Interesting. I was just reading about endosymbiosis last night after being lead there by research into The Force from Star Wars (midichlorians is supposed to be a portmanteau of mitochondria and chloroplast).
What I find interesting is that mitochondria suffer damage but still continue to fission and serve our needs. I know we think of it as the cause of death in use, but in reality, that collection of chemicals represents a single lifeform that has lived continuously for billions of years. That's all but immortal. Actually, all our cells are like that, in some sense, although we don't think of it that way.
However, I don't know that endosymbiosis must occur in all life-forms. It may be possible for alien life to develop those characteristics in-house. Even if they don't develop it through direct mutation and evolution, they might obtain it using a plasmid type of genetic transfer or via viruses or viroids.
Et tu BAUT? Quantum mutatus ab illo.