Differential transcriptomic responses to heat in wild-type and heat-evolved Symbiodiniaceae in corals

Heat-evolved Symbiodiniaceae can improve the physiological performances of their coral host under heat stress, but their gene expression responses to heat remained unknown. We explore here the transcriptomic basis of differential thermal stress responses between in hospite wild-type and heat-evolved Cladocopium proliferum strains and their coral host Platygyra daedealea. Three P. daedalea colonies were fragmented into nubbins. Once recovered, the vast majority of native symbionts was removed through chemical bleaching (exposure to menthol and diuron). Once bleached, the nubbins – kept in separate tanks – were inoculated with either a wild-type C. proliferum strain (WT10), heat-evolved C. proliferum strains (SS1 or SS8) or native symbionts (RIC, controlling for the effect of chemical bleaching). Additionally, unbleached and uninoculated nubbins (C) were maintained in parallel as experimental controls. After symbiosis re-establishment, half of the corals were subjected to heat stress (ramp up to 32.2°C), while the remaining half were at ambient temperature (27°C). After 4.9 eDHWs, ten nubbins were sampled from each colony at each temperature. For each colony, the ten nubbins were taken across the five treatments (i.e., two samples per treatment per colony per temperature treatment). In some cases, we were only able to recover one nubbin per colony per treatment.