Secondary and tertiary endosymbiosis

Eukaryotic cells evolved the ability to harness the energy of sunlight by the process of endosymbiosis. Well after the endosymbiotic origin of mitochondria, an ancestor of modern-day cyanobacteria was taken up by a heterotrophic eukaryote and gradually became a fully integrated subcellular organelle within the cytoplasm of its host. This event led to the differentiation of three algal lineages: red algae, glaucophyte algae, and green algae (as well as their land plant derivatives), all of which possess double membrane-bound plastids. On multiple occasions after these three lineages diverged, their plastids were acquired by other eukaryotes by the process of secondary endosymbiosis, that is, the engulfment and retention of a primary plastid-bearing alga by a second, nonphotosynthetic eukaryote host. Secondary plastids are characterized by the presence of either three or four outer membranes and, in some cases, the retention of the nucleus of the algal endosymbiont in a residual cytoplasmic compartment. This article gives a detailed overview of the genetic, molecular, and cell biological aspects of secondary endosymbiosis, summarizes the diversity of secondary endosymbioses, and reviews what is known about the tempo and mode of secondary endosymbiosis and its role in the diversification of photosynthetic life. The phenomenon of tertiary endosymbiosis is also discussed.