Transcriptome Analysis in a Scleractinian Coral, Acropora tenuis, during the Spawning Season With Reference to the Gonadal Condition

Synchronous spawning is a striking feature of coral. Although it is important for reproductive success, corals reallocate energy for reproduction to growth when they are damaged by external stimuli. To assess the transcriptome before and after spawning in the scleractinian coral Acropora tenuis, we tagged three colonies (one bleached and two unbleached) in the field around Sesoko Island (Okinawa, Japan) in November 2016, sampled them monthly from May to July 2017, and performed RNA sequencing (RNA-Seq) analysis. Histological analysis revealed that the previously bleached colony possessed gametes in June, by which time the other two colonies had already spawned. In RNA-Seq analyses, multi-dimensional scaling based on gene expression similarity among the samples reflected the differences between colonies and between months except for the sample of a non-spawned colony in May, which was similar to the samples in June. The similarity of the non-spawned colony sample in May to the samples in June was also shown in hierarchical clustering based on the expression patterns of the genes that were differentially expressed between months in the spawned colonies. These results suggest that non-spawning was already decided in May, and that the physiological condition in a non-spawned colony in May was advanced to June. RNA-Seq analysis also showed that genes related to gametogenesis and those related to apoptosis were upregulated before and after spawning, respectively.