What Has Paulinella Taught us About Endosymbiont Metabolic Integration?

Primary endosymbiosis between a cyanobacterium and a protist marked the origin of photosynthetic eukaryotes, profoundly impacting the history of life on our planet. This event occurred 1.6–2.1 Bya and gave rise to the putative single common ancestor of the Archaeplastida (i.e., glaucophytes, red algae, and green algae plus land plants). A second independent primary endosymbiotic event took place more recently (~120 Mya) and resulted in photosynthetic Paulinella species that harbor primary plastids known as chromatophores. Whereas there are multiple examples of plastid secondary and tertiary endosymbiosis, fueling massive diversification of photosynthetic eukaryotes on Earth, instances of primary plastid origin are fleetingly rare. This review summarizes what has been learned about organellogenesis using the nascent Paulinella system, emphasizing mechanisms and constraints with respect to host–endosymbiont metabolic integration. Similarities and differences between the nascent Paulinella symbiosis with the more ancient Archaeplastida system are described, and possible convergent outcomes of these systems are discussed.