M. tardigradum survives high doses of ionizing radiation in both hydrated and anhydrobiotic states, but irradiation with >1000 Gy makes them sterile.
Studies on the ability of multicellular organisms to tolerate specific environmental extremes are relatively rare compared to those of unicellular microorganisms in extreme environments. Tardigrades are extremotolerant animals that can enter an ametabolic dry state called anhydrobiosis and have high tolerance to a variety of extreme environmental conditions, particularly while in anhydrobiosis. Although tardigrades have been expected to be a potential model animal for astrobiological studies due to their excellent anhydrobiotic and extremotolerant abilities, few studies of tolerance with cultured tardigrades have been reported, possibly due to the absence of a model species that can be easily maintained under rearing conditions. We report the successful rearing of the herbivorous tardigrade, Ramazzottius varieornatus, by supplying the green alga Chlorella vulgaris as food. The life span was 35 +/- 16.4 d, deposited eggs required 5.7 +/- 1.1 d to hatch, and animals began to deposit eggs 9 d after hatching. The reared individuals of this species had an anhydrobiotic capacity throughout their life cycle in egg, juvenile, and adult stages. Furthermore, the reared adults in an anhydrobiotic state were tolerant of temperatures of 90 degrees C and -196 degrees C, and exposure to 99.8% acetonitrile or irradiation with 4000 Gy (4)He ions. Based on their life history traits and tolerance to extreme stresses, R. varieornatus may be a suitable model for astrobiological studies of multicellular organisms.
The possible mechanism of a radiation-induced bystander response was investigated by using a high-LET heavy particle microbeam, which allows selected cells to be individually hit with precise numbered particles. Even when only a single cell within the confluent culture was hit by one particle of 40Ar (approximately 1260 keV/microm) or 20Ne (approximately 380 keV/microm), a 1.4-fold increase of micronuclei (MN) was detected demonstrating a bystander response. When the number of targeted cells increased, the number of MN biphasically increased; however, the efficiency of MN induction per targeted cell markedly decreased. When 49 cells in the culture were individually hit by 1 to 4 particles, the production of MN in the irradiated cultures were approximately 2-fold higher than control levels but independent of the number and LET of the particles. MN induction in the irradiated-culture was partly reduced by treatment with DMSO, a scavenger of reactive oxygen species (ROS), and was almost fully suppressed by the mixture of DMSO and PMA, an inhibitor of gap junctional intercellular communication (GJIC). Accordingly, both ROS and GJIC contribute to the above-mentioned bystander response and GJIC may play an essential role by mediating the release of soluble biochemical factors from targeted cells.
Evidence has accumulated showing that ionizing radiations persistently perturb genomic stability and induce delayed reproductive death in the progeny of survivors; however, the linear energy transfer (LET) dependence of these inductions has not been fully characterized. We have investigated the cell killing effectiveness of gamma rays (0.2 keV/microm) and six different beams of heavy-ion particles with LETs ranging from 16.2 to 1610 keV/microm in normal human fibroblasts. First, irradiated confluent density-inhibited cultures were plated for primary colony formation, revealing that the relative biological effectiveness (RBE) based on the primary 10% survival dose peaked at 108 keV/microm and that the inactivation cross section increased proportionally up to 437 keV/microm. Second, cells harvested from primary colonies were plated for secondary colony formation, showing that delayed reproductive death occurred in a dose-dependent fashion. While the RBE based on the secondary 80% survival dose peaked at 108 keV/microm, very little difference in LET was observed in the RBE based on secondary survival at the primary 10% survival dose. Our present results indicate that delayed reproductive death arising only during secondary colony formation is independent of LET and is more likely to be dependent on initial damages having been fixed during primary colony formation.
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