New insights into polyp bail-out and future applications
Based on our results, it appears that the TNF receptor-mediated extrinsic apoptotic pathway, FGF-mediated ECM degradation, and anti-apoptotic/survival signals are involved in polyp bail-out (Fig. 4). This finding suggests the feasibility of inducing polyp bail-out using ligands or agonists specifically targeting these pathways, which, by avoiding exerting systemic stresses on the corals, shall facilitate acquisition of viable polyps. The discreteness of pathways leading to apoptosis and ECM degradation also suggests possible independence of coenosarc degradation and polyp detachment in bail-out. Indeed, separation of these processes has been reported in previous studies using different treatments (Domart-Coulon et al., 2004; Fordyce et al., 2017; Kvitt et al., 2015) and was achieved in this study by subjecting corals to different levels of hyperosmotic stress (Fig. S1). Also, since detachment of polyps is mediated by degradation of ECM, we hypothesize that it might be possible to induce polyp resettlement either by stimulating ECM secretion or by providing suitable ECM components to the polyps. In support of our hypothesis, previous studies have found that fibronectin-like peptide facilitated attachment of solitary polyps to resettlement substrates, although after resettlement, polyps tended to dissociated into single cells (Domart-Coulon et al., 2004; Puverel et al., 2005). Future studies of resettlement processes are expected to optimize conditions for resettlement and maintenance of coral polyps, potentiating their use for mass production of coral colonies for reef restoration.
Considering that some pocilloporid corals, such as P. damicornisand S. pistillata , have been widely used as model coral species for both physiological and genetic studies, this study of polyp bail-out mechanisms provides additional information for future coral studies at cellular and molecular levels. Moreover, as pocilloporid corals are dominant reef-building corals in shallow water, these findings may help to understand polyp bail-out in nature and its role in shaping coral reef ecosystems in response to anticipated global climate change.