Plastid Endosymbiosis. Sources and Timing of the Major Events.

This chapter reviews the current ideas regarding the origin of plastids in eukaryotes and the timing of these events, with particular emphasis on the initial source of eukaryotic photosynthesis. First, a general introduction is provided on plastid endosymbiosis. The currently available evidence suggests that a single primary endosymbiosis gave rise to the Plantae, comprising the glaucophytes, red algae, and Viridiplantae. The chapter looks in detail at the evidence regarding the source and timing of the plastids that have resulted from primary, secondary, and tertiary endosymbiosis. The greatest attention is paid to the primary endosymbiosis. Primary plastid's origin and Plantae monophyly are discussed in detail. The unique and relatively late appearance of photosynthetic eukaryotes has important implications for understanding the early biosphere and its fossil record. Nuclear phylogeny supports the monophyly of photosynthetic eukaryotes containing a primary plastid, and molecular clock estimates provide a timeline for reconstructing the early evolutionary history of the Plantae. Together with the fossil and geochemical records, these data provide an increasingly resolved view of early eukaryotic photosynthesis and, by extension, important features in evolutionary and Earth history. Plastid endosymbiosis has clearly been a driving force in eukaryotic evolution and instrumental to the success of many eukaryotic groups and has influenced the evolution of other organisms that utilize these organisms for food or habitat. Ongoing studies of Paulinella may shed light on the early stages of primary endosymbiosis, a process that has profoundly impacted evolution of life on Earth.