Talk:Endosymbiotic theory

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Rated "top" as important theory of cell evolution. The article could be improved by adding a figure. It should be expanded with a section on the endosymbiotic theory of the origin of the nucleus and its problems. - tameeria 23:41, 18 February 2007 (UTC)

As per a discussion with mav on the Endosymbiont page, I've created this page on the Endosymbiotic Hypothesis, by editing the content of a previous version of the Endosymbiont page (circa late Dec 2002).

Currently, I think the evidence I offer in support of the Endosymbiotic Hypothesis is fairly weak, and much of the evidence seems consistent with a variety of other hypotheses. For example, here's an obvious alternate hypothesis (I'm not saying this is a good one, just that it's an alternative):

  • the ancestor of eukaryotes started to sequester ATP synthesis into certain regions of the cell, e.g. to make the reactions more efficient (I'm no biochemist at all, but I understand there are advantages)
  • eventually a membrane evolved around this ATP synthesis region, forming a compartment, and isolating the reactions further in an organelle
  • some fragment of the nuclear genome and machinery broke off into the new organelle and formed the minimal genome that mitochondria currently have
  • chloroplasts subsequently formed through a similar process

I'm sure others can propose variations on the (not especially good) hypothesis I just stated, or other alternate hypotheses.

Ideally, this article should cite the evidence that more-convincingly indicates that the Endosymbiotic Hypothesis is probably correct, and other hypotheses (like the one I just made up) are significantly less plausible. Currently the only evidence I offer that gets at this is that phylogenetic evidence indicates that chloroplasts are more closely related to cyanobacteria than anything else. (Well, perhaps we could argue that ancient eukaryotes got DNA from cyanobacteria through horizontal transfer of DNA.) It's certainly my perception that this theory is generally accepted, but I feel I don't have all the arguments.

Another thing that would be nice is a historical discussion of what prompted Dr. Margulis to form the Endosymbiotic Hypothesis.

Finally, I think there might be some overlap with the very extensive (but IMHO not entirely desirably) article on evolution of flagella, but maybe that's something for another day.

Anyway, I don't know much about this theory, so I'm unlikely to be able to do this, but I hope what I've said is motivating to someone who knows more.

--User:zashaw

Thanks for laying all this out. The article as is also looks good - of course much more can and should eventually be written. If I find time I'll brush-up a bit on my cellular evolution and give it a go. --mav 04:05 Feb 23, 2003 (UTC)

I added this: Protista with chloroplasts are often more closely related to forms lacking than to each other, suggesting chloroplasts appeared more than once. This could be expanded on - I think the presence of nucleomorphs in the chloroplasts of cryptomonads and chlorarachniophytes has been considered a spectacular piece of evidence for the endosymbiotic hypothesis, though it shows chloroplasts secondarily derived from eukaryotic rather than prokaryotic ancestors. However, chances are a full discussion of the various sorts of chloroplasts (like the probably confusing blurb I wrote on that page) would simply be distracting. Does anyone have ideas on how this could be worked in?

Contents

[edit] Credit for Endosymbiotic Hypothesis

This article states that: "The endosymbiont theory of mitochondria and chloroplasts was proposed by Lynn Margulis." This is not true. The endosymbiont theory of mitochondria and chloroplasts was proposed by scientists in the 19th century (including Russian scientists). Wallin wrote a book in 1927 called Symbionticism and the Origin of Species. yo. Dr. Margulis's role has been to popularize the endosymbiont theory of mitochondria and chloroplasts.

Bob Bloodgood Department of Cell Biology University of Virginia school of Medicine

hello

I just checked. I wrote that. I doubt I invented it, so if what you say is right, I obviously used a source which was putting untrue claims. Please do edit the article accordingly. I will check on my side what I can find on the topic. Anthère


Indeed. I found the name of one of the author C. Mereschkovsky. Would you by any chance know more precisely the nature of this biologist initial claim, so we could see on in which direction Margulis work has been extensive ?

