Identification of zooxanthellate genes that are involved in symbiosis with a reef coral, and analyses of their function and expression

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Hermatypic (or reef-building) corals live in obligate mutualistic symbiosis with the symbiotic dinoflagellates Symbiodinium spp. (generally known as zooxanthellae). With the purpose of identifying and characterizing Symbiodinium mRNAs whose expression levels are enhanced in the symbiotic state than in ex hospite conditions, attempts were made to establish a model symbiosis system consisting of newly metamorphosed polyps of the reef coral Acropora tenuis and a monoclonal population of zooxanthellae. To this end, infectivity of several cultured Symbiodinium cell lines was tested on naturally aposymbiotic polyps of A. tenuis. Two clade A strains (PL-TS-1 and CCMP2467) infected the juveniles at high densities, and promoted growth of the host. To identify PL-TS-1 (or A. tenuis) genes involved in the establishment or maintenance of symbiosis, suppression subtractive hybridization was performed. Approx. 50 cDNA clones in the subtracted library were analyzed, but symbiotically induced mRNAs of PL-TS-1 were not represented in the analyzed cDNA population. However, two mRNAs, the expression levels of which were augmented more than twofold by the presence of the symbionts, could be identified. The mRNAs, named AtSym-01 and -02, encode a syndecan family protein with one trans-membrane domain, and an Na^+-independent sulfate transporter of the SLC26A11 family, respectively (it was the first identification on such coral mRNAs). Two actin mRNAs (SyAct-A1 and -A2) in PL-TS-1 were analyzed for the expression levels in the in and ex hospite conditions, and the result strongly suggested that the expression of the SyAct-A2 mRNA was higher in symbiosis with the coral than in in vitro culture.