ANALYSIS OF AN EST LIBRARY FROM THE DINOFLAGELLATE (SYMBIODINIUM SP.) SYMBIONT OF REEF-BUILDING CORALS 1 William Leggat 3 , Ove Hoegh-Guldberg, Sophie Dove Centre for Marine Studies, ARC Centre of Excellence for Coral Reef Studies, University of Queensland, Queensland 4072, Australia and David Yellowlees 2 Biochemistry and Molecular Biology, ARC Centre of Excellence for Coral Reef Studies, James Cook University, Townsville, Queensland 4811, Australia Dinoflagellates (Symbiodinium sp. Freud.) are an obligatory endosymbiont of the reef-building corals. Recent changes to the environment surrounding coral reefs (e.g., global warming) have demonstrated that this endosymbiotic relationship between corals and Symbiodinium is particularly sensitive to environ- mental changes. Therefore, understanding gene expression patterns of Symbiodinium is critical to understanding why coral reefs are susceptible to glo- bal climate change. This study identified 1456 unique expression sequence tags (ESTs) generated for Symbi- odinium (clade C3) from the staghorn coral Acropora aspera following exposure to a variety of stresses. Of these, only 10% matched previously reported dinofla- gellate ESTs, suggesting that the conditions used in the construction of the library resulted in a novel transcriptome. The function of 561 (44%) of these ESTs could be identified. The majority of these genes coded for proteins involved in posttranslational mod- ification, protein turnover, and chaperones (12.3%); energy production and conversion (12%); or an unknown function (18.6%). The most common tran- script found was a homologue to a bacterial protein of unknown function. This algal protein is targeted to the chloroplast and is present in those phototrophs that acquired plastids from the red algal lineage. An additional 48 prokaryote-like proteins were also iden- tified, including the first glycerol-phosphate:phos- phate antiporter from dinoflagellates. A protein with similarity to the fungi–archael–bacterial heme cata- lase peroxidases was also found. A variety of stress genes, in particular heat-shock proteins and proteins involved in ubiquitin cascades, were also identified. This study is the first transcriptome from the unicellu- lar component of a eukaryote–eukaryote symbiosis. Key index words: coral; coral bleaching; dinofla- gellate; heat-shock protein; stress; Symbiodinium; ubiquitin; zooxanthellae Abbreviations: COG, clusters of orthologous groups; ESTs, expression sequence tags; FSW, filtered seawater; Hsp, heat-shock protein; PCP, peridinin-chlorophyll- binding protein; UTR, untranslated region Dinoflagellates play key ecological roles in a num- ber of ecosystems, from freshwater lakes to the open ocean, where they account for a significant amount of carbon fixation worldwide. They have also been implicated in red tides and human diseases such as ciguatera, but perhaps the most iconic ecological role that dinoflagellates play is in the symbiotic association of the dinoflagellate Symbiodinium with scleractinian corals. This symbiosis is the biological driving force behind the establishment and mainte- nance of tropical coral reefs, which dominate inshore tropical and subtropical waters. Coral reefs have been persistent features of these waters for over 300 million years (Hoegh-Guldberg 2004). Despite this fact, there is ample paleontological evi- dence that coral reefs have experienced periods of low abundance and have been subject to periodic extinctions at the local level (Veron 1995, Hoegh- Guldberg 2004). Anthropogenic influences are cur- rently leading to a sharp decline in coral-reef abun- dance worldwide, with all corals reefs impacted to some degree by human disturbances (Pandolfi et al. 2003). Climate change and the consequent increase in seawater temperature is now acknowledged as the major threat to the survival of coral reefs globally (Hughes et al. 2003). When corals are exposed to temperatures only slightly above long-term maxi- mum summer averages, they undergo a phenome- non known as ‘‘coral bleaching’’ (for reviews see Douglas 2003, Hoegh-Guldberg 2004). Bleaching occurs when elevated temperatures result in a whit- ening of the coral tissues, due either to a direct loss of symbiotic dinoflagellates from the coral host or a decrease in photosynthetic pigment concentration (Takahashi et al. 2004, Dove et al. 2006). These episodic coral-bleaching events were first recorded in the late 1970s (Hoegh-Guldberg 2004). Since 1 Received 14 April 2006. Accepted 9 April 2007. 2 Author for correspondence: e-mail mody@jcu.edu.au. 3 Present address: Biochemistry and Molecular Biology, James Cook University, Townsville, Queensland 4811, Australia. J. Phycol. 43, 1010–1021 (2007) Ó 2007 Phycological Society of America DOI: 10.1111/j.1529-8817.2007.00387.x 1010