Control of cell division of the intracellular Chlorella symbionts in green Hydra.

Green hydra maintains within its digestive cells a population of symbiotic algae which remains constant in normal culture conditions, although potentially the algae have a much higher growth rate than their animal hosts. Numbers of algae per cell vary along the body column, cells in the gastric region containing more than those of the head or peduncle. This relationship is disturbed in excised, regenerating peduncles and heads; a transitory increase in algal numbers occurs, the decline of which may be prevented by application of the mitotic inhibitor vinblastine sulphate. No increase is seen in the already high numbers of algae in the gastric regions of regenerating animals. A close link between host and symbiont mitosis may explain this phenomenon. Nondividing host cells with a full complement of algae inhibit symbiont mitosis; the inhibition is removed when the host cell is stimulated to divide, as in regenerating peduncles and heads. Algae divide more rapidly than host cells, so a transient increase in algal numbers occurs. During host cell division, algae are parcelled between daughter cells, which reimpose inhibition once the normal population of algae is reached. There may be no increase in algal numbers in regenerating gastric regions because extensive mitosis already occurs. The nature of the restriction on algal growth remains obscure, but uncoupling of animal and algal mitosis during regeneration suggests a useful experimental approach to the problem.