Ever wonder how the two most important energy producing machines on our planet, the organelles mitochondrion and plastid got their start? Well, it was through a process made famous by the late Lynn Margulis, that is known as primary endosymbiosis; but they needed help. In endosymbiosis one cell engulfs another (a bacterium) and over evolutionary time (i.e., millions of years) reduces it to an organelle. The problem with this scenario is that when cells enter other cells, it is usually as food or as an unwanted pathogen seeking host resources. In the first case, the cell is rapidly digested, and in the second case, if possible, destroyed. This latter case of self-defense was an early innovation on our planet and therefore makes the simple endosymbiosis model difficult to, as they say, digest. As a possible solution to this conundrum Debashish Bhattacharya and colleagues Steven Ball from Lille, France and Andreas Weber from Duesseldorf, Germany propose in their Perspectives piece in Science magazine that both cases of endosymbiosis were made possible by the role of pathogens. Here, they mean specialized cells well suited to escape host defenses that could act as longer-term symbionts persisting in the protected, nutrient-rich intracellular environment. For the mitochondrion, it was likely that an archael (prokaryotic) cell captured a Rickettsiales-like alphaproteobacterium that became the organelle (see image), and in the case of the plastid, a parasite related to present day chlamydiae sheltered the cyanobacterium that ultimately became the photosynthetic organelle. A lot of detailed science supports these ideas, as cited in the article, and a lot more needs to be done to test them. How did this theory initially take wing? It turns out that the sequencing of genomes of bacterial cells (i.e., related to Rickettsiales and Chlamydiales) that occur in natural symbioses with protists is critical. These cells have larger genomes than their cousins that are full-fledged human parasites. Environmental genomes such as these inform us about how cells persist inside their hosts and what these longer-term interactions mean for the process of primary endosymbiosis.
Current understanding of the origin of eukaryotic organelles. The host cell was derived from a member of the Lokiarchaeota that underwent primary endosymbiosis (filled violet circle) with a Rickettsiales-like obligate intracellular bacterium, giving rise to the mitochondrion. Thereafter, the Archaeplastida ancestor underwent primary endosymbiosis (filled green circle) with a cyanobacterium, putatively aided by a chlamydial infection (red cell).