Temporal relationships of host cell and algal mitosis in the green hydra symbiosis

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Abstract
In fed hydra or excised regenerating peduncles there are increases in the mitotic indices of both digestive cells and the symbiotic algae that reside within them. Conversely, algal and digestive cell mitotic indices decrease in starved hydra. The temporal relationships of algal and host cell division differ in fed hydra and regenerating peduncles. After feeding, algal and digestive cell mitotic indices both reach a peak at about the same time; during regeneration, first the algae and then the digestive cells divide. Thus, mitotic digestive cells in regenerating peduncles contain more algae than those in gastric regions of fed hydra. However, in both cases mitotic digestive cells contain more algae than non-mitotic cells. The algae appear to be partitioned at random between daughter digestive cells at teleophase. It is suggested that the division of the symbiotic algae is closely related to that of the digestive cells in which they are maintained. Mitosis of algae is stimulated by host cell mitosis, but in non-dividing cells algal mitosis is restricted. Possible mechanisms by which the host digestive cells could restrict algal division are discussed.