Analysis of an EST library from the dinoflagellate (Symbiodinium sp.) symbiont of reef‐building corals1

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 environmental changes. Therefore, understanding gene expression patterns of Symbiodinium is critical to understanding why coral reefs are susceptible to global climate change. This study identified 1456 unique expression sequence tags (ESTs) generated for Symbiodinium (clade C3) from the staghorn coral Acropora aspera following exposure to a variety of stresses. Of these, only 10% matched previously reported dinoflagellate 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 modification, protein turnover, and chaperones (12.3%); energy production and conversion (12%); or an unknown function (18.6%). The most common transcript 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 identified, including the first glycerol‐phosphate:phosphate antiporter from dinoflagellates. A protein with similarity to the fungi–archael–bacterial heme catalase 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 unicellular component of a eukaryote–eukaryote symbiosis.