As Lynn Margulis elegantly explained, some eukaryotic organelles — such as mitochondria and chloroplasts — are the product of an ancient endosymbiosis event. Free living prokaryotes were absorbed by primitive eukaryotes and, over many generations, become entangled in an obligate host-symbiont interaction. There are other examples of such interactions between eukaryotes and intracellular symbionts, such as those found within deep sea tubeworms, sponges, and plant roots.
The structures we currently call organelles were, at one point, merely endosymbionts. Where do we draw the line between endosymbiont and organelle? Ursula Theissen and Bill Martin lay out the ground rules for what we can call an endosymbiont and what we can call an organelle:
Formerly endosymbiotic organelles are double-membrane-bound intracellular structures.
All of the cytosolic proteins in an endosymbiont are encoded by the endosymbiont’s genome. Most organellar proteins are encoded by nuclear genes and translated by the host’s ribosomes.
Organelles must import most of their proteins using evolved protein importation apparatuses. Theissen and Martin argue that this is “rate-limiting step in the transition from endosymbionts to organelles”.
Merely being an obligate host-parasite interaction does not make an endosymbiont an organelle. Only when the endosymbiont abandons most of its genes in favor of those encoded by the nuclear genome does it gain the status of organelle. In order to do so, it must be able to import proteins across its double membrane.
Theissen and Martin’s article is inspired by four publications that describe an obligate endosymbiont as a plastid (1, 2, 3, 4). Two authors from the previous studies reply to Theissen and Martin, pointing out that the host in question regulates the number of symbionts within the cell as well as their replication and segregation. They argue that this justifies calling the endosymbiont a plastid. In the end, it’s all just semantics, right?