Origin and Evolution of Plastids: Genomic View on the Unification and Diversity of Plastids

The notion “plastid” unifies the diversity of various plastids in diverse photosynthetic eukaryotes and certain non-photosynthetic parasites. Plastid diversity can be seen in the photosynthetic or metabolic capacities, the photosynthetic accessory pigments, the architecture of plastid membranes, and the size and content of plastid genomes. The central unifying principle is that all plastids are bound by two envelope membranes and possess plastid DNA, which was inherited from an ancestral cyanobacterial endosymbiont. Although phylogenetic analysis of the relationship between cyanobacteria and plastids does not identify the cyanobacterial species nearest to the plastid origin or the branching order of various plastids lineages, the radiation of both extant cyanobacteria and plastids is estimated to have occurred on a similar geological timescale. In addition to two major secondary endosymbiogenesis each involving a red and a green algal endosymbionts, tertiary endosymbiotic events have been proposed to explain the origin of diverse dinoflagellates. A new concept of plants suggests that all hosts of secondary or tertiary plastid endosymbiogenesis had once possessed primary plastids and subsequently lost them, and thereby they had been prepared to accept new plastids. In spite of these recent developments in plastid phylogeny that demonstrate continuity of plastid genomes, discontinuous evolution of plastid genomic machinery is another aspect of plastid evolution. Plastids gained various regulatory mechanisms from their host organisms during the evolution of land plants such that the genomic machinery that runs the plastid genome of the flowering plants no longer looks like red algal counterparts, nor even the cyanobacterial genomic machinery.