Coral spawning synchronicity: The role of circadian genes in circalunar rhythms

Broadcast spawning corals reproduce synchronously on one or more nights, with several species releasing gametes during specific timing windows. The tight intraspecies synchrony involved in these events has been of much interest, with the mechanisms driving these behaviours still largely unknown. This thesis is investigating the role of light in regulating spawning synchrony, with the hypothesis that endogenously controlled biological clocks are involved in the timing of this process.;Mass spawning corals release gametes on one or two evenings within a defined window from year to year. By artificially changing sunset time, colonies of Montastraea franksi were monitored for changes in spawning time. 2-hour and 1-hour earlier sunsets caused a corresponding shift in spawn time, indicating that the time of spawning at night is not controlled by an entrained biological clock but is directly controlled by local solar light cycles.;Although a circadian clock did not set the time at night for spawning, biological rhythms could still play an important role in controlling which night corals spawn. Candidate circadian genes were tested for rhythmic expression in the absence of light in larvae of Acropora millepora. Several genes were found to maintain rhythmic expression, while others experienced reduced expression suggesting they were not under the control of an endogenous clock. Transcriptome-wide changes in response to light and dark were also found using high-throughput Illumina sequencing, with over 1,000 genes differentially expressed, and several biologically relevant gene sets were enriched during day or night.;Many of the circadian genes tested in larvae showed rhythmic expression patterns over a lunar cycle in adults. Most circadian genes did not maintain rhythmicity over the lunar cycle when lunar light was removed or kept constant, although Six may have demonstrated rhythmic expression in the absence of lunar light. On the night of coral spawning, all candidate circalunar genes displayed increasing levels of expression following low expression levels experienced on the Full Moon, and may be involved in triggering a threshold response. RNA-Seq was also used to identify changes between the Full Moon and New Moon and found 86 genes differentially expressed.