Symbiosis modulates gene expression of symbionts, but not hosts, under thermal challenge

Abstract

AbstractIncreasing ocean temperatures are causing dysbiosis between coral hosts and their symbionts. Previous work suggests that coral host gene expression responds more strongly to environmental stress compared to their intracellular symbionts; however, the causes and consequences of this phenomenon remain untested. We hypothesized that symbionts are less responsive because hosts modulate symbiont environments to buffer stress. To test this hypothesis, we leveraged the facultative symbiosis between the scleractinian coralOculina arbusculaand its symbiontBreviolum psygmophilumto characterize gene expression responses of both symbiotic partnersinandex hospiteunder thermal challenges. To characterize host andin hospitesymbiont responses, symbiotic and aposymbioticO. arbusculawere exposed to three treatments: 1) control (18°C), 2) heat (32°C), and 3) cold (6°C). This experiment was replicated withB. psygmophilumcultured fromO. arbusculato characterizeex hospitesymbiont responses. Both thermal challenges elicited classic environmental stress responses (ESRs) inO. arbuscularegardless of symbiotic state, with hosts responding more strongly to cold challenge. Hosts also exhibited stronger responses thanin hospitesymbionts.Inandex hospite B. psygmophilumboth downregulated genes associated with photosynthesis under thermal challenge; however,ex hospitesymbionts exhibited greater gene expression plasticity and differential expression of genes associated with ESRs. Taken together, these findings suggest thatO. arbusculahosts may buffer environments ofB. psygmophilumsymbionts; however, we outline the future work needed to confirm this hypothesis.