The environmental risk assessment of chronic exposure to ionizing radiation (natural and ubiquitous phenomenon enhanced by human activities) has become a major concern. Few studies relating to chronic exposure over several generations - essential knowledge to better understand the disruption caused by ionizing radiation and its possible consequences on the population - exist. In addition, it has become necessary to understand the mechanisms of disturbances related to ionizing radiation at the molecular and cellular level. Without this mechanistic understanding, it is difficult to extrapolate the effects observed between the different levels of biological organization and between different species.
The aim of this PhD was to study the multigenerational effects of chronic gamma radiation in an integrated manner (to the life history traits from the subcellular mechanisms) in a model organism, the nematode Caenorhabditis elegans. A two-step strategy was implemented. First, studying the effects of chronic gamma radiation on the life history traits of C. elegans was performed. The objective of this experiment was to test the hypothesis of an increase of the sensitivity according generations. For that, three generations have been exposed to different dose rates. In parallel, two generations have been placed in "control" environment after parental exposure to test a possible transmission of maternal effects. The second part of this thesis aimed to characterize the different subcellular mechanisms that could explain the observed effects on the life history traits after multigenerational exposure.
The results showed that (i) the cumulative number of larvae was the most sensitive endpoint to gamma radiation, (ii) an increase in radiosensitivity was observed over three exposed generations and (iii) the effects of the parental generation were transmitted to the non-exposed generations. An increase in apoptosis, a reduction in the stock of sperm, and to a lesser extent, a decrease in the number of mitotic cells, could explain the observed decrease in the number of laid eggs for the exposed generations. Only a decrease in sperm number was observed in parallel with a reduction in the cumulative number of larvae in the non-exposed generations. Other processes, including epigenetic processes, could also explain the observed effect on reproduction in these generations. This research contributes to our knowledge on the multigenerational effects of gamma irradiation and shows the importance of an integrated approach to better understand the mechanisms of action related to the action of a pollutant and improve the environmental risk assessment.
