Variation in antioxidant gene expression in the scleractinian coral Acropora millepora under laboratory thermal stress

The effects of climate change over the last few decades have pushed the majority of reef-building corals close to their upper thermal limit. Upon exposure to thermal stress, the intra-cellular formation of harmful oxygen intermediates can lead to the disruption of the obligate symbiosis between the coral host and their dinoflagellate endosymbionts (zooxanthellae), a process known as coral bleaching. Applications of molecular techniques to cnidarian research have recently enhanced our understanding of the magnitude of corals’ transcriptional response to various stressors. In the present study, we developed a quantitative real-time polymerase chain reaction assay to assess expression levels of 4 genes involved in the corals’ oxidative stress response (HSP70, MnSOD, ferritin, Zn2+-metalloprotease) after exposure to laboratory-controlled thermal stress. Using the Indo-Pacific reef coral Acropora millepora, our study provides the first population-scale analysis of antioxidant gene expression in coral. Despite the significant up-regulation of those 4 genes in the thermally stressed samples relative to non-stressed samples, our results show that there is an enormous intra- as well as inter-colony variation in transcript abundance at a particular point in time. We discuss the potential roles of ferritin and Zn2+-metalloprotease in the break down of the intra-cellular Fe2+-homeostasis and in coral host cell detachment, respectively, during bleaching conditions. Our results emphasise the importance of measuring inter-individual variation to gain an insight into the population response to a common and increasingly encountered environmental stressor

A multilocus, temperature stress-related gene expression profile assay in Acropora millepora, a dominant reef-building coral

We report an accurate multiplex reverse transcription quantitative polymerase chain reaction (RT–qPCR) assay, capable of reproducing gene expression profiles from 16 target genes [12 genes of interest (GOIs) and four reference genes (RGs)] in Acropora millepora, a common reef-building model coral species. The 12 GOIs have known or putative roles in the coral bleaching response, yet the method is not restricted to this particular assay and gene set. The procedure is based on the Beckman Coulter (Fullerton, CA, USA) GenomeLab™ GeXP Genetic Analysis System and bridges the gap between quantitative real-time PCR (qPCR) expression analysis of a single or a small number of genes and microarray gene expression surveys of thousands of genes. Despite large variation among biological replicates, the majority of GOIs were up-regulated (up to 4000%) in most colonies during a laboratory-based thermal stress experiment. Two genes, Nf-kβ2 and MnSod, were consistently up-regulated in all colonies tested, and we therefore propose these as candidate markers useful for population-level evaluations of thermal stress. Our assay provides an important new tool for coral bleaching studies; because of the lower cost, labour and amount of cDNA required compared with singleplex qPCR, population-level studies with large biological replication are feasible