Figure 4. Mixotrophy hypothesis for the origin of the primary plastid in algae and plants.

The ancestor of Plantae was a phagotrophic protist that consumed cyanobacteria as food and was parasitized by environmental Chlamydiae. Gene transfers from both prokaryotic sources to the host nucleus either resulted in their decay and loss or occasional gene replacement as observed in modern-day protists [25], [43]. The transition to mixotrophy was facilitated by the transfer and activation of key genes from the cyanobacterium such as those that regulate the cell cycle. The mixotrophic Plantae ancestor continued to consume bacterial prey but over time, harvesting genes from Chlamydiae and eventually developed a regulated metabolic connection (e.g., for the export of fixed carbon compounds) between the newly established endosymbiont and the host cytosol [21] and a system for protein import into the endosymbiont using the host secretory system [11]. These developments cemented the relationship and led to selection for massive gene loss in the endosymbiont and EGT to the host nucleus. Activation of at least 37 genes recruited from Chlamydaie further enhanced plastid functions. The final transition occurred in a prey-poor environment that favored phototrophy. Thereafter this ancestral alga lost the ability for phagotrophy and diversified into the extant lineages of green, red, and glaucophyte algae.