De novo assembly of the transcriptome of scleractinian coral, Anomastraea irregularis and analyses of its response to thermal stress.

Rising seawater temperatures cause coral bleaching. The molecular responses of the coral holobiont under stress conditions, determine the success of the symbiosis. Anomastraea irregularis is a hard coral commonly found in the harsh intertidal zones of the south coast of KwaZulu-Natal (KZN), South Africa, where it thrives at the very margins of hard coral distribution in the Western Indian Ocean. To identify the possible molecular and cellular mechanisms underlying its resilience to heat stress, experimental and control nubbins were exposed to temperatures of 29 and 19 °C respectively for 24 h. The transcriptome was assembled de novo from 42.8 million quality controlled 63 bp paired-end short sequence reads obtained via RNA sequencing (RNA-seq). The assembly yielded 333,057 contigs (> 500 bp = 55,626, Largest = 6341 bp N50 = 747 bp). 1362 (1.23%) of the transcripts were significantly differentially expressed between heat stressed and control samples. Log fold change magnitudes among individual genes ranged from - 4.6 to 7.2. Overall, the heat stress response in the A. irregularis constituted a protective response involving up regulation of apoptosis and SUMOylation. Gene ontology (GO) analyses revealed that heat stress in the coral affected the metabolism, protein synthesis, photosynthesis, transport and cytoskeleton. This is the first study to produce a reference transcriptome of this coral species and analyze its response to heat stress. The assembled transcriptome also presents a valuable resource for further transcriptomic and genomic studies.