Endosymbiosis fascinates me and seems to have this effect on many people. This week in class, we introduced the evolution of the eukaryotic lineage with a focus on two key events – the evolution of mitochondria, which are key organelles in nearly all eukaryotic cells, and the evolution of chloroplasts, and thus, plants.
I can see from a few Google searches that endosymbiosis is considered a theory. That does not mean it didn’t happen! It simple means that we do not know precisely how it happened. And that is what really fascinates me.
According to the theory, an aerobic bacterium (one that can respire using oxygen as a final electron acceptor, and thus, derive much more ATP from a single molecule of sugar than one that must simply rely on anaerobic glycolysis), came to live within a larger anaerobic bacterium. How did that happen? Was it swallowed? Or did it sneak inside, more like a covert operation? And did it happen just once, and that single, random event gave rise to the entire eukaryotic lineage? Or did it happen many times simultaneously? In either scenario, the advantage must have been tremendous for this event to have survived and propagated. The alternative, of course, would have been that the swallowed or invaded organisms might have evolved mechanisms to avoid the other. Most of life as we know it would have been over before it even got started.
And then it happened again, with the precursors of chloroplasts. Imagine the course of evolution if photosynthesis and the ability to fix carbon from carbon dioxide never made it beyond a few single celled prokaryotes. We certainly wouldn’t be here to theorize about all this.
When we look at contemporary cells, we realize that there is no turning back. Since mitochondria and chloroplasts were derived from independently living bacteria, it is no surprise that they possess their own genomes, can replicate and transcribe these genes, and have the machinery to translate their own proteins. What nearly blows my mind is not that they can make their own proteins. That makes sense. However, mitochondria and chloroplasts synthesize only about 10% of their own proteins. The rest are imported into these organelles from nuclear encoded genes. How did these genes get into the nuclear genome??? The fact that these genes are there, means of course, that a mitochondrion or chloroplast can no longer survive on its own, outside of the cell. No more than a liver or a heart or a lung can survive for long outside of the body. Hence, we call these subcellular structures, organelles. What about the converse, could a eukaryotic cell survive without mitochondria? Probably, at least under pampered conditions, but once the energy requirements exceeded a certain threshold, that cell would waste away for lack of sufficient ATP.
To try to make sense of all this, I try to think of useful metaphors for endosymbiosis. Is endosymbiosis like a long-term romantic relationship? Not a very functional relationship, in my opinion, with one partner sequestering the other from the rest of the world. “I’m only trying to protect you, darling.” The other partner gets even in a sort of passive-aggressive way by controlling the ATP production. “Tired, honey? Here, let me make you feel better.” No, that metaphor is not working at all. Perhaps endosymbiosis is better compared to a corporate merger, a highly functional one. I have friends in the business world who have worked for little companies that were taken over by larger companies. They tell me that their working groups still largely function as a company-within-a-company. They perform a specialized role within the larger operation. However, over time, the boundaries soften a bit. A few employees move in and out between this group and the larger organization. The big company provides the health insurance, the pension and the overall vision. However, if the little subcompany were to pick up and leave, the operation as a whole would come to a standstill, or nearly so.
I can understand how this all happens in the business world. People talk, deals are made, many lawyers are involved. But how this happened eons ago to give rise to eukaryotic cells in all their glory, still baffles and amazes me.