Canonical and cellular pathways timing gamete release in Acropora digitifera, Okinawa, Japan.

Natural light cycles are important for synchronizing behavioural and physiological rhythms over varying time periods in both plants and animals. An endogenous clock, regulated by positive and negative elements, interacting in feedback loops controls these rhythms. Many corals exhibit diel cycles of polyp expansion and contraction entrained by solar light patterns and monthly cycles of spawning or planulation that correspond to nocturnal lunar light cycles. However, despite considerable interest in studies of coral reproduction, there is currently not enough molecular information about the cellular pathways involved with synchronizing spawning/planulation in broadcast spawners and brooders. To determine whether the endogenous clock is implicated in the regulation of reproductive behaviour in corals, we characterized the transcriptome of Acropora digitifera colonies at twelve time points over a 2-month period of full and new moons, starting with the day of spawning in June 2014. We identified 608 transcripts with differential expression only on the spawning night during the coral setting phase and gamete release. Our data revealed an upregulation of light-sensing molecules and rhodopsin-like receptors that initiate signalling cascades, including the glutamate, SMAD signalling and WNT signalling pathways, neuroactive ligand-receptor interactions and calcium signalling. These are all involved in cell cycling, cell movement, tissue polarity, focal adhesion and cytoskeleton reorganization and together lead to gamete release. These findings can improve the understanding of many time-based cycles and extend our knowledge of the interplay between exogenous signals and the endogenous clock in cnidarians.

The transcriptomic response of the coral Acropora digitifera to a competent Symbiodinium strain: the symbiosome as an arrested early phagosome.

Despite the ecological significance of the relationship between reef-building corals and intracellular photosynthetic dinoflagellates of the genus Symbiodinium, very little is known about the molecular mechanisms involved in its establishment. Indeed, microarray-based analyses point to the conclusion that host gene expression is largely or completely unresponsive during the establishment of symbiosis with a competent strain of Symbiodinium. In this study, the use of Illumina RNA-Seq technology allowed detection of a transient period of differential expression involving a small number of genes (1073 transcripts; <3% of the transcriptome) 4 h after the exposure of Acropora digitifera planulae to a competent strain of Symbiodinium (a clade B strain). This phenomenon has not previously been detected as a consequence of both the lower sensitivity of the microarray approaches used and the sampling times used. The results indicate that complex changes occur, including transient suppression of mitochondrial metabolism and protein synthesis, but are also consistent with the hypothesis that the symbiosome is a phagosome that has undergone early arrest, raising the possibility of common mechanisms in the symbiotic interactions of corals and symbiotic sea anemones with their endosymbionts.