Signaling pathways of heat- and hypersalinity-induced polyp bailout in Pocillopora acuta

Abstract

Polyp bailout is a drastic response to acute stress where coral coloniality breaks down and polyps detach. We induced polyp bailout in Pocillopora acuta with heat stress and tested for differential gene expression using RNAseq and a qPCR assay. Furthermore, we induced polyp bailout with hypersalinity and compared the results to identify stressor-independent signals and pathways active during polyp bailout. Both stressors led to the onset of polyp bailout and the detachment of vital polyps. We observed activation of microbe-associated molecular pattern receptors and downstream signaling pathways of the innate immune system. Further, we detected growth factors and genes active during Wnt-signaling potentially contributing to wound healing, regeneration, and proliferation. Upregulation of several genes encoding for matrix metalloproteinases and the fibroblast growth factor signaling pathway are the most likely involved in the remodeling of the extracellular matrix, as well as in the detachment of polyps from the calcareous skeleton during polyp bailout. Expression of genes of interest in our qPCR assay of vital polyps from our heat-stress experiment, showed a trend for a normalization of gene expression after polyp bailout. Our results provide new insights into the signaling cascades leading to the observed physiological responses during polyp bailout. Comparison between the two stressors showed that certain signaling pathways are independent of the stressor and suggested that polyp bailout is a general response of corals to acute stress. Furthermore, immune system responses during polyp bailout indicate that microbe-associated partners of corals may lead to the polyp bailout response.

Bacterial Community Shifts during Polyp Bail-Out Induction in Pocillopora Corals

“…As both represented onset of polyp detachment, samples at day 5 and day 7 were pooled (denoted days 5 to 7) in subsequent analyses (so too, corresponding samples in the control group). Detached polyps in both experiments showed intact morphological features similar to those reported in earlier studies ( 31 , 33 ), including round to cylindrical bodies and intact tentacles, indicating successful induction of polyp bail-out instead of tissue sloughing or polyp death. No samples in the control groups showed any signs of bleaching, bail-out, or any other stress response in either experiment.…”

“…Polyp bail-out may be a strategy of certain scleractinian corals to facilitate dispersal. Contemporary studies have documented polyp bail-out in response to different stressors ( 15 , 18 , 22 , 33 – 35 ). In the present study, we conducted two experiments to induce polyp bail-out in P. acuta , a scleractinian coral commonly used in polyp bail-out research, and examined bacterial community changes during bail-out induction ( 21 , 31 , 33 , 36 ).…”

“…Asynchronous expression of tumor necrosis factor (TNF) and tumor necrosis factor receptor (TNFR) genes in Pocillopora corals during bail-out induction led to a hypothesis of a microbial trigger for initiation of polyp bail-out ( 31 , 32 ). Activation of the Toll-like receptor signaling pathway and immune responses also implies changes of coral-microbiome interactions during polyp bail-out progression ( 33 ). However, to our knowledge no study has so far attempted to characterize microbes responsible for initiation of polyp bail-out.…”

Inducing Polyp Bail-out in Coral Colonies to Obtain Individualized Micropropagates for Laboratory Experimental Use

“…However, polyps in many studies, including the present, have been able to present zooxanthellae cells inside their tissues and external morphologies with intact oral-aboral polarization and tentacles weeks after bailout 25 , 27 , 32 , 36 . Previous studies have also found that, after being relieved from acute stress, released coral polyps exposed to highly saline or heated seawater have been able to recover the expression of genes related to processes such as apoptosis, proteolysis, and cell division to levels similar to the ones found before bail-out 32 , 36 and even to increase the expression of genes related to tissue healing 36 .…”

Characterizing transcriptomic responses to sediment stress across location and morphology in reef-building corals

“…In corals, Rho GTPases participate in cytoskeleton remodeling during phagocytosis, as well as cell division of endosymbionts within symbiotic gastrodermal cells (Li et al 2014). Rho GTPase pathways have been identified in coral polyp bailout responses to heat stress and hyperosmosis, as well as in bleached corals in response to low flow environments (Chuang and Mitarai 2020; Fifer et al 2021; Gösser et al 2021). In this study, downregulation of Rho protein pathways suggests that minimal cytoskeleton maintenance, assembly and organization is occurring and that the corals may not be able to properly maintain their cellular structures under sedimentation.…”