The Chromalveolate Hypothesis
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Two putative “supergroups” of anciently derived photosynthetic eukaryotes exist in the tree of life, the Archaeplastida (i.e., Plantae; red, green [including plants], and glaucophyte algae) and the Chromalveolata (cryptophytes, haptophytes, and stramenopiles, alveolates). It is widely accepted that the photosynthetic organelle (plastid) of Plantae traces its origin to primary endosymbiosis, whereby a unicellular protist (the ‘host’) engulfed and retained a photosynthetic cyanobacterium (the endosymbiont). This momentous step in evolution likely occurred in the late Paleoproterozoic about 1.5 billion years ago with the resulting proto-alga being the putative common ancestor of this eukaryotic supergroup. Under the most parsimonious scenario, a single Plantae ancestor underwent the exceptional process of primary endosymbiosis described above that was ultimately driven by ecological pressures acting on the host genome. Once established, the primary plastid has apparently never been lost by Plantae hosts. In contrast to Plantae, the chromalveolates gained their widespread plastid through secondary endosymbiosis (i.e., eukaryote-eukaryote), whereby under the most prominent hypothesis, a red alga was engulfed and reduced to a secondary plastid. This event occurred about 1.2 billion years ago and gave rise to the putative photosynthetic ancestor of this supergroup.