Abstract
Anthozoans are a class of Cnidarians that includes scleractinian corals, anemones, and their relatives. Despite a global rise in disease epizootics impacting scleractinian corals, little is known about the immune response of this key group of invertebrates. To better characterize the anthozoan immune response, we used the model anemone Exaiptasia pallida to explore the genetic links between the anthozoan–algal symbioses and immunity in a two‐factor RNA‐Seq experiment using both symbiotic and aposymbiotic (menthol‐bleached) Exaiptasia pallida exposed to the bacterial pathogen Vibrio coralliilyticus. Multivariate and univariate analyses of Exaiptasia gene expression demonstrated that exposure to live Vibrio coralliilyticus had strong and significant impacts on transcriptome‐wide gene expression for both symbiotic and aposymbiotic anemones, but we did not observe strong interactions between symbiotic state and Vibrio exposure. There were 4,164 significantly differentially expressed (DE) genes for Vibrio exposure, 1,114 DE genes for aposymbiosis, and 472 DE genes for the additive combinations of Vibrio and aposymbiosis. KEGG enrichment analyses identified 11 pathways—involved in immunity (5), transport and catabolism (4), and cell growth and death (2)—that were enriched due to both Vibrio and/or aposymbiosis. Immune pathways showing strongest differential expression included complement, coagulation, nucleotide‐binding, and oligomerization domain (NOD), and Toll for Vibrio exposure and coagulation and apoptosis for aposymbiosis.
Highlights
Cnidarians represent one of the earliest animal groups (Steele, David, & Technau, 2011) and are ideal systems to study the origins of genetic processes like innate immunity (Bosch, 2013; Hemmrich, Miller, & Bosch, 2007; Lehnert, Burriesci, & Pringle, 2012)
We explore the genetic links between the anthozoan–algal symbioses and immunity in a two‐factor RNA‐Seq experiment using both symbiotic and aposymbiotic Exaiptasia exposed to the bacterial pathogen Vibrio coralliilyticus
Seven pathways were enriched for both Vibrio and aposymbiosis (VA), four pathways were enriched for Vibrio only (V), and no pathways were enriched for aposymbiosis only
Summary
Cnidarians represent one of the earliest animal groups (Steele, David, & Technau, 2011) and are ideal systems to study the origins of genetic processes like innate immunity (Bosch, 2013; Hemmrich, Miller, & Bosch, 2007; Lehnert, Burriesci, & Pringle, 2012). Bleaching occurs when the symbionts are degraded or expelled by the coral host due to factors like thermal stress (Fitt, Brown, Warner, & Dunne, 2001), UV exposure (Gleason & Wellington, 1993), and disease (Libro et al, 2013) In addition to these naturally occurring stressors, chemical agents have been identified to deliberately induce bleaching in the laboratory for manipulative studies. Menthol bleaching was used to compare symbiotic (untreated) versus aposymbiotic (menthol‐treated) anemones where the hypothesized mechanism of menthol bleaching is thought to be the activation of autophagic digestion of Symbiodinium cells (symbiophagy) as part of host innate immunity (Dani et al, 2016). The two‐factor design comparing Vibrio and aposymbiosis as factors allowed us to identify gene expression patterns that were due to Vibrio and/or symbiotic state as well as any interactions between pathogen exposure and symbiotic state
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