Plant-like proteins in protozoa, metazoa and fungi imply universal plastid endosymbiosis.
In recent years, plant-like proteins in protozoa, metazoa and fungi have been identified. Analysis of them suggests that for millions of years universal plastid endosymbiosis and gene transfer occurred in ancestors of metazoa/fungi, and some transferred fragments have been reserved till now even in modern mammals. Most eukaryotes once contained plastids in the ancient era, and some of them lost plastids later. Functions of homologues in cyanobacterial genomes and eukaryotic genomes are in consensus, and are most involved in organic compound metabolism. With emergence of organelles and subcellular structures in the eukaryotic cell, the locations of these proteins diversified. Furthermore, some novel functions were adopted, especially in vertebrates. Analysis also implies that plastids acquired through a mechanism of secondary endosymbiosis may be preserved even until the multicellular era in simple animals. Phylogenetic trees of some proteins suggest that in ancient times the common ancestor of photosynthetic protist Euglena and parasite Trypanosoma once engulfed a green alga, and then it lost the plastid, but recently some euglenids engulfed algae again. Plastid endosymbiosis is a more general process than we originally thought, and may happen more than one time in one species.