Gamma rays induce DNA damage and oxidative stress associated withimpaired growth and reproduction in the copepod Tigriopus japonicus

Abstract

Nuclear radioisotope accidents are potentially ecologically devastating due to their impact on marineorganisms. To examine the effects of exposure of a marine organism to radioisotopes, we irradiated theintertidal copepod Tigriopus japonicus with several doses of gamma radiation and analyzed the effectson mortality, fecundity, and molting by assessing antioxidant enzyme activities and gene expressionpatterns. No mortality was observed at 96 h, even in response to exposure to a high dose (800 Gy) ofradiation, but mortality rate was significantly increased 120 h (5 days) after exposure to 600 or 800 Gygamma ray radiation. We observed a dose-dependent reduction in fecundity of ovigerous females; eventhe group irradiated with 50 Gy showed a significant reduction in fecundity, suggesting that gamma raysare likely to have a population level effect. In addition, we observed growth retardation, particularlyat the nauplius stage, in individuals after gamma irradiation. In fact, nauplii irradiated with more than200 Gy, though able to molt to copepodite stage 1, did not develop into adults. Upon gamma radiation,T. japonicus showed a dose-dependent increase in reactive oxygen species (ROS) levels, the activities ofseveral antioxidant enzymes, and expression of double-stranded DNA break damage genes (e.g. DNA-PK,Ku70, Ku80). At a low level (sub-lethal dose) of gamma irradiation, we found dose-dependent upregulationof p53, implying cellular damage in T. japonicus in response to sub-lethal doses of gamma irradiation,suggesting that T. japonicus is not susceptible to sub-lethal doses of gamma irradiation. Additionally,antioxidant genes, phase II enzyme (e.g. GSTs), and cellular chaperone genes (e.g. Hsps) that are involvedin cellular defense mechanisms also showed the same expression patterns for sublethal doses of gammairradiation (50-200 Gy). These findings indicate that sublethal doses of gamma radiation can induceoxidative stress-mediated DNA damage and increase the expression of antioxidant enzymes and proteinswith chaperone-related functions, thereby significantly affecting life history parameters such as fecundityand molting in the copepod T. japonicus.