Tracking the early events of photosymbiosis evolution

Oxygenic photosynthesis evolved in cyanobacteria around 3.2 giga-annum (Ga) ago and was acquired by eukaryotes starting around 1.8 Ga ago by endosymbiosis. Photosymbiosis results either from integration of a photosynthetic bacteria by heterotrophic eukaryotes (primary photosymbiosis) or by successive integration of photosymbiotic eukaryotes by heterotrophic eukaryotes (secondary photosymbiosis). Primary endosymbiosis is thought to have been a rare event, whereas secondary and higher-order photosymbiosis evolved multiple times independently in different taxa. Despite its recurrent evolution, the molecular and cellular mechanisms underlying photosymbiosis are unknown. In this opinion, we discuss the primary events leading to the establishment of photosymbiosis, and we present recent research suggesting that, in some cases, domestication occurred instead of symbiosis, and how oxygen and host immunity can be involved in symbiont maintenance.