Abstract
BackgroundCoral reef ecosystems are renowned for their diversity and beauty. Their immense ecological success is due to a symbiotic association between cnidarian hosts and unicellular dinoflagellate algae, known as zooxanthellae. These algae are photosynthetic and the cnidarian-zooxanthellae association is based on nutritional exchanges. Maintenance of such an intimate cellular partnership involves many crosstalks between the partners. To better characterize symbiotic relationships between a cnidarian host and its dinoflagellate symbionts, we conducted a large-scale EST study on a symbiotic sea anemone, Anemonia viridis, in which the two tissue layers (epiderm and gastroderm) can be easily separated.ResultsA single cDNA library was constructed from symbiotic tissue of sea anemones A. viridis in various environmental conditions (both normal and stressed). We generated 39,939 high quality ESTs, which were assembled into 14,504 unique sequences (UniSeqs). Sequences were analysed and sorted according to their putative origin (animal, algal or bacterial). We identified many new repeated elements in the 3'UTR of most animal genes, suggesting that these elements potentially have a biological role, especially with respect to gene expression regulation. We identified genes of animal origin that have no homolog in the non-symbiotic starlet sea anemone Nematostella vectensis genome, but in other symbiotic cnidarians, and may therefore be involved in the symbiosis relationship in A. viridis. Comparison of protein domain occurrence in A. viridis with that in N. vectensis demonstrated an increase in abundance of some molecular functions, such as protein binding or antioxidant activity, suggesting that these functions are essential for the symbiotic state and may be specific adaptations.ConclusionThis large dataset of sequences provides a valuable resource for future studies on symbiotic interactions in Cnidaria. The comparison with the closest available genome, the sea anemone N. vectensis, as well as with EST datasets from other symbiotic cnidarians provided a set of candidate genes involved in symbiosis-related molecular crosstalks. Altogether, these results provide new molecular insights that could be used as a starting-point for further functional genomics studies.
Highlights
Coral reef ecosystems are renowned for their diversity and beauty
In order to maximize the diversity of genes expressed under both normal and stress conditions within the symbiotic association, sea anemones were subjected to different environmental conditions before RNA extraction and cDNA library construction
Trimmed masked ESTs were further subjected to cluster analysis using the TIGRTGICL pipeline with default parameters [32]
Summary
Coral reef ecosystems are renowned for their diversity and beauty Their immense ecological success is due to a symbiotic association between cnidarian hosts and unicellular dinoflagellate algae, known as zooxanthellae. In most cases, such symbiont algae are dinoflagellates from the genus Symbiodinium, commonly referred to as zooxanthellae This association is a trophic endosymbiosis and is considered to be mutualistic because the zooxanthellae provide their cnidarian host with reduced organic carbon resulting from their photosynthetic activity [1] while the host provides the zooxanthellae with inorganic carbon [2], inorganic nitrogen [3,4] and inorganic phosphate [5], as well as a refuge from herbivory. Endosymbioses are highly complex associations, implying intimate interactions between host and symbionts as well as constraints, such as hyperoxic conditions generated by symbiont photosynthesis, and transfer of inorganic carbon to the symbiont [8]
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