N-Acetyl-d-Glucosamine-Binding Lectin in Acropora tenuis Attracts Specific Symbiodiniaceae Cell Culture Strains

Many corals establish symbiosis with Symbiodiniaceae cells from surrounding environments, but very few Symbiodiniaceae cells exist in the water column. Given that the N-acetyl-d-glucosamine-binding lectin ActL attracts Symbiodiniaceae cells, we hypothesized that corals must attract Symbiodiniaceae cells using ActL to acquire them. Anti-ActL antibody inhibited acquisition of Symbiodiniaceae cells, and rearing seawater for juvenile Acropora tenuis contained ActL, suggesting that juvenile A. tenuis discharge ActL to attract these cells. Among eight Symbiodiniaceae cultured strains, ActL attracted NBRC102920 (Symbiodinium tridacnidorum) most strongly followed by CS-161 (Symbiodinium tridacnidorum), CCMP2556 (Durusdinium trenchii), and CCMP1633 (Breviolum sp.); however, it did not attract GTP-A6-Sy (Symbiodinium natans), CCMP421 (Effrenium voratum), FKM0207 (Fugacium sp.), and CS-156 (Fugacium sp.). Juvenile polyps of A. tenuis acquired limited Symbiodiniaceae cell strains, and the number of acquired Symbiodiniaceae cells in a polyp also differed from each other. The number of Symbiodiniaceae cells acquired by juvenile polyps of A. tenuis was correlated with the ActL chemotactic activity. Thus, ActL could be used to attract select Symbiodiniaceae cells and help Symbiodiniaceae cell acquisition in juvenile polyps of A. tenuis, facilitating establishment of symbiosis between A. tenuis and Symbiodiniaceae cells

Possible involvement of Tachylectin-2-like lectin from Acropora tenuis in the process of Symbiodinium acquisition

Most reef-building corals in tropical and subtropical areas symbiose with microalgae from the genus Symbiodinium (dinoflagellate) and depend on the photosynthate produced by the microalgae. The majority of corals acquire Symbiodinium from the surrounding environment through horizontal transfer, but the molecular mechanisms involved in the acquisition of Symbiodinium remain unknown. It has been hypothesized that carbohydrate-binding proteins, or lectins, of the host coral recognize cell surface carbohydrates of Symbiodinium in the process of acquiring symbionts. Thus, we examined the molecular mechanisms involving lectins and carbohydrates using model organism Acropora tenuis, a common reef-building coral, and Symbiodinium culture strains. Juvenile polyps acquire more cells of Symbiodinium strain NBRC102920 at 72–96 h of metamorphosis induction than in any other period. Glycosidase treatment of Symbiodinium inhibited the acquisition of Symbiodinium by juvenile coral polyps. The presence of carbohydrates D-galactose, N-acetyl-D-galactosamine, and N-acetyl-D-glucosamine at 10 mM also tended to decrease Symbiodinium acquisition. We isolated two N-acetyl-D-galactosamine binding lectins with apparent molecular masses of 14.6 and 29.0 kDa from A. tenuis, and de novo sequencing and cDNA cloning showed that the 29.0 kDa protein is Tachylectin-2-like lectin (AtTL-2). The anti-Tachylectin-2 antibody is suggested to bind specifically to AtTL-2. The antibody also inhibited binding of AtTL-2 to N-acetyl-D-galactosamine-resin and the acquisition of Symbiodinium by juvenile A. tenuis polyps. Based on these results, AtTL-2 is likely involved in the process of Symbiodinium acquisition