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What is the scenario for the endosymbiotic theory?

The endosymbiotic theory seems easy to understand, a bacterium engulfed by another cell. Apart from the problems in identifying the host, there is ambiguity whether the invader was single- or double-membraned but they distinguish between two different scenarios.  

The first is a eukaryotic cell that engulfed a bacterium by invagination of the membrane (here), thereby providing the newly derived endosymbiont with the extra membrane. I cannot take this scenario seriously, although it appears in most major textbooks. In genetic evolution, it is essential that new functions are reflected by genomic changes. In this case, the encapsulation should also persist after numerous cell divisions, and the eukaryotic cell should thus be able to put an extra membrane around dividing bacteria. This supposed event is clearly Lamarckian since the developmentally acquired outer membrane would have to been passed to the germline, clearly in contrast with genetic determinism. Moreover, it assumes evolutionary fitness advantages over having an endosymbiont for both parties while it is difficult to imagine how a bacteria would benefit from being encapsulated into a new membrane. On the other hand, what benefits would encapsulated bacteria into the cytoplasm bring, if they would survive. Moreover the alpha-proteobacteria Rickettsia, which is the supposed closest relative to mitochondria is gram-negative and already has two membranes.

The second possibility in the endosymbiotic hypothesis is that the entire double-membraned prokaryote invaded a cell (e.g. here) and evolved into a endosymbiont. In this scenario, both the inner and outer membrane of the endosymbiont were bacterial in nature and it predicts that the endosymbiont would still harbor some characteristics of the bacterial membrane.

Earlier evidence for the endosymbiotic theory included reports that the outer membrane was eukaryotic (still here), while the inner membrane had more prokaryotic characteristics although this seems to be abandoned later. Even though some shared characteristics may be found, we have to realize that bacterial and eukaryotic membranes are fundamentally different. It seems virtually impossible to change all fundamental bacterial membrane characteristics and replace them with a eukaryotic counterpart without loosing membrane integrity.

The ease with which textbooks print either scenario without discussing the mechanistic differences this makes shows the disregard for the mechanistic nature of organisms and its evolution. 

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