Fig. 5.—

A proposed model of the endosymbiotic interaction between the two Hydra species and algae. The endosymbiotic hydra is exposed to reactive oxygen species (ROS) extracted from both mitochondria (Mt) and the symbiont (Sy). H. viridissima represses ROS by downregulation of respiratory chain genes of mitochondria (indicated by (1)), and respond to the stress by extensive upregulation of ascorbate peroxidase (indicated by (2). Moreover, the disruption of ${\text{Ca}}_{\text{2}}^{\text{+}}$homeostasis is repressed by the upregulation of ${\text{Ca}}_{\text{2}}^{\text{+}}$binding and cell adhesion genes (indicated by (3) and (4)). Although H. vulgaris also responds to the stress by upregualtion of ascorbate peroxidase (indicated by (2)), it still experiences oxidative stress that induces disruption of the ${\text{Ca}}_{\text{2}}^{\text{+}}$homeostasis and cell adhesion (indicated by (3) and (4)), which contributes to the cell death. The genes/systems are shown as upregulated (red) and downregulated (blue).