Proteomic and transcriptional analyses of coral larvae newly engaged in symbiosis with dinoflagellates.

Many marine cnidarians, such as stony corals, engage in intracellular symbiosis with dinoflagellates, forming the trophic and structural foundation of the coral reef ecosystem. Very little is known about the cellular and molecular mechanisms that are at work in these associations. In this study, we examined changes in both the soluble proteome and the transcriptome of larvae of the Hawaiian solitary coral Fungia scutaria during the onset of symbiosis with Symbiodinium sp. We conducted a comparative host coral proteome analysis using 2-dimensional gel electrophoresis, as well as a comparative host coral transcriptome analysis using a cDNA subtracted library and real-time quantitative PCR. Age-related changes in the larval protein profiles were detected, however virtually no symbiosis-related changes in the protein profiles were evident. Symbiosis-related differences in gene expression were found using a subtracted library, although the differences were very subtle and almost undetectable using real-time quantitative PCR.

Two atypical carbonic anhydrase homologs from the planula larva of the scleractinian coral Fungia scutaria.

In cnidarians, the enzyme carbonic anhydrase (CA) is important to inorganic carbon (Ci) flux in processes including calcification and dinoflagellate symbiont photosynthesis. Although CA is known to function in Ci delivery to symbionts in adults with mature symbioses, it is not known when CA becomes active in this capacity during the onset of symbiosis in developing hosts. We identified two CA cDNA sequences from the planula larvae of the Hawaiian scleractinian coral Fungia scutaria. Expression of these larval CAs did not differ between infected and uninfected larvae or vary over the course of infection. Bioinformatic analyses of the two homologs showed that the sequences are unusually short and are missing some residues that support active site structure in other CAs. This is the first description of a short form of CA. Phylogenetic analyses of the larval CAs grouped them with membrane-bound homologs from vertebrates. Studies in other calcifying cnidarians have identified membrane-associated CAs as functioning in calcification, and therefore the two larval CAs could play a role in the onset of calcification during metamorphosis. A longer CA isoform was amplified from adult F. scutaria cDNA but not from larvae, suggesting that the longer form is not expressed in larvae. The longer form grouped with cytosolic CAs including a symbiotic anemone homolog implicated in Ci delivery to dinoflagellate symbionts. The apparent absence of this "symbiosis" CA in larvae suggests that the Ci supply mechanism is not active during the initial onset of the association.

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.