Chlorarachniophytes With Complex Secondary Plastids of Green Algal Origin

Abstract Diverse algae have evolved ‘secondary’ plastids through parallel endosymbiotic uptakes of photosynthetic eukaryotes either of green or red algae, and these events are referred to as secondary endosymbioses. Chlorarachniophytes are a group of marine unicellular algae with four-membrane-bound secondary plastids that originated from a green algal endosymbiont. Remarkably, chlorarachniophyte plastids possess a vestigial nucleus termed a ‘nucleomorph’ in the periplastidal compartment between the second and third membranes that corresponds to the remnant cytoplasm of the endosymbiont. Given that endosymbiont nuclei have disappeared in most secondary plastid-bearing algae, chlorarachniophytes appear to represent an intermediate stage of secondary endosymbiosis and thereby offer an interesting opportunity to study complex plastid evolution. In this chapter, I summarise the current understanding of the evolutionary history of chlorarachniophytes in terms of morphology, phylogeny, and genomics.