Diurnal and nocturnal transcriptomic variation in the Caribbean staghorn coral, Acropora cervicornis.

Reef-building corals experience large diel shifts in their environment, both externally due to changes in light intensity, predator activity and prey availability, and internally as a result of diel fluctuations in photosynthesis by their endosymbiotic algae, Symbiodinium. Diel patterns of tentacle behaviour, skeletal growth and gene expression indicate reactions of the coral animal in response to light and through circadian regulation. Some corals, such as the Caribbean Acroporas, have strong within-colony division of labour, including specialized fast-growing apical polyps, accompanied by large gene expression differences. Here we use RNA-seq to evaluate how diel changes in gene expression vary within the branching Caribbean staghorn coral, Acropora cervicornis, between branch tips and branch bases. Multifactor generalized linear model analysis indicated that 6% (3005) of transcripts were differentially expressed between branch tips and bases, while 1% (441) of transcripts were differentially expressed between day and night. The gene expression patterns of 220 transcripts were affected by both time of day and location within the colony. In particular, photoreceptors, putative circadian genes, stress response genes and metabolic genes were differentially expressed between day and night, and some of these, including Amcry1, tef and hebp2, exhibited location-specific regulation within the coral colony as well. These findings indicate that the genetic response of the coral to day and night conditions varies within the colony. Both time of day and location within the colony are factors that should be considered in future coral gene expression experiments.