Competition between Chlorellae in chimeric infections of Hydra viridis: the evolution of a stable symbiosis.

Aposymbiotic polyps of Hydra viridis were infected with one or two of the following strains of Chlorella: the native strain obtained from green H, viridis, and the originally non-symbiotic strains Fs and 211/8p cultured in vitro. Larvae of Artemia served as infecting vectors. Chimeric infections were obtained with two different Chlorella strains cohabiting in the same cells and polyps. In time, the chimeric infections disappeared and mixed populations of Hydra were formed with different strains of Chlorella in different polyps. We suggest that the Chlorella/Hydra symbiosis was initiated originally by an infection of preadapted hydra by preadapted chlorellae. Through intracellular interalgal competition and competition between dissimilar infected cells and polyps, the present-day stable symbiosis has evolved.