Interesting Path to Eukaryotic Cell Complexity Proposed

In summary, the Inside-Out hypothesis, proposed in the paper (open access), explains the origin of several eukaryotic cellular structures based on cell geometry resulting from expansions of cell protrusions becoming the cell's non-nuclear cytoplasm. These relationships can be tested by comparing proteins found in different cell structures and there may be a phylogenetic sequence of their evolution. This hypothesis also offers an explanation for the differences in cell division between bacteria and eukaryotes. While this topic is vast and constantly evolving, it is an interesting and promising idea that could potentially become an enduring finding.
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BillTre
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A paper, published in 2014, proposed how many eukaryotic cell features originated. It involves the elaboration of cell protrusions that have been observed recently in relatives of the eukaryotic host cell.
The Inside-Out hypothesis, proposed in this paper (open access), explains the origin of several eukaryotic cellular structure based upon the cell geometry resulting from expansions cell protrusions becoming the cell's non-nuclear cytoplasm.

Here is their summary diagram:
Screen Shot 2019-12-17 at 4.06.57 PM.png


and their caption:
Screen Shot 2019-12-17 at 4.09.23 PM.png


This quite similar to the scenario recently proposed in the paper describing the culturing of Lokiarchaebacteria. Here is their summary diagram:
Screen Shot 2019-10-19 at 3.41.35 PM.png
This results in the following proposed relationships:
  • The original archaeal cell body of the host cell becomes that eukaryotic cell nucleus.
  • The base of the protrusions evolve into nuclear pores, connecting the nucleus with the cytoplasm.
  • The original archaeal cell membrane becomes the inner nuclear membrane of the eukaryotic nuclear envelope (a double membrane).
  • the membrane apposed to the cell membrane of the original archaeal cell membrane becomes the outer nuclear membrane.
  • The protrusion (or bleb) derived membranes opposed to each other in the cytoplasm become endomembranes (like the Endoplasmic Reticulum (ER)). Their lumen is the lumen of the endomembrane system.
  • What the outer mitochondrial membrane is derived from is not clear to me. A layer of membrane is lost going from: the double bacterial (future mitochondrial) membrane plus to the archaeal cell membrane opposed the mitochondria get reduced to just two membranes around the eukaryotic mitochondria somehow.

Many of these relationships are testable by comparing proteins found in particular cell structures, such as proteins at the base of the protrusions evolving into components of the nuclear pore.
In addition, there should be a phylogenetic sequence of the evolution of sets of proteins used in different parts of eukaryotic cells that parallels when they arise, based on this scheme. For example, proteins of the ER probably evolved before proteins of the Golgi complex.
Sets of proteins (toolkits) involved in controlling membrane trafficking and function may be derived from those required in the archaeal cell protrusions for its structure and functions.

Cell division differs between bacteria and eukaryotes. In bacteria (and presumably archaea also?) the DNA circular chromosome is attached to the cell membrane. The two copies go to opposite ends of the cell and the cell divides partitioning the two DNA molecules to the two resulting daughter cells.
In the newly formed Archaeal/bacterial cell composite, cell division can proceed in a similar manner until the cytoplasmic blebs merge together into a single mass. As the Archaeal cell body (which becomes the nucleus) divides, the cytoplasmic components will be dragged along by their connections at the base of the former protrusions. However, once the plasma membrane is sealed up with a single all-enveloping membrane, new mechanisms have to arise to separate the duplicated DNA and divide the cell.

I find this to be a very interesting scenario.
Here is a discussion of these ideas.
It is more appealing to me than many others since it is able to explain a lot of novel eukaryotic cellular components by simple geometry.

Previous PF threads on this general issue are:
Archaeal host cell structure
Lokiarchaea as the eukaryotic host cell
Hypotheses of metabolic exchange in proto-eukaryotes
 

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I was thinking it might be an Insight, but to me it seems like less of an enduring finding and more of an update to a rapidly moving field which may or may not turn out to be well supported in the long run.
Thus I choose to a post a thread rather than an Insight.

I had preliminarily typed up about 5-10 pages of notes on this subject (which is vast) and made several figures.
I then took a lot of time to pare it down to a not too long (IMO) post, as well as wording it in an interesting, but not overly certain manner.

I am wondering: what are relative viewing rates of an Insight vs. a thread?
 
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BillTre said:
I am wondering: what are relative viewing rates of an Insight vs. a thread?
For article type threads, Insights get more views as I am able to promote them more and Google respects the wordpress article platform more than a random forum thread.
 
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BillTre said:
I had preliminarily typed up about 5-10 pages of notes on this subject (which is vast) and made several figures.
I then took a lot of time to pare it down to a not too long (IMO) post, as well as wording it in an interesting, but not overly certain manner.
Why summarise and post in a thread? This could easily (and should) be an insight.
 

Related to Interesting Path to Eukaryotic Cell Complexity Proposed

1. What is the proposed path to eukaryotic cell complexity?

The proposed path to eukaryotic cell complexity is called the "inside-out" model, which suggests that eukaryotic cells evolved from within prokaryotic cells through a process called endosymbiosis.

2. What evidence supports this proposed path?

There is evidence from comparative genomics, structural biology, and biochemistry that supports the endosymbiotic theory. For example, mitochondria and chloroplasts have their own DNA and ribosomes, similar to prokaryotic cells, and their genetic sequences are more closely related to bacteria than eukaryotic cells.

3. How does this proposed path explain the complexity of eukaryotic cells?

The "inside-out" model proposes that eukaryotic cells gained their complexity through the incorporation of prokaryotic cells, which provided additional functions and abilities. This allowed for the development of complex cellular structures, such as the nucleus, endoplasmic reticulum, and Golgi apparatus.

4. Are there any alternative theories to explain the evolution of eukaryotic cells?

Yes, there are alternative theories such as the "outside-in" model, which suggests that eukaryotic cells evolved from the fusion of multiple prokaryotic cells. However, the endosymbiotic theory has more evidence and support from the scientific community.

5. How does understanding the evolution of eukaryotic cells impact our understanding of life on Earth?

Understanding the evolution of eukaryotic cells can provide insights into the origins of complex life on Earth. It also helps us understand the relationships between different organisms and how they have evolved over time. This knowledge can also have practical applications in fields such as medicine and biotechnology.

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