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.