Radiation tolerance and bystander effects in the eutardigrade species Hypsibius dujardini (Parachaela: Hypsibiidae)

Some tardigrade species can tolerate high radiation doses. Radiation-induced bystander effects - damage in unirradiated cells associated with irradiated cells - have yet to be reported in tardigrades. We investigated radiation tolerance and radiation-induced bystander effects on long-term survival in the eutardigrade species Hypsibius dujardini. To study direct radiation effects, groups were irradiated with gamma radiation at levels of 3 and 5 kGy. To study radiation-induced bystander effects, unirradiated groups were exposed to a single irradiated (3 or 5 kGy) individual. Survivorship was monitored every 2 days until all individuals had died. Direct radiation decreased survivorship in a dose-dependent manner. Survivorship in bystander groups differed significantly from survivorship in groups irradiated directly with 5 kGy but insignificantly from survivorship in groups irradiated directly with 3 kGy, suggesting that radiation-induced bystander effects had saturated. The eutardigrade species H. dujardini can tolerate high radiation doses, with radiation-induced bystander effects that manifest as a threshold response.