Understanding Endosymbiotic Theory

[8link48]

In terms of endosymbiotic theory, is it still possible for organisms to "merge" if their environmental conditions are too hazardous for both organisms? Even if previosly the relationship between the two organisms was that of predator and prey? If so, are there any examples of said organisms throughout history?

[Endy0816]

12 hours ago, 8link48 said:

In terms of endosymbiotic theory, is it still possible for organisms to "merge" if their environmental conditions are too hazardous for both organisms? Even if previosly the relationship between the two organisms was that of predator and prey? If so, are there any examples of said organisms throughout history?

Can you elaborate a bit on what you mean by environmental conditions?

Most probably exist(ed) as intracellular parasites before taking up permanent residence.

[8link48]

9 hours ago, Endy0816 said:

Can you elaborate a bit on what you mean by environmental conditions?

 

I'm thinking something related to a close representation of prehistoric earth's atmospheric conditions, like somewhere near a geyser or a volcano.

 

9 hours ago, Endy0816 said:

Most probably exist(ed) as intracellular parasites before taking up permanent residence.

Do you have any examples of said parasites, I'm not finding any good examples to reference to.

[mistermack]

Some Anglerfish mate by fusing the tiny male to the much bigger female, becoming merged physically if not genetically. 

https://en.wikipedia.org/wiki/Anglerfish   

[CharonY]

On 10/30/2018 at 7:23 AM, 8link48 said:

In terms of endosymbiotic theory, is it still possible for organisms to "merge" if their environmental conditions are too hazardous for both organisms? Even if previosly the relationship between the two organisms was that of predator and prey? If so, are there any examples of said organisms throughout history?

The issue here is that forming endosymbiosis is typically a very slow process. I.e. if environmental situations become hazardous, both would die before a symbiosis had stabilized. A simple model how these symbioses form is the avoidance of phagocytosis in amoeba. Specifically you could look up Dictyostelium which is a model to study symbioses (mostly in terms of parasitic ones, but the lines are blurry).

Typically amoeba and other eukaryotes consume bacteria as prey. However, over time some bacteria may obtain genetic mechanisms allowing them to survive the uptake (phagocytosis) from the predator. They then exist in a membrane-surrounded compartment within the host. At this point there is no actual benefit for each partner. In a parasitic interaction, the bacterium may have additional so-called virulence factors, that allow them not only to survive the host, but potentially also steal nutrients and other things from the host. Here, we have a parasitic symbiosis. However, in arms races between host and bacterium, over time the bacterium may settle down a bit (so to say) and instead of exploiting the host, it may couple its reproductive success with that of the host. When that happens, anything benefiting the host also benefits the bacterium. So over time,  the bacterium may start contribute to the nutrition of the host, rather than just take away. Only at this point real mutualism starts. Thus it is unlikely that species suddenly merge to overcome a harmful situation. Rather, over time mutualism may develop that allow them to exploit previously unfavourable niches.

Note that the "endosymbiotic theory" as a term generally refers to a specific endosymbiotic case, which is the formation of organelles in the evolution of eukaryotes.

3 hours ago, mistermack said:

Some Anglerfish mate by fusing the tiny male to the much bigger female, becoming merged physically if not genetically. 

https://en.wikipedia.org/wiki/Anglerfish   

Symbioses generally refers to interspecies interaction. I.e. the luminiscent bacteria on anglerfish would be considered a symbiotic partner, but the male would not.

[Endy0816]

16 hours ago, 8link48 said:

Do you have any examples of said parasites, I'm not finding any good examples to reference to.

Rickettsia is the main one and nearest relative of the mitochondria. Still making itself at home within cells to this day.

May want to look at Paulinella Chromatophora too. Relatively recently took in a cyanobacteria.

[CharonY]

1 hour ago, Endy0816 said:

Rickettsia is the main one and nearest relative of the mitochondria. Still making itself at home within cells to this day.

