From Polyps to Colonies: Applying Polyp Bail-Out to Study Coral Coloniality

Colonial lifestyles have been adopted by the majority of shallow-water stony corals (Cnidaria: Scleractinia), as they facilitate coral responses to environmental changes. Polyp bail-out, a coral stress response featuring colony dissociation and polyp detachment, offers a platform to study coloniality in stony corals. However, employing bailed-out polyps in coral research requires greater understanding of the biology of this stress response. This thesis investigates the molecular basis of polyp bail-out in Pocillopora acuta, a branching coral that is common in the tropical waters of the Indo-Pacific Ocean, and examines bailed-out polyps to study the biological foundation of coral coloniality. First, I probe molecular mechanisms involved in hyperosmosis-induced polyp bail-out, based upon a P. acuta transcriptome assembly. Then, I monitor morphological and genetic changes of bailed-out P. acuta polyps after the induced bail-out response. Finally, I explore transcriptional profiles of bailed-out polyps and those of polyps in normal colonies in order to identify differences between corals at different levels of structural and social complexity. Based on transcriptomic data, activation of tumor necrosis factor and fibroblast growth factor signaling pathways was revealed during initiation of polyp bail-out, possibly linking these pathways to colony dissociation and polyp detachment, respectively. Under ambient conditions, about half of bailed-out polyps displayed morphological recovery and genetic resumption of fundamental cellular processes within five days. Compared with recovered bailed-out polyps, normal colonies showed activation of genes for neurological and circulatory system development. Furthermore, in response to environmental stresses, few genetic changes were shared by bailed-out polyps and colonies, suggesting that coloniality promotes distinctively different stress responses, probably enhancing fitness in stony corals. Interestingly, transcriptomic data also revealed differential expression of angiotensin-converting and endothelin-converting enzymes in the transition between normal colonies and bailed-out polyps. This thesis presents a robust polyp bail-out induction protocol and develops a foundation for its application to coral research. Using this new research model, this thesis presents the first molecular-level study of coral coloniality and identifies genes potentially participating in development of coral colonies, which are expected to be fruitful topics for future studies.Okinawa Institute of Science and Technology Graduate Universit