- The intracellular mutualism between cnidarians and photosynthetic dinoflagellates (genus Symbiodinium) is responsible for the physical and trophic structure of diverse coral reef ecosystems. This relationship, based on nutrient exchange, allows for high productivity in tropical waters, which are generally nutrient-poor environments. Numerous environmental stressors currently threaten the health of corals, most notably elevated seawater temperatures due to global climate change, many of which can cause coral bleaching, or symbiosis collapse. Despite this, relatively little is known about the mechanisms underpinning the onset and maintenance of the association. In this dissertation, I studied the onset of cnidarian-dinoflagellate symbiosis using ecological, molecular, and genomic approaches.
First, I examined effects of elevated seawater temperature on coral larvae (Fungia scutaria) during the period of symbiosis establishment (Chapter 2). I found that larvae exposed to a 2-4°C increase in temperature were significantly impaired in their ability to form the symbiosis. These results are the first to quantify the effect of elevated temperature on coral symbiosis onset and are important in light of projected increases in seawater temperatures.
Next, I created a cDNA microarray from non-symbiotic and newly symbiotic F.
scutaria larvae to identify host transcripts that were differentially expressed in response to symbiosis onset (Chapter 3). Analyses revealed very few changes in the larval transcriptome as a result of infection with its homologous symbiont. I hypothesize that Symbiodinium sp. has evolved mechanisms to suppress or circumvent cnidarian host responses to colonization similar to those seen in the invasion of animal cells by protozoan parasites.
Finally, I explored a family of genes (tumor necrosis factor receptor associated factors, or TRAFs), which are key signal transducers in pro-inflammatory innate immune pathways, in cnidarian genomes (Chapter 4). Phylogenetic analyses identified 8 major lineages of TRAFs, including 3 new subfamilies, each with cnidarian TRAF sequences, indicating that the TRAF gene family was fully diversified prior to the divergence between cnidarians and bilaterians. I also cloned TRAF6-like genes from two model symbiotic cnidarians, Aiptasia pallida and F. scutaria, laying the groundwork for future functional studies that can examine the role of TRAF6 in cnidarian immunity, and a possible role for TRAF6 in regulating cnidarian-dinoflagellate mutualisms.