Primary, secondary, and tertiary endosymbiosis.

(a) In primary endosymbiosis, a host cell without a plastid gains one from a cyanobacterial endosymbiont. During the evolution of a primary plastid, the bacterial cell wall is lost and most endosymbiont genes are transferred to the host nucleus. (b) In secondary endosymbiosis, a host cell gains a plastid from a eukaryotic endosymbiont that contains one or more primary plastids. During the evolution of a secondary plastid, most components of the endosymbiont cell are lost, but the plastid is retained within an envelope of endoplasmic reticulum. (c) In tertiary endosymbiosis, a host cell gains a plastid from a eukaryotic endosymbiont that has secondary plastids. Tertiary endosymbiosis occurs among dinoflagellates.