Introduction
Corals in the family Pocilloporidae (Cnidaria: Anthozoa: Scleractinia) are the major reef-building corals in the Indo-Pacific Ocean (Pinzon & LaJeunesse, 2011). Pocilloporid corals generally inhabit shallow water, which renders them susceptible to thermal and osmotic fluctuations (Poquita-Du et al., 2019; Schmidt-Roach, Miller, Lundgren, & Andreakis, 2014). Being representatives of the “robust corals” branch, studies on pocilloporid corals complement our understanding derived from the Acroporidae, the extensively studied corals in the “complex corals” branch, about impacts of anthropogenic climate change on coral reef ecosystems (Traylor-Knowles et al., 2011). Compared to acroporid corals, some pocilloporids have been recognized as being relatively resistant to environmental stresses. However, bleaching, the systematic dissociation of symbiotic zooxanthellae from coral host, can still occur in these corals once stresses reach critical levels (Kvitt, Rosenfeld, Zandbank, & Tchernov, 2011). As with most stony corals, pocilloporids rely on photosynthesis by symbiotic algae as their major energy source and cannot live as heterotrophs for prolonged periods. Extended bleaching can therefore cause mortaility of pocilloporid corals, even though the stressors might not be directly lethal to the coral hosts (Traylor-Knowles et al., 2011).
In recent decades, a novel stress response called “polyp bail-out” has been increasingly reported in pocilloporid corals (Domart-Coulon, Tambutté, Tambutté, & Allemand, 2004; Fordyce, Camp, & Ainsworth, 2017; Kvitt et al., 2015; Sammarco, 1982; Shapiro, Kramarsky-Winter, Gavish, Stocker, & Vardi, 2016; Wecker et al., 2018). Unlike coral bleaching, polyp bail-out is characterized by dissociation of coral colonies via coenosarc degradation and detachment of zooxanthellate polyps from the calcareous skeletons (Sammarco, 1982). In natural environments, polyp bail-out has been reported in the Great Barrier Reef and reefs along the Pacific coast of Costa Rica, although the triggers are still uncertain (Sammarco, 1982; Wild et al., 2014). Under laboratory conditions, induction of polyp bail-out has been demonstrated with different treatments, including thermal stress, acidification, and hyperosmosis (Domart-Coulon et al., 2004; Fordyce et al., 2017; Kvitt et al., 2015; Serrano, Coma, Inostroza, & Serrano, 2018; Shapiro et al., 2016; Wecker et al., 2018). As detached polyps can be maintained in laboratory conditions for periods of weeks to months (Capel, Migotto, Zilberberg, & Kitahara, 2014; Serrano et al., 2018; Shapiro et al., 2016), these polyps offer a different subject material for in situ studies of coral cellular biology and of symbiotic relationships between corals and symbiotic algae. Moreover, since detached polyps are thought to resettle and resume skeletogenesis under favorable conditions, polyp bail-out can be considered as an asexual reproductive method in stony corals and may provide an alternative approach of mass production of coral colonies for reef restoration (Sammarco, 1982; Shapiro et al., 2016).
Identifying signaling pathways participating in polyp bail-out can help to develop methods to induce the response without stressing the corals, which would facilitate survival and resettlement of detached polyps. Furthermore, understanding molecular mechanisms underlying polyp bail-out may help us to better understand its occurrence in nature and its ecological significance. Recently, genomes of some pocilloporid corals have been published (Cunning, Bay, Gillette, Baker, & Traylor-Knowles, 2018; Voolstra et al., 2017), enabling a more thorough understanding of this response from a molecular perspective. A recent transcriptomic study of Pocillopora damicornis during polyp bail-out demonstrated overexpression of many caspase-encoding genes in concert with coenosarc degradation, supporting the hypothetical link between polyp bail-out and tissue-specific apoptosis (Kvitt et al., 2015; Wecker et al., 2018). In that study, it was also proposed that proteolytic enzymes, such as cathepsins, trigger degradation of the extracellular matrix (ECM) between coral polyps and the calcareous skeleton, resulting in detachment of individual polyps (Wecker et al., 2018). However, our understanding of signaling pathways activating the apoptotic and proteolytic responses in polyp bail-out is still in its infancy.
In the present study, we applied hyperosmotic stress to induce bail-out in P. acuta , a species closely related to P. damicornis , according to recent phylogenic classifications (Johnston et al., 2017; Schmidt-Roach et al., 2014). Based on both transcriptomic analysis and qPCR assays, we sought to identify the signaling pathways that lead to polyp bail-out. Results show involvement of multiple signaling pathways in polyp bail-out, including the tumor necrosis factor (TNF) and fibroblast growth factor (FGF) signaling pathways, which likely independently trigger apoptotsis and ECM degradation, respectively.