That was assumed sometime around the 2000s. However with more genome sequences out there this has become disputed. It is still likely found among the alphaproteobacteria but it is far from clear whether it is actually Rickettsia. There is some evidence that mitochondria arose after the split of Rickketsiales from other alphaproteobacteria.

[Endy0816]

7 hours ago, CharonY said:

That was assumed sometime around the 2000s. However with more genome sequences out there this has become disputed. It is still likely found among the alphaproteobacteria but it is far from clear whether it is actually Rickettsia. There is some evidence that mitochondria arose after the split of Rickketsiales from other alphaproteobacteria.

Okay, interesting. Did notice they were slimmed down in terms of some of their code.

Edited November 1, 2018 by Endy0816

[CharonY]

2 hours ago, Endy0816 said:

Okay, interesting. Did notice they were slimmed down in terms of some of their code.

Yes, but that is fairly typical for intracellular parasites.

[jimmydasaint]

Are some of the genes for replication of mitochondria and chloroplasts found in the cellular chromosomes?

One would need to explain why the cell's chromosomes also contribute in a pivotal way to the replication of organelles and how the genes got there in the first place. 

Or are the DNA plasmids found in the organelles enough for replication?

[CharonY]

6 minutes ago, jimmydasaint said:

Are some of the genes for replication of mitochondria and chloroplasts found in the cellular chromosomes?

One would need to explain why the cell's chromosomes also contribute in a pivotal way to the replication of organelles and how the genes got there in the first place. 

A lot (most) of mitochondrial (and plastid) DNA has been incorporated into the the host genome, though the extent can vary between different groups of organisms. Many of functions for replication are typically transferred to the host, though a few may still be retained. Typically the mitochondrial genomes lack the complete set of  genes required for independent replication. There are several models and evidence of DNA transfer during the early phases of organelle origins. After establishing the core functions of organelles the rise of protein import/export functions have slowed down the transfer of the rest.

[jimmydasaint]

2 minutes ago, CharonY said:

A lot (most) of mitochondrial (and plastid) DNA has been incorporated into the the host genome, though the extent can vary between different groups of organisms. Many of functions for replication are typically transferred to the host, though a few may still be retained. Typically the mitochondrial genomes lack the complete set of  genes required for independent replication. There are several models and evidence of DNA transfer during the early phases of organelle origins. After establishing the core functions of organelles the rise of protein import/export functions have slowed down the transfer of the rest.

Is there a particular model which is more plausible than others about the movement of DNA from the organelle to the nuclear chromosomes? I am making an assumption here that the genes for mitochondrial replication are spread amongst several chromosomes as a "buffer" to avoid deleterious mutations, but I could be corrected...

[CharonY]

19 hours ago, jimmydasaint said:

Is there a particular model which is more plausible than others about the movement of DNA from the organelle to the nuclear chromosomes? I am making an assumption here that the genes for mitochondrial replication are spread amongst several chromosomes as a "buffer" to avoid deleterious mutations, but I could be corrected...

There are different mechanisms known intracellular gene transfer. But if you refer to reasons why the transfer stabilized, it is a bit more complicated. If it was to avoid mutations, keeping it in mitochondria would be better, after all there would be more copies around per cell. If transferred into the nucleus, there is only one point of attack. 

However, there is something related to that is called "Muller's ratchett", a hypothesis that in asexual reproduction deleterious mutations can accumulate irreversibly and hence a nuclear transfer can, by providing access to sexual reproduction, relieve these effects. However, especially in plants there is minimal evidence for that. Another hypothesis is genome streamlining, which is often found in parasites. By not having to carry a gene, a mitochondrium would outcompete those that still carry it.

There are also a number of hypotheses that are non-selective, and have just ended in the nucleus due to DNA escape and uptake, i.e. mostly mechanisms-driven with subsequent pruning, for example. In different lineages different factors could have contributed, to make things more complicated.

[jimmydasaint]

/nice one mate. Thank you for the information.