THERE IS AN ECOLOGICAL BASIS FOR HOST/SYMBIONT SPECIFICITY IN CHLORELLA/HYDRA SYMBIOSES

Abstract

Summary: The Hydra/Chlorella symbiosis is an important model system for examining host/symbiont specificity in alga/inverteb rate systems. The capability of free-living Chlorella to establish symbioses correlates with tolerance of growth to low pH. This agrees with preliminary evidence that pH levels in perialgal vacuoles in aposymbiotic hydra drop to values of 3.5 - 4.0 within several hours after infection. In contrast, excretion of maltose or other sugars and the binding capacity of several lectins is unrelated to infectivity. Formation of successful endosymbioses also appears to be species-specific. Small subunit rRNA sequence comparisons showed that native symbionts of H. viridissima descend from at least two symbiotic events. Symbionts of the 'European'-, 'Swiss'- and 'Jerusalem' strain are closely related to the C. vulgaris/soro kiniana group, and Strain HvT from Israel is very similar to C. protothecoides. We conclude from the above data that ecological factors given by the envi­ ronment inside the phagosomes of the hydra determine host/symbiont specificity rather than 'recognition' processes.