Figure 1.

A schematic outline of the acquisition, reduction, and loss of genomes and compartments during evolution. Black arrows indicate evolutionary pathways; white arrows indicate endosymbiotic events in the host cell. Endosymbiotic event 1 occurred at the origin of eukaryotes. The proteobacterial endosymbiont gave rise to mitochondria (the smaller organelles in the bottom part of the diagram). Endosymbiotic event 2 occurred at the origin of plastid-containing cells. Endosymbiotic event 3 represents the secondary and higher-order endosymbioses giving rise to numerous algal phyla, as well as apicomplexans (such as Plasmodium) which have residual plastids, and to trypanosomes, which have no plastid at all. Black, filled circles indicate nuclei or nucleomorphs; ellipses within organelles indicate bacterially derived genomes, which may be reduced or lost completely. More than one kind of host cell and of endosymbiont is involved in the secondary, and in the higher-order, symbioses. The genome of the Archaebacterium is not represented in the diagram.