Figure 3.

Hypothetic stages of plastid origin and establishment. The stages (as discussed in the text) are displayed as follows: (a,b) first stage; (c) second stage; (d) third stage; and (e) fourth stage. White, yellow, and green colors show α-proteobacterial (mitochondrial), chlamydial, and cyanobacterial endosymbionts, as well as genes and proteins of their respective origins. Arrows directly from the endosymbiont point to the symbiotic partner that receives the benefit, and the thickness of the arrow indicates the degree of benefit. Dashed lines indicate directions of intracellular gene transfer, whereas solid lines show protein targeting of the transferred genes. Crosses indicate chlamydial endosymbiont and gene transfer processes that might not exist in extant photosynthetic eukaryotes. Note that chlamydial endosymbiont was initially a bacterial parasite in the first stage, but it had a transient mutualistic relationship with the host cell in the second and third stages, and then might have degenerated in modern photosynthetic eukaryotes. Note also that the cyanobacterial endosymbiont was initially captured to solely benefit the host cell (panel b), and then received metabolites from the host cell (a process facilitated by the chlamydial endosymbiont) and was gradually transformed into a plastid organelle in the host cell (panels d and e).