SRA

The origin of the photosynthetic organelle (plastid) in eukaryotes >1 billion years ago led to the evolution of algae and plants, which form the base of the food chain for life on our planet. The plastid is derived from a captured cyanobacterial endosymbiont that is now dependent on the “host” for survival. Here we analyzed a draft genome and transcriptome assembly from the anciently diverged alga Cyanophora paradoxa and provide multiple lines of evidence supporting a single origin of the plastid in eukaryotes. Cyanophora retains ancestral features of starch biosynthesis, fermentation, and plastid protein translocation common to plants and algae but lacks typical light harvesting complex proteins. Surprisingly, the Cyanophora gene inventory includes “genomic fossils” that indicate the involvement of environmental Chlamydiae in primary endosymbiosis. These intracellular symbionts donated genes that allow export of photosynthate from the cyanobiont and its polymerization into storage polysaccharide in the host cytosol.