Survey of cnidarian gene expression profiles in response to environmental stressors: Summarizing 20 years of research, what are we heading for?

Coral research has come a long way since the pioneering coral biology studies of thermal tolerance dating back to the turn of the previous century. In great contrast, at the present time, the currently available in silico technologies enable the entire transcriptome to be surveyed in a high-throughput manner following an array of stress manipulations. Deepsequencing is expected to revolutionize the way we study gene expression and holds the potential to answer prominent questions regarding cnidarian cellular pathways following global change scenarios. In this review we focus on cnidarian responses to environmental stressors in general and to global climate change in particular, focusing on the gene expression levels. A wide characterization of studies conducted in cnidarians following environmental stress revealed that most of the studies investigated a single stress factor and mostly thermal stress, were short-term and focused on branching corals. Subsequently, there is a lack of gene expression knowledge concerning massive corals that are known to be less susceptible to bleaching comparing to branching corals. In this review, we present a detailed list of differentially expressed genes in branching/massive corals under eight types of environmental stress. A conceptual model was constructed of the main processes occurring within the coral host cell under heat, ocean acidification and UV stress. The tables and the pathways of this review emphasize gaps in knowledge and can assist in guiding future research as they suggests which genes/processes one should look at in order to achieve a greater understanding of the cnidarians molecular processes affected by global anthropogenic stress.