I found a reference, and have changed the page accordingly. It really looks like this page relies heavily on Margulis-based popularizations, and I think it might miss many important ideas. I'm not sure if discussing her ideas about the cooperative biosphere is entirely relevant. Josh

[edit] Endosymbiotic Theory

can we redirect this to Endosymbiotic Theory, i have never heard it referred to as a the "hypothesis", the standard term is theory. rhyax 04:52, 7 Sep 2004 (UTC)

I'm neutral. Google seems to agree with you:
You might want to talk with User:mav, who suggested breaking this article up from the Endosymbiont article I made; he seems to follow evolution stuff, so he may have an opinion.
Zashaw 23:33, 7 Sep 2004 (UTC)
Yea, I think it definitely deserves it's own article, I was actually looking for it and couldn't find it due to the name. rhyax 05:06, 8 Sep 2004 (UTC)


When I was taught this it was still a hypothesis. But it seems to have gained greater acceptance since then. So yes, move the article. --05:07, 8 Sep 2004 (UTC)

[edit] eucaryotic engulfment leading to endosymbiosis

yes, Life on a Young Planet by Andrew? Knoll has a good summary of how various diatoms,euglenids etc and dinoflagellates etc formed through repeat endosymbioses

[edit] minor correction

I changed "Some genes encoded in the nucleus are transported to the organelle" to "Some proteins encoded in the nucleus are transported to the organelle". As you will know, genes are not transported from the nucleus, they are transcribed (structural genes are) to mRNA, translated to proteins outside the nucleus and then the proteins may be transported to their final destination. Those working on this article might find this reference interesting: Ancient Invasions: From Endosymbionts to Organelles. By S. D. Dyall et al., Science 304, 253 2004

[edit] another minor correction

I removed the external link

because it was broken. If this article/page is indispensible, and you know of its new url, please fix it.

Safay 07:49, 8 May 2005 (UTC)

[edit] Helotry

This word is new to me. I clicked on the Wikipedia link provided but it is only an historical explaination of the Helots. Wiktionary does not have an entry for this word. Mirriam-Webster gives the same historical definition and gives it as a synonym for slavery. Is there any reason not to use the word slavery here, if that's what is meant?

Incidentally, I would argue that this kind of endosymbiosis leading to organelle is not slavery. The host is a habitat. Often endosymbionts are maintained at population levels far above what they could acheive without a host. Once you start to get genomic integration, the individual identities of the two, host and symbiont, get eroded and the holobiont starts to become a new kind of fused individual. I was initially excited about the word "helotry" because I thought maybe it elegantly captured this process, but upon looking it up was dissapointed to discover it only meant slavery.

Safay 08:04, 7 November 2005 (UTC)

[edit] Redirect

Does anyone feel that pages like Cellular evolution or Evolution of cells should redirect to this page or any others. RENTASTRAWBERRY FOR LET? röck 01:13, 29 November 2005 (UTC)

Endosymbiotic theory should be a topic within Evolution of cells, if you ask me. Safay 09:05, 1 December 2005 (UTC)

[edit] Corrections

Corrected some info re: who came up with what. Fixed the flagella not being thought to result from endosymbiosis (which is false). A lot of the info is only half true or big generalisations, which I might fix later if I can do it without being long winded. One of the big assumptions in this article was that whatever was engulfed and became part of the host needed to have DNA. This isn't really true since something bigger (a multicellular protist that needs to eat other simpler organisms to survive) can engulf a simple photosynthetic algae and take apart that algae and basically reassign the use of the major algae components for whatever it wants. I.e. take off the flagella and use it for locomotion, and keep the photosynethic algae bit in another part, etc. Once it divides only one daughter cell gets to keep the original algae, but the flagella might go to another daughter cell, so then you'd have a protist without any algae inside but with a flagella.--Scyfer 15:02, 8 February 2006 (UTC)

Flagella are closely tied in with the cytoskeleton as a whole, and their structure and arrangement is fairly constant within major groups. In any case, transfers are not the same thing as an endosymbiotic origin, which does require them having DNA-containing ancestors (e.g. spirochaetes, as Margulis suggests). From all the references I can find, this seems to have extremely limited support.

