Radiation tolerance in the tardigrade Milnesium tardigradum

Purpose: Tardigrades are known to survive high doses of ionizing radiation. However, there have been no reports about radiation effects in tardigrades under culture conditions. In this study, we investigated tolerance of the tardigrade, Milnesium tardigradum, against gamma-rays and heavy ions by determining short-term or long-term survival, and reproductive ability after irradiation. Materials and methods: Hydrated and anhydrobiotic animals were exposed to gamma-rays (1000-7000 Gy) or heavy ions (1000-8000 Gy) to evaluate short-term survival at 2, 24 and 48 h post-irradiation. Long-term survival and reproduction were observed up to 31 days after irradiation with gamma-rays (1000-4000 Gy). Results: At 48 h after irradiation, median lethal doses were 5000 Gy (gamma-rays) and 6200 Gy (heavy ions) in hydrated animals, and 4400 Gy (gamma-rays) and 5200 Gy (heavy ions) in anhydrobiotic ones. Gamma-irradiation shortened average life span in a dose-dependent manner both in hydrated and anhydrobiotic groups. No irradiated animals laid eggs with one exception in which a hydrated animal irradiated with 2000 Gy of gamma-rays laid 3 eggs, and those eggs failed to hatch, whereas eggs produced by non-irradiated animals hatched successfully. Conclusion: M. tardigradum survives high doses of ionizing radiation in both hydrated and anhydrobiotic states, but irradiation with > 1000 Gy makes them sterile.