Hermatypic (or reef-building) corals live in obligatory Mutualistic symbiosis with the symbiotic dinoflagellates Symbiodinium spp. (generally known as zooxanthellae). In an attempt to establish a model symbiosis system consisting of a coral host and a monoclonal population of zooxanthellae, infectivity of five cultured Symbiodinium cell lines was tested on naturally aposymbiotic juveniles of Acropora tenuis. A clade A3 strain (PL-TS-1) infected the juveniles at high density and promoted growth of the host. To identify host genes involved in the establishment or maintenance of symbiosis, mRNA expression patterns were compared between aposymbiotic and PL-TS-1-infected juvenile polyps using the suppression subtractive hybridization technique. Two mRNAs, the expression levels of which were augmented more than twofold by the presence of the symbionts, were thereby identified. One of the mRNAs, AtSym-02, encodes a novel protein of 322 amino acids which is predicted to be a glycosylated trans-membrane protein. (c) 2005 Elsevier Inc. All rights reserved.