Chloroplast evolution--ancient and modern.

The traditional theory of serial endosymbiosis envisages the origin of all chloroplasts from prokaryotic algal symbionts. Of the two membranes that immediately surround plastids, the inner is considered homologous with the plasma membrane of the symbiont and the outer homologous with the vacuolar membrane provided by the host. This theory has been modified to suggest that those chloroplasts that are surrounded by more than two membranes were derived, following a second act of symbiosis, from eukaryotic rather than prokaryotic symbionts; these may have been whole eukaryotic algae or chloroplasts isolated from them. This suggestion is partly based on homologies between the various membranes that surround chloroplasts of different taxonomic groups and those that surround photosynthetic symbionts known today. If chloroplasts did indeed evolve from photosynthetic symbionts, then algal phylogeny must take account not only of the individual characteristics of the host cells and their symbionts but also of modifications of these characteristics arising from interactions between host and symbiont.