ABSTRACT.Protozoa may be thought of as preadapted to serve as hosts for cellular endosymbionts by virtue of their widespread ability to take up particles by endocytosis. The absence of the cell wall so characteristic of plants and fungi and the commonly large size of most protozoa are additional factors favoring protozoan cells for endosymbioses. The conversion of symbiont into a cellular organelle (e.g. a mitochondrion or chloroplast) is more complicated, especially since the latter do not code for all of their own proteins. Thus, such conversions are held to be rare. Among protozoa, numerous foraminifera appear to have characteristics making them very favorable as hosts for certain algae. Such adaptations, both physiological and morphological in nature, are discussed. Also discussed in this paper are the ways by which (present‐day) chloroplasts and mitochondria may have been derived from early endosymbionts: a single ancestral cyanobacterium, in the first case, and a single ancestral purple‐nonsulfur bacterium, in the second. Mechanisms for insertion of proteins into and across the organellar membranes had to be evolved for all genes transferred from the symbionts into the host nucleus.

CAVALIER‐SMITH, T., & LEE, J. J. (1985). Protozoa as Hosts for Endosymbioses and the Conversion of Symbionts into Organelles1,2. The Journal of Protozoology, 32(3), 376–379. Portico.