What do you think is the single best criterion for telling an endosymbiont of a eukaryotic cell from an organelle?
Post Script
I must come clean and confess that I formulated this question after having read a most stimulating review by John McCutcheon and Nancy Moran that touches on this subject. Some of the responses you provided are related to considerations in this article. Read and enjoy!
—Elio
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how far back you want to look? when we look that far back can we decide when its a eukaryotic cell? prokaryotes share all the time...
I am not sure what the criterion would be exactly, but I think it would involve the degree of dependency on genes encoded in the host nucleus.
I'm not sure about this, but could dependence on host DNA polymerase be a distinguishing factor?
Prokaryotic surface features including peptidoglycan, (lipo)teichoic acids, and capsular protieins/peptides
An endosymbiont can generate an electrochemical potential across their own membrane,
[from metabolism within their cytoplasm] without importation of nucleus-encoded and imported proteins.
I think that the presence of evolutionary echoes of independence is the key point. Peptidoglycan is not present in all bacteria (or among the archaea at all). Metabolic pathways can be co-opted and harnessed, blurring distinctions. I guess I would be inclined to stick with evidence of the "organelle/endosymbiont" having some kind of genetic blueprint: some DNA or RNA. And I guess we can add "compartmentalization" to the Venn diagram of this conundrum.
Membranes of endosymbionts are double layered?
may be, endosymbionts do not rely on imported host proteins for major processes, e.g. transcription, translation, DNA replication
If the eukaryotic cell can fulfill its life cycle without the organelle, it's an endosymbiont.
–bks
Might there be a continuum from endosymbiont to organelle? ... are there some that are difficult to classify one way or the other? (asking out of ignorance)
Perhaps the criteria is disappointingly contextual: if the endosymbiont/organelle is found only in special organs, or only in somatic tissues, it is probably an endosymbiont, not an organelle.
If you already know that the organism inside the cell is an endosymbiont, you can perform an in situ stain against the conserved segments of the bacterial ribosome and only the bacteria will show up
With respect to the postscript
http://download.bioon.com.cn/view/upload/201112/19191116_6450.pdf
–bks