Abstract
Tardigrades inhabiting terrestrial environments exhibit extraordinary resistance to ionizing radiation and UV radiation although little is known about the mechanisms underlying the resistance. We found that the terrestrial tardigrade Ramazzottius varieornatus is able to tolerate massive doses of UVC irradiation by both being protected from forming UVC-induced thymine dimers in DNA in a desiccated, anhydrobiotic state as well as repairing the dimers that do form in the hydrated animals. In R. varieornatus accumulation of thymine dimers in DNA induced by irradiation with 2.5 kJ/m2 of UVC radiation disappeared 18 h after the exposure when the animals were exposed to fluorescent light but not in the dark. Much higher UV radiation tolerance was observed in desiccated anhydrobiotic R. varieornatus compared to hydrated specimens of this species. On the other hand, the freshwater tardigrade species Hypsibius dujardini that was used as control, showed much weaker tolerance to UVC radiation than R. varieornatus, and it did not contain a putative phrA gene sequence. The anhydrobiotes of R. varieornatus accumulated much less UVC-induced thymine dimers in DNA than hydrated one. It suggests that anhydrobiosis efficiently avoids DNA damage accumulation in R. varieornatus and confers better UV radiation tolerance on this species. Thus we propose that UV radiation tolerance in tardigrades is due to the both high capacities of DNA damage repair and DNA protection, a two-pronged survival strategy.
Highlights
Tardigrades are invertebrate animals found in oceans, freshwater, and terrestrial environments
Egg production was confirmed even 25 d after irradiation in R. varieornatus irradiated with 2.5 kJ/m2 of UVC radiation in the hydrated state and all doses in the anhydrobiotic state (Fig. 2)
In the present study we found that R. varieornatus had higher survival ability against UVC radiation than H. dujardini when exposed in the hydrated state
Summary
Tardigrades are invertebrate animals found in oceans, freshwater, and terrestrial environments. A small fraction of individuals (, 3%) of terrestrial tardigrade Milnesium tardigradum in the anhydrobiotic state survived a 10-day space flight exposure experiments at low Earth orbit even after exposure to 7577 kJ/m2 of UV radiation (116.5–400 nm wavelength) [8]. Since such massive exposure to UV radiation usually causes lethal DNA damage in organisms, tardigrades must bear enhanced capabilities for repairing heavily damaged DNA and/or protecting DNA against UV radiation. To examine thymine dimers formed in DNA by UV irradiation is of importance to estimate whether tardigrades cope with UV radiation exposure by protecting DNA against UV radiation or by repairing UV-induced DNA damage
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