Plastid Origin and Evolution

Abstract

Plastids (or chloroplasts in plants) are organelles within which photosynthesis takes place in eukaryotes. The origin of the widespread plastid traces back to a cyanobacterium that was engulfed and retained by a heterotrophic protist through a process termed primary endosymbiosis. Subsequent (serial) events of endosymbiosis, involving red and green algae and potentially other eukaryotes, yielded the so‐called ‘complex’ plastids found in photosynthetic taxa such as diatoms, dinoflagellates and euglenids. The field of plastid research also includes nonphotosynthetic organelles (apicoplasts) within the parasitic apicomplexans and the temporary sequestration (‘theft’) of plastids by heterotrophic organisms (kleptoplasty) such as the sea slug Elysia chlorotica . The gain and loss of plastids, and nonlineal gene transfer (associated with endosymbiosis) are key aspects of algal evolution that have decisive impacts on inference of their phylogenetic positions in the tree of life. Deciphering plastid origin therefore provides general insights into the evolution of eukaryote lineages. Key Concepts: The plastid organelle in eukaryotes originated from primary endosymbiosis involving a cyanobacterium. The secondary (and tertiary) plastids show a history of multiple (serial) endosymbiosis involving red and/or green algae. Plastid research is complicated by the presence of nonphotosynthetic plastids in parasitic apicomplexans and by the theft of temporary plastids referred to as kleptoplasty. Understanding plastid origin enhances our understanding of eukaryote evolution.