S0168-9525(02)02777-4

interchange.ubc.ca It is now well established that plastids, the light-harvesting organelles of photosynthetic eukaryotes, are the product of an ancient symbiosis between a eukaryote and a cyanobacterium. Together with the endosymbiosis that gave rise to mitochondria, the origin of plastids ranks as one of the most significant events in the evolution of the eukaryotic cell, because it gave rise to all phototrophic eukaryotes. Studies based on gene sequences encoded in the mitochondria [1], plastids [2–4] and nuclei [5,6] of plants and algae reveal that all plastids ultimately trace back to a single endosymbiotic event with a cyanobacterium, called the primary endosymbiosis (Box 1). However, primary-plastid-containing plant