without a nucleus. Please go back and re-read those posts.

 

And yes, the enzymes for catabolism in the early eukaryotes could either have been 1) glycolytic or 2) aerobic but the individual enzymes diffused thruout the cytoplasm. This would have been inefficient compared to cell membrane based aerobic metabolism in the bacteria that were endosymbiosed to become mitochondria. Which would explain the selective advantage to the first eukaryote that endocytosed.

 

okay so the first 2 parts of the endosymbiotic theory are:

*mitochondria are the result of endocytosis of aerobic bacteria.

*chloroplasts are the result of endocytosis of photosynthetic bacteria.

 

i can't seem to find the other 2 parts. :-C

can anyone help me?

thanks

-hilary.

 

What "other 2 parts"?

 

Chloroplasts are a subset of "plastids" in plants. It is thought that all plastids arose from endosymbiosis.

 

The other organelles mentioned by CharonY are not considered to have arisen by endosymbiosis. There are a few papers showing that endosymbiosed bacteria lose their flagella. http://mbe.oxfordjournals.org/cgi/content/abstract/msn153

Edited September 30, 2008 by lucaspa
multiple post merged