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- TL;DR Summary
- 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:
and their caption:
This quite similar to the scenario recently proposed in the paper describing the culturing of Lokiarchaebacteria. Here is their summary diagram:
This results in the following proposed relationships:
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
Here is their summary diagram:
and their caption:
This quite similar to the scenario recently proposed in the paper describing the culturing of Lokiarchaebacteria. Here is their summary diagram:
- 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
