Melatonin in the evolution of plants and other phototrophs

Melatonin is present in numerous phototrophic eukaryotes, not only in plants in the meaning of Archaeplastida or of Viridiplantae. It is also formed in members of other superclades, such as Excavata and SAR clade. Typically, their respective phototrophs have acquired chloroplasts from phototrophic eukaryotes, either by taking them up as endosymbionts or by chloroplast capturing. It has been the aim of this overview to trace the phylogenetic relationships between the various phototrophs according to actual, genetically based taxonomy. This includes the consideration of primary heterotrophs that also exist within the same groups and some secondary heterotrophs that have lost functional chloroplasts. The presence of melatonin in these different taxa is discussed under the aspects of its cyanobacterial or α-proteobacterial origins, as transmitted by plastidial or mitochondrial ancestors, or by horizontal gene transfer. Peculiarities of melatonin metabolism that have evolved in some of these groups are also addressed.