Josh, I agree. While the flagellum may have evolved from a spriochete (which there is not much evidence for, or support for as a theory in the scientific community, as far as I am aware) I don't think that there is any evidence or even anyone out there suggesting that endosymbiosis has resulted in a kind of adoption of modular parts of cells, as suggested by Scyfer. The cell would still need the genetic information somewhere for it to be a structure passed along in the lineage. When we do have structures like hydrogenosomes that are presumed to come from endosymbiosis events but do not contain DNA, it is thought that indeed the endosymbiont did have DNA, but that DNA was transferred to the nucleus over evolutionary time. Safay 00:28, 4 March 2006 (UTC)
Are here other criteria? If so, suggest editing to show less confdence in DNA as the criterion. DGG 03:28, 19 September 2006 (UTC)


[edit] Viral Eukaryogenesis?

Should this theory (which states that the cell nucleus evolved from a large DNA virus) be included here? It seems to be gaining support as ever larger, more complex DNA viruses (such as Mimivirus) are discovered. archola 01:26, 9 February 2006 (UTC)

Probably not here, but I've added mention of it to eukaryote under origins, along with several other theories. It's difficult to tell how important the competing hypotheses are.

I've also removed some text from this page. Here's why.

  • In other words, the endosymbiotic theory suggests that eukaryotic cells first appeared when a prokaryotic cell was absorbed into another cell without being digested. Biologists still debate the nature of the cell that acquired mitochondria, and the means by which it did so. I think we should discuss those things separately from their endosymbiotic origin, which is uncontroversial. Plastids were definitely acquired by a eukaryotic cell.
  • However in 2005, Okamato & Inouye found a case of a multicellular heterotropic flagellate host engulfing a unicellular photosyntheic green algae; the algae lost its flagella and cytoskeleton and the host gained the ability to photosynthesize and the ability to move toward light.[1] This in vitro result illustrates that endosymbiotic components need not have DNA themselves to become embedded in a host - as components of single celled organisms can be separated and re-assigned functions upon engulfment. This is a good reference, as far as evidence goes; but it doesn't seem to be evidence that flagella are spirochete-derived, or address the main criticisms of that proposal.

[edit] Evolution and natural selection

Did Darwin (or anyone else for that matter) ever say that natural selection "drove" evolution, in any sense of the word?

I thought rather that there was not considered to by any driving force behind evolution, but rather that (1) species came into being purely at random and that (2) species which were better adapted to their environment would simply produce more descendants, while those more poorly adapted would produce fewer. Natural selection would then blindly "select" better species once they had come into existence but had nothing to do with bringing them into existence.

Am I close, or completely misunderstanding the science here, or what? --Uncle Ed 18:27, 3 April 2006 (UTC)

You misunderstand evolution. Darwin's writings are not particularly clear to modern readers because he tried to fit evolution into the framework of inheritance that was hypothesized at the time (mostly, blending inheritance, although many contemporary biologists questioned that).
Here is the (very simplified) process:
  • mutation is random, causing genetic variation.
  • natural selection works on the genetic variation. This is not entirely random, but is dictated by the environment and genetic variation available.
  • this eventually leads to speciation. The speciation is related to random events (mutation), but is given direction and order by natural selection & the environment. And don't forget reproductive isolation.
Basically, it is very difficult (but not impossible) to have speciation without natural selection. Speciation is the results of many evolutionary forces, including natural selection and mutation, but also including drift, founder effect, migration (and lack of migration), etc. Ted 04:20, 7 April 2006 (UTC)

[edit] Problems Section

The Problems section is very poorly written, and lacks any citations. The first bullet states: "Some form of transfer between nuclear and mitochondrial/plastid DNA must have taken place..." [emphasis mine], then goes on to say "...or, alternatively, a mechanism for turning off portions of early proto-mitochondrial/plastid DNA must have been in place in early symbiotes." [emphasis mine] If the first one must have happened, then how could the second be an alternative, which also must have happened? For the second point, I'm not sure any reasonable person can say the problem is "easily accounted for" by stating DNA transfer had to have taken place. I'm not up on all the research in this area, but I don't recall ever seeing anyone give a decent explanation as to how this transfer occurs (retrotransposition has problems, in my opinion). If there is, then it should be cited -- even retrotranspositions, if that's what we have.

