Lectin/glycan interactions play a role in recognition in a coral/dinoflagellate symbiosis

Recognition is an important stage in the establishment of highly specific mutualistic associations. Yet, for the majority of symbioses, very few of the mechanisms involved in recognition and specificity are known. In this study, we provide evidence for a recognition mechanism at the onset of symbiosis between larvae of the coral Fungia scutaria and their endosymbiotic dinoflagellate algae. This recognition step occurs during initial cellular contact between the symbiotic partners through a lectin/glycan interaction. We determined that an intact algal cell surface was required for successful infection of F. scutaria larvae. Modification of the algal cell surface by enzymatic digestion with trypsin or N-glycosidase significantly reduced infection success, and implicated algal cell surface glycans in recognition. Using flow cytometry, alpha-mannose/alpha-glucose and alpha-galactose residues were identified as potential recognition ligands on the algal cell surface. Finally, inhibition of these cell surface glycans significantly reduced infection of F. scutaria larvae by the algae. These data provide evidence that the algal cell surface contains glycan ligands, such as alpha-mannose/alpha-glucose and alpha-galactose, which play a role in recognition during initial contact at the onset of symbiosis with F. scutaria larvae.