THE CELL BIOLOGY OF DINOFLAGELLATE SYMBIOSIS IN A COELENTERATE

This dissertation chronicles the cellular events involved in establishment of a symbiotic association between a dinoflagellate and the marine scyphozoan jellyfish Cassiopeia xamachana. Eleven strains of the alga Symbiodinium microadriaticum were electrophoretically defined according to isoenzyme patterns. C. xamachana distinguished between strains, selectively establishing an association with only some.;In addition to freshly isolated algae, endodermal cells will ingest a wide variety of particles including killed-symbionts, food particles, colloidal gold and carmine particles. However, live, acceptable strains of algae persist and proliferate in host tissues, in contrast to all other aforementioned particles which are subsequently removed from host tissues by either egestion or digestion.;Cellular events that occur after phagocytosis determine whether a symbiosis will be established. After phagocytosis, even algae belonging to successful strains experienced a decline in cell numbers. Some of the phagosomes containing algae fused with secondary lysosomes, but whether the decline was due to digestion is unclear.;The key feature which distinguishes symbiotic algae from all other particles is that they persist and proliferate within mesogleal cells. Mesogleal cells are derived from endodermal cells containing algae that move into the mesoglea. The hypothesis is proposed that within these mesogleal cells successful symbionts selectively avoid digestion, persist, proliferate, and induce strobilation in C. xamachana.;C. xamachana usually contains its algal symbionts in two cell types, endodermal cells and mesogleal cells. Animals were grown without symbionts and experimentally infected by incubation with appropriate strains of algae. The stages during the infection process when hosts and symbionts distinguish between appropriate and inappropriate partners were studied by comparing the cellular fates of "acceptable" and "unacceptable" strains of algae. When algae were freshly isolated from their respective hosts and presented to C. xamachana, all strains were taken into the coelenteric cavity and phagocytosed by endodermal cells. The rates of phagocytosis of such cells were high and directly correlated with the presence of a membrane, thought to be the host vacuolar membrane that surrounds the freshly isolated algae. Cultured algae lack this membrane and were phagocytosed at very low or undetectable rates; although they eventually proliferate and establish an association.