The green hydra symbiosis. II. The biology of the establishment of the association

In laboratory experiments, ‘European’ green hydra established a permanent symbiosis with five different strains of Chlorella . A number of other Chlorella strains were unable to establish a symbiosis; some were rejected rapidly, but others showed varying capacities to persist in digestive cells for relatively long periods before eventually disappearing. It was con­cluded that specificity of green hydra for its algae was moderately broad, with no sharp distinction between its reaction to acceptable and that to unacceptable algae. Chlorella strains that possessed sporopollenin and the capacity for mesotrophic growth were, in general, more acceptable than those lacking these characteristics. During re-establishment of the symbiosis, large numbers of algal symbionts were initially taken into digestive cells, but, after 6-12 h, the num­ber of algae per hydra began to decline, reaching a minimum value 2-6 days after reinfection. This ‘resorting phase’ was then followed by a rapid increase in symbiont number until the normal population size was achieved. The rate of initial uptake of algae into digestive cells varied greatly between different Chlorella strains, but there was no correlation between uptake rate and eventual capacity to establish a symbiosis. Latex spheres could also enter digestive cells, and their rate of entry could be substan­tially increased by suspending them in glycogen. There was no evidence that specific recognition of potential symbionts occurred at first surface contact. A general difference between acceptable and unacceptable algae after phagocytosis is that the former remain in individual vacuoles and migrate to the distal end of digestive cells, while the latter aggregate into large apical vacuoles and are ejected. However, if latex spheres were suspended in bovine serum albumin and injected into the coelenteron of starved aposymbiotic hydra, they remained in individual vacuoles after uptake into digestive cells, and many of them also migrated to the distal end of the cells. This observation casts doubt upon whether the ability of algae to remain in individual vacuoles and to migrate to the distal end is necessarily associated with the specific recognition of symbionts. ‘Florida’ hydra behaved similarly to ‘European’ hydra, except that they were distinctly more difficult to infect and were not quite so broad in specificity. Digestive cells of brown hydra were unable to take in algae. It is concluded that there may be no single stage in the establishment of the symbiosis in green hydra that constitutes a single, positive act of specific recognition. Rather, the establishment of the symbiosis may be better regarded as a process of ecological colonization. The fact that only a few kinds of algae can become symbiotic may simply reflect the severity of the obstacles that a symbiont has to overcome in colonization and not necessarily indicate the existence of a definite mechanism for recognizing symbionts.