Secondary endosymbiosis

Secondary endosymbiosis occurs when a eukaryotic cell engulfs and retains a photosynthetic eukaryote, converting its plastid into a plastid with multiple surrounding membranes.

Endosymbiosis: A lasting association in which one organism lives inside another. Secondary endosymbiosis is a eukaryote-within-eukaryote form of this broader process.
Primary endosymbiosis: The origin of plastids when a eukaryote engulfed a cyanobacterium. It produced the algal plastid later transferred in secondary endosymbiosis.
Euglenids: A group of flagellated eukaryotes, including photosynthetic species with green-algal plastids. Their plastids exemplify secondary acquisition from a green alga.
Tertiary endosymbiosis: The acquisition of a eukaryotic alga that had itself acquired a plastid through secondary endosymbiosis. It adds another eukaryotic host to an already layered plastid history.
Plastid: A membrane-bound organelle in eukaryotes, including chloroplasts and related nonphotosynthetic forms. The engulfed alga’s plastid is the organelle retained by the host.
Archaeplastida: A eukaryotic group that includes red algae, green algae, and land plants. Its members include the primary plastid-bearing lineages that served as secondary endosymbionts.
Chlorarachniophytes: A group of amoeboid algae with green-algal plastids and a nucleomorph. Their plastids retain a reduced nucleus from the engulfed green alga.
Serial endosymbiosis: The acquisition of successive endosymbionts by a lineage over evolutionary time. Multiple acquisitions and replacements complicate reconstruction of plastid ancestry.
Nucleomorph: A highly reduced nucleus retained from an engulfed eukaryotic alga. Its presence preserves direct evidence that a eukaryote became an endosymbiont.
Red algae: A diverse group of photosynthetic eukaryotes, many of which contain red-algal plastids. Red algae were engulfed in several independent origins of complex plastids.

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