Global gene expression patterns in response to white patch syndrome: Disentangling symbiont loss from the thermal stress response in reef-building coral

The mechanisms resulting in the breakdown of the coral symbiosis once the process of bleaching has been initiated remain unclear. Distinguishing symbiont loss from the abiotic stress response may shed light on the cellular and molecular pathways involved in each process. This study examined physiological changes and global gene expression patterns associated with white patch syndrome (WPS) in P. lobata, which manifests in localized bleaching independent of thermal stress. In addition, a meta-analysis of global gene expression studies in other corals and anemones was used to contrast differential regulation as a result of abiotic stress from expression patterns correlated with symbiotic state. Symbiont density, chlorophyll a content, holobiont productivity, instant calcification rate, and total host protein content were uniformly reduced in WPS relative to healthy tissue. While expression patterns associated with WPS were secondary to fixed effects of source colony, specific functional enrichments suggest that the viral infection putatively giving rise to this condition affects symbiont rather than host cells. The meta-analysis revealed that expression patterns in WPS-affected tissues were significantly correlated with prior studies examining short-term thermal stress responses. This correlation was independent of symbiotic state, as the strongest correlations were found between WPS adults and both symbiotic adult and aposymbiotic coral larvae experiencing thermal stress, suggesting that the majority of expression changes reflect a non-specific stress response. Across studies, the magnitude and direction of expression change among particular functional enrichments suggests unique responses to stressor duration, and highlights unique responses to bleaching in an anemone model which engages in a non-obligate symbiosis.