Immunocytochemical evidence that symbiotic algae secrete potential recognition signal molecules in hospite

Cnidarian–dinoflagellate symbioses are not well understood at the molecular level. Observed specificity between partners during initiation, establishment, and maintenance of the relationship strongly implies a role for chemical signaling. This report presents biochemical and immunocytochemical evidence for potential signaling molecules, as large molecular weight glycoproteins, secreted by Symbiodinium dinoflagellates both in culture and in symbiosis. Polyclonal antibodies directed against recovered exudate from S. microadriaticum, the natural endosymbiont of Cassiopea xamachana, the upside–down jellyfish, were highly specific in recognizing exudates from Symbiodinium species that can successfully induce developmental metamorphosis in the host but did not recognize exudates from Symbiodinium species that do not. Immunoblot analyses showed S. microadriaticum exudate to be protease sensitive. Release of antigenic material by symbiotic S. microadriaticum was demonstrated through light and electron microscopy using immunogold-labeled anti-S. microadriaticum (anti-Sm-XuLg) antibodies as probes. These secreted, symbiont-derived glycoconjugates may be candidates for interspecific molecular signals.