More importantly, there are no citations. This is crucial when trying to answer problems with the theory. Right now, this section says, "the reason these problems aren't serious is because it must have happened as indicated." Ted 17:10, 23 April 2006 (UTC)

Removed an incoherent paragraph on chloroplast-mitochondrial cross influences (or at least that's what I think it was about. And the even more absurd one by the same web address, 70.187.235.73, both of which you can see in the history. If it was a joke, it was a rather good one. DGG 00:02, 20 September 2006 (UTC)

The Problems: introns section is incorrect and I am removing it. Organellar introns are of a different type (Group I or II introns) from nuclear eukaryotic introns (spliceosomal introns) and derived from bacterial type introns (eg see Xu et al Science, Vol 250, Issue 4987, 1566-1570), so the the proposed paradox is artificial.--Spamburgler 01:47, 24 April 2007 (UTC)

[edit] Diagram needs a caption

The diagram badly needs a caption. Is this a genuine hypothesis of the evolution of some organism? If so, which? (Or is it just illustrative of the possibilities?) Which organisms are indicated by the letters A to I? What do the numbers 1 to 4 mean? Gdr 11:07, 29 May 2006 (UTC)

I agree. Maybe the article should have a diagram that includes alternative hypotheses such as the idea that both prokaryotes and Archaea may have been involved in the origin of eukaryotic cells. The Endosymbiotic theory article could expand on the Hydrogen hypothesis. There could also be mention of other ideas such as the proposal that viral introductions of genes have been important in the origin of eukaryotic cells. --JWSchmidt 14:43, 23 June 2006 (UTC)

Without a caption, the diagram is useless. I have deleted it. TedTalk/Contributions 15:15, 17 September 2006 (UTC)

[edit] Polydnavirus?

Should a link to Polydnavirus be added to the See Also section? It's not precisely an example of endosymbiotic theory, but it is related. Kabu 20:31, 2 June 2006 (UTC)

[edit] plastid or chloroplast

Why is most of the article usingthe term "plastid" , when also saying it is equivalent to chloroplast? Has "plastid" ever been used in a broader sense? Has any other organelle but the chloroplast ever been called a "plastid" If not, chloroplast is the more familiar word. DGG 03:28, 19 September 2006 (UTC) Yes Plastid is a broader term. Chloroplast specifically describes green plastids whose main role is photosynthesis.--Spamburgler 01:53, 24 April 2007 (UTC)

[edit] Minor Correction

I've removed the part in the beginning where it says ",now generally accepted by biologists," because a theory is a hypothesis which has broad acceptance and has withstood the trials of time. I believe to state that the endosymbiotic theory has broad acceptance is saying the same thing twice. Also, I am a biology student in high school (going into university next year) and I have never heard of this being called a hypothesis; it has always been called a theory in all the textbooks I've read. MickeyK 00:28, 23 January 2007 (UTC)

Actually, the labelling of an hypothesis as a theory does not require that it has general acceptance, the term 'general acceptance' meaning referring to the state of being accepted by the majority. There are many theories which are rejected by the majority. Some examples include the theory that Elvis is still alive, the theory that aliens landed at Roswell, the theory that the ancient Egyptians were black, the theory that human races evolved from seperate species of ape-men, etc, etc. Also, notice the use of the word 'an' rather than 'a' preceding the word 'hypothesis.' Also, if you want to do a biology degree, be prepared to do at least a master's or a PhD. I have a neuroscience degree and now I'm doing a programming degree (double degree, but the neuroscience part finished first). Just the bachelor's degree alone will likely get you nowhere or just working as a research assistant, etc. You should look into it. Holymolytree2
A scientific theory is one which is almost unanimously accepted by scientists. "Theory" in common speech is what you are referring to, I believe. MickeyK 22:29, 19 June 2007 (UTC)
I don't believe that's true. You can have conflicting theories on the same thing, as long as they are both consistent with the evidence. Holymolytree2 05:47, 24 July 2007 (UTC)

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