Transcriptomic differences between bleached and unbleached hydrozoan following the 2015-2016 ENSO in the Mexican Caribbean.

The 2015-2016 El Niño-southern oscillation or "ENSO" caused many colonies that live in the Mexican Caribbean to experience extensive bleaching. The purpose of this work was to analyze the effect of bleaching on the cellular response of , employing a transcriptomic approach with RNA-seq. As expected, bleached specimens contained a significantly lower chlorophyll content than unbleached hydrocorals. The presence of algae of the genera and was only found in tissues of unbleached , which could be associated to the greater resistance that these colonies exhibited during bleaching. We found that 299 genes were differentially expressed in bleached colonies following the 2015-2016 ENSO in the Mexican Caribbean. The differential expression analysis of bleached specimens evidenced enriched terms for functional categories, such as ribosome, RNA polymerase and basal transcription factors, chaperone, oxidoreductase, among others. Our results suggest that the heat-shock response mechanisms displayed by include: an up-regulation of endogenous antioxidant defenses; a higher expression of heat stress response genes; up-regulation of transcription-related genes, higher expression of genes associated to transport processes, inter alia. This study constitutes the first differential gene expression analysi of the molecular response of a reef-forming hydrozoan during bleaching.