The origin and spread of eukaryotic photosynthesis: Evolving views in light of genomics

Plants and algae acquired photosynthesis through the assimilation of a prokaryotic endosymbiont related to the ancestors of modern-day cyanobacteria. This landmark event, known as the primary endosymbiotic origin of plastids, is generally thought to have occurred only once during the history of eukaryotes and to have given rise to the plastids of green algae, land plants, red algae and glaucophyte algae through vertical evolution. Plastids have also spread horizontally across the tree of eukaryotes by "secondary" endosymbioses involving heterotrophic host eukaryotes and both green and red algal endosymbionts. Here I provide an overview of current research in the area of plastid evolution, focusing on the latest advances in the field of algal comparative genomics. Recent genome-scale analyses of both photosynthetic and non-photosynthetic eukaryotes have provided fresh new insight into the pattern and process of secondary endosymbiosis, although it is still not possible to discern with confidence the number of endosymbiotic events that gave rise to the known spectrum of eukaryotic phototrophs. In fact, with more genomic data has come the intriguing possibility that the nuclear genomes of some secondary plastid-containing algae are a mosaic of genes derived from multiple endosymbioses, adding yet another layer of complexity to the convoluted evolutionary history of these fascinating organisms.