Abstract
Tardigrades are highly tolerant to desiccation and ionizing radiation but the mechanisms of this tolerance are not well understood. In this paper, we report studies on dose responses of adults and eggs of the tardigrade Hypsibius dujardini exposed to gamma radiation. In adults the LD50/48h for survival was estimated at ~ 4200 Gy, and doses higher than 100 Gy reduced both fertility and hatchability of laid eggs drastically. We also evaluated the effect of radiation (doses 50 Gy, 200 Gy, 500 Gy) on eggs in the early and late embryonic stage of development, and observed a reduced hatchability in the early stage, while no effect was found in the late stage of development. Survival of juveniles from irradiated eggs was highly affected by a 500 Gy dose, both in the early and the late stage. Juveniles hatched from eggs irradiated at 50 Gy and 200 Gy developed into adults and produced offspring, but their fertility was reduced compared to the controls. Finally we measured the effect of low temperature during irradiation at 4000 Gy and 4500 Gy on survival in adult tardigrades, and observed a slight delay in the expressed mortality when tardigrades were irradiated on ice. Since H. dujardini is a freshwater tardigrade with lower tolerance to desiccation compared to limno-terrestrial tardigrades, the high radiation tolerance in adults, similar to limno-terrestrial tardigrades, is unexpected and seems to challenge the idea that desiccation and radiation tolerance rely on the same molecular mechanisms. We suggest that the higher radiation tolerance in adults and late stage embryos of H. dujardini (and in other studied tardigrades) compared to early stage embryos may partly be due to limited mitotic activity, since tardigrades have a low degree of somatic cell division (eutely), and dividing cells are known to be more sensitive to radiation.
Highlights
Tardigrades are small aquatic invertebrates known to tolerate a number of severe stressors, including very high and low temperatures, complete desiccation under vacuum and high doses of ionizing radiation [1]
We suggest that the higher radiation tolerance in adults and late stage embryos of H. dujardini compared to early stage embryos may partly be due to limited mitotic activity, since tardigrades have a low degree of somatic cell division, and dividing cells are known to be more sensitive to radiation
The present study adds a freshwater species, H. dujardini, to the list of tardigrades showing a very high tolerance to gamma radiation in the active hydrated state, with an LD5048h of ~4500 Gy. This is in the same range of tolerance as previously reported for the limnoterrestrial tardigrades Milnesium tardigradum (5000 Gy [4]) and Richtersius coronifer (3500 Gy/22 hours; [3])
Summary
Tardigrades are small aquatic invertebrates known to tolerate a number of severe stressors, including very high and low temperatures, complete desiccation under vacuum and high doses of ionizing radiation [1]. Beginning with the seminal paper by May et al [2] several studies on radiation tolerance of both adult tardigrades and their eggs have been published These studies include both low-LET radiation (X-rays, [2]; gamma rays, [3,4]), high-LET radiation (alpha particles, [4]; protons, [5]), UV radiation [6,7], and combinations of cosmic and UV radiation under space conditions [8]. The results from these studies position tardigrades among the most radiation-tolerant animals, with LD50 values of adults within the first 2 days after irradiation in the range of 3 to 10 kGy for ionizing radiation. Most species studied belong to the ecological category of “limno-terrestrial” species inhabiting microenvironments that regularly dry out, and the tolerance to extreme unnatural stressors such as ionizing radiation is supposed to be a by-product of adaptations to survive in dry environmental conditions [12]
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