Abstract
Ecosystems worldwide are suffering from climate change. Coral reef ecosystems are globally threatened by increasing sea surface temperatures. However, gene expression plasticity provides the potential for organisms to respond rapidly and effectively to environmental changes, and would be favored in variable environments. In this study, we investigated the thermal stress response in Pocillopora coral colonies from two contrasting environments by exposing them to heat stress. We compared the physiological state, bacterial and Symbiodiniaceae communities (using 16S and ITS2 metabarcoding), and gene expression levels (using RNA-Seq) between control conditions and heat stress (the temperature just below the first signs of compromised health). Colonies from both thermal regimes remained apparently normal and presented open and colored polyps during heat stress, with no change in bacterial and Symbiodiniaceae community composition. In contrast, they differed in their transcriptomic responses. The colonies from Oman displayed a more plastic transcriptome, but some genes had a higher basal expression level (frontloading) compared to the less thermotolerant colonies from New Caledonia. In terms of biological functions, we observed an increase in the expression of stress response genes (including induction of tumor necrosis factor receptors, heat shock proteins, and detoxification of reactive oxygen species), together with a decrease in the expression of genes involved in morpho-anatomical functions. Gene regulation (transcription factors, mobile elements, histone modifications and DNA methylation) appeared to be overrepresented in the Oman colonies, indicating possible epigenetic regulation. These results show that transcriptomic plasticity and frontloading can be co-occurring processes in corals confronted with highly variable thermal regimes.
Highlights
Earth is undergoing unprecedented global environmental changes with major effects on biodiversity (Barnosky et al 2011)
In terms of biological functions, we observed an increase in the expression of stress response genes, together with a decrease in the expression of genes involved in morpho-anatomical functions
Comparison of the response to an ecologically realistic heat stress of corals from the same genus but pertaining to different species hypotheses thriving in two contrasting thermal environments sheds light on the molecular basis of thermotolerance
Summary
Earth is undergoing unprecedented global environmental changes with major effects on biodiversity (Barnosky et al 2011). The ongoing erosion of the most vulnerable ecosystems due to current environmental degradation is worrying and is only a premise to what scientists have called the sixth mass extinction (Barnosky et al 2011). The extinction or even major decrease of corals would have dramatic repercussions on the overall associated communities (Hughes et al 2017a). It has been shown that populations inhabiting in zones with more variable temperature regimes display better tolerance to heat stress from local (Kenkel et al 2013) to geographical scales (Hughes et al 2003; Riegl et al 2011; Coles & Riegl 2013)
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