Endosymbiosis in Hydra and the evolution of internal defense system

Certain species of Chlorella have exploited an intracellular habitat and occur naturally as cytobionts in Hydra viridissima. The algae evoke phagocytosis by Hydra digestive cells and are sequestered in individual phagosomes that migrate to the base of the host cells and resist fusion with lysosomes. The ability to resist digestion is closely correlated with release of extracellular carbohydrate (maltose) by the algae. The established population of algae grows at an average rate equal to or greater than that of the host and a constant population density is maintained. The host regulates algal population density by expelling or digesting excess algae, or by controlling algal cell division. The control mechanism is unknown but can be breached by addition of inorganic ions to the Hydra culture medium with the result that the algae overgrow the Hydra.The Hydra-Chlorella symbiosis is probably mutually beneficial, but conditions such as prolonged darkness (with or without feeding) can reduce the competitive fitness of the host since this condition results in heterotrophy by the algae at the expense of selected host substrates. The evolution of selective permeability to organic substrates is a major feature of the successful colonization of the intracellular habitat by symbiotic Chlorella.