| Algal-invertebrate associations demonstrate specificity in their establishment and persistence. The determinants of specificity are unknown. One hypothesis states that receptor-ligand interactions are involved. A competing hypothesis states that "ecological factors given by the environment" inside the host determine specificity rather than "recognition" processes. Yet, all known cellular interactions are in some way receptor mediated. The symbiosis between the dinoflagellate Symbiodinium microadriaticum and the mangrove jellyfish Cassiopeia xamachana was employed as a model to investigate the molecular basis of recognition in algal-invertebrate associations. In order to complete its polyphasic life history, the host must establish a stable association with an appropriate species of microalgae. Aposymbiotic C. xamachana remain in developmental arrest as scyphistomae (asexually reproductive polyps). Numerous species of symbiotic dinoflagellates are maintained in culture in the laboratory. The cell walls of the algae contain cellulose as well as species-specific SDS-extractable proteins/glycoproteins which could serve as signals in cellular interactions between host and symbiont. In addition, all of the algae studied exude water-soluble high molecular weight glycoconjugates to the growth medium. The proteins/glycoproteins associated with the cell walls may be intrinsic cell wall macromolecules or exudates in transit. These molecules are also possible candidates as molecular signals (ligands). The exudates are heterogeneous and include proteins/glycoconjugates of M{dollar}sb{lcub}rm r{rcub}{dollar} 200kDa to 14kDa as determined by SDS-PAGE. HPLC analyses of the sugar and amino acid composition of exudates from five species of symbiotic dinoflagellate algae demonstrated the presence of glucose, galactose, and their respective amines, fucose, ribose, and mannose, and virtually all the essential amino acids. The uronic acid content of exudates from three algal species that infect scyphistomae of the jellyfish C. xamachana were higher than those of two species which did not infect. Highly specific polyclonal antibodies directed against exudate from S. microadriaticum, were produced in a rabbit. Using the antibodies as probes, an immunohistochemical study at was done at both the LM (light microscopic) and EM (electron microscopic) levels. The results show that nitrogen-rich macromolecules released by symbiotic dinoflagellates in culture, are also released in hospite. These previously undetected molecules have the potential to serve as molecular signals, structural molecules, and/or a rich nutritional source of high quality nitrogen, both essential and non-essential amino acids, and carbohydrate. |
