Thermal stress-related gene expression in corals with different Symbiodinium types

The endosymbiotic relationship between scleractinian corals and Symbiodinium spp underpins the biodiversity and productivity of coral reefs worldwide. The genetic and physiological characteristics of Symbiodinium have large effects on coral host physiology and thermal tolerance; however, the degree to which molecular responses to thermal and oxidative stress vary among corals with different symbiont types is still not well understood. We examine gene expression in response to laboratory-based thermal stress in 1 year-old juveniles of Acropora millepora hosting different dominant Symbiodinium types. We detected significant changes in symbiont dominance through time, with 59.7% of coral juveniles changing their symbiont type during a 12-month growth period in the wild and 22% hosting two types simultaneously. Only three of 50 genes with a putative role in heat and oxidative stress were differentially expressed. Heat Shock Proteins 70 and 90 were expressed at higher levels in juveniles hosting multiple symbiont types during early stages of heat stress, whereas a NOS-interacting gene (a gene regulating nitric oxide production) was up-regulated concurrently with a decline in maximum quantum yield during heat stress. Our results support an important role for symbiont complements in the transcriptomes of corals and highlight high variability among individuals