Cartoon representation of major events during the evolution of streptophyte phytochromes. D indicates the duplication of phytochrome genes that gave rise to A- and B-type phytochromes. X indicates loss of A-type phytochromes; R indicates the rearrangement of PGP and PAS/PAS/HK in the evolution of gymnosperm P-type phytochromes. Secondary endosymbiosis is indicated by the curved arrow. The overall domain arrangement of diatom (heterokont, left) and streptophyte phytochrome is also shown.