Why Do so Few Animals Form Endosymbiotic Associations with Photosynthetic Microbes?

A survey of modern associations in which protists or invertebrates are hosts shows that very few of the many species of photosynthetic microbes are adapted to an endosymbiotic existence. None occurs as intracellular symbionts in animals structurally more complex than cnidarians and platyhelminths. Photosynthetic symbionts are not usually capable of being the sole food source for hosts because they do not provide a balanced diet; most hosts therefore retain holozoic feeding. Interactions between hosts and intracellular symbionts are complex, and have to include mechanisms for inducing release of photosynthate from symbionts as well as controlling symbiont cell division. Possession of symbionts imposes a measurable cost on hosts. For the great majority of animals, the costs of adapting to herbivory or other forms of nutrition are probably less than that of hosting photosynthetic symbionts, especially when the need for exposure of a large surface area to light is borne in mind. Once hosts become multicellular, it is virtually impossible for any photosynthetic symbionts they possess to evolve into organelles because they are restricted to specific host cell types. After the evolution of the eukaryotic ancestors of plants in the late Precambrian, the major significance of endosymbiosis to the evolution of plant-animal interactions has been the development of gut symbioses in some major groups of herbivores.