Three’s a crowd: plastid establishment did not require a chlamydial partner

The plastids of plants and algae trace their origin to a single, primary endosymbiosis, but the selective driver of this foundational event remains unclear. A recent hypothesis proposes that the initial role of the cyanobacterial endosymbiont was to provide energy to a host cell whose reserves were depleted by a chlamydial pathogen, and that proteins secreted by the chlamydia forged the initial metabolic link between host and symbiont through integration of bacterial metabolites into host energy stores. Here, we test this hypothesis against phylogenies of the key enzymes involved. Our analyses reveal a mosaic origin for plant energy metabolism in which genes were obtained from the host lineage, the cyanobacterial endosymbiont, and other bacterial groups. Crucially, we find no evidence for a chlamydial origin for these genes under the best-fitting phylogenetic models, suggesting there is no need to invoke a chlamydial partner in the establishment of the primary plastid endosymbiosis.