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

The Hydra/Chlorella symbiosis is an important model system for examini ng host/symbiont specificity in alga/invertebrate systems. The capabil ity of free-living Chlorella to establish symbioses correlates with to lerance of growth to low pH. This agrees with preliminary evidence tha t pH levels in perialgal vacuoles in aposymbiotic hydra drop to values of 3.5 - 4.0 within several hours after infection. In contrast, excre tion of maltose or other sugars and the binding capacity of several le ctins is unrelated to infectivity. Formation of successful endosymbios es also appears to be species-specific. Small subunit rRNA sequence co mparisons 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/sorokini ana group, and Strain HvT from Israel is very similar to C. prototheco ides. We conclude from the above data that ecological factors given by the environment inside the phagosomes of the hydra determine host/sym biont specificity rather than 'recognition' processes.