origin of eukaryotic cell

[manohara]

hi,
I was trying to understand the origin of eukaryotic cell and what cells could have been their ancesters. What I gathered in some literature is that eukaryote may have evolved by a merger of an archebacteria with an eubacteria. The evidence quoted was that a protein FtsZ ( which is involved in bacterial cell division) is similar in its sequence to tubulin - the cytoskeletal elements found only in eukaryotes. But what I also found out was that anothr protein FtsA may have given rise to actin another cytoskeletal element. But Fts A has not been found in any of the archea so far. Does that mean that the merger theory mentioned earlier has to be reconsidered?? Please can somebody shed more light on this? ( we cannot avoid not having archea because they contain some other genes which are unique to eukaryotes)

[mamoru]

I thought the main evidence for thinking that the host cell of the endosymbiosis that created eukaryotes was an archea was more due to Eukaryotes have more Archea-like transcription and translation machinery. That is what Carl Woese said in 2002, and Martin & Russell use that information for their model of Eukaryote evolution. For more in depth reading:

• Woese, C.R., 2002. On the evolution of cells. Proceedings of the National Academy of Sciences of the United States of America, 99(13), 8742-7. Available at: http://www.pnas.org/content/99/13/8742.abstract.

• Martin, W. & Russell, M.J., 2003. On the origins of cells: a hypothesis for the evolutionary transitions from abiotic geochemistry to chemoautotrophic prokaryotes, and from prokaryotes to nucleated cells. Philosophical transactions of the Royal Society of London. Series B, Biological sciences, 358(1429), 59-83; discussion 83-5. Available at: http://rstb.royalsocietypublishing.org/content/358/1429/59.abstract.

Both of those have full text PDFs available for free download.

[edit]
More specific to your question, if it is not in fact present in any archaea but is present in eubacteria, is there any reason why FtsA genes couldn't have been inherited from the eubacterial endosymbiont via horizontal gene transfer along with the great many other genes that are now in the eukaryotic nuclear genome (as opposed to the mitochondrial genome)? If this is the case, then it seems that the endosymbiont theory is still well supported.

[manohara]

There is no reason not to agree with the endosymbiotic theory. But what is unsettling is that there is very little similarity between the purported precursor of FtsZ and the tubulin itself. If sequence similarity is any measure of evolutionary relatedness then the divergence in FtsZ and tubulin will place the archea and eubacteria away from eukarya.

No doubt there are lot of evidences to support the merger theory but what has not been clearly established is the origin of cytoskeletal elements in eukaryotes. Because FtsZ is poor precursor of tubulin like FtsA is for actin. So from where did the cytoskeltal elements originate.

Why could it not be hypothesised that there was a third party involved which already contained cytoskeletal elements within it. and the merger of both archea and eukarya happened within it. It may be asked that if that is the case, why dont we see those third party organisms any more. Perhaps they were overwhelmed by the emergence of this new merger candidate.

Ofcourse this is only an hypothesis and a closer look at the genomes is needed. One explanation that could bypass all these difficulties could be Horizontal gene trasnfers as has been suggested of cytoskeletal genes from unknown sources