Journal of Cell Science, Vol 88, 571-578, Copyright © 1987 by Company of Biologists

Submitted on June 2, 1987
Accepted on August 10, 1987

Growth of endosymbiotic algae in the green hydra, Hydra viridissima

¹ Department of Ecology and Evolution, State University of New York, Stony Brook, Seio York, SY 11794, USA; Department of Pathology, College of Physicians and Surgeons, Columbia University, New York, NY 10032, USA

Stable endosymbiosis depends upon balanced growth of the symbionts. In green hydra, coincident patterns of host and algal mitotic index suggest that coordinated reproduction provides for balanced growth. However, when hydra shrink during food shortage, the population of endosymbiotic algae in a green hydra must likewise decline in size. Thus far, no mechanism of reducing the size of the endosymbiont population has been described. Algal mitosis was found here to be stimulated by host feeding and clumped in its distribution among host cells, supporting the notion of some degree of control of algal mitosis exercised at the level of the host cell. However, comparisons of the rates of algal mitosis with the realized rates of algal population growth show that substantial numbers of algae disappear from hydra, in numbers in excess even of those necessary to accommodate host shrinkage. Only a small proportion of these lost algae was found to be expelled by hydra. Microscopic observations of the cells of macerated hydra show evidence of algal disintegration in nearly 50 % of the digestive epithelial cells of regularly fed hydra. Coincidence of remnants of algal cells and food-derived materials within the same vacuoles suggests that algae are digested by host cells.

Key words: symbiosis, hydra, Chlorella

Submitted on June 2, 1987
Accepted on August 10, 1987