Trans-generational immune priming (TGIP) is defined as the plastic enhancement of offspring's immunocompetence following an immune challenge of the females of the parental generation. In vertebrates, this phenomenon is well described, and is achieved by the maternal transfer of antibodies. In invertebrates however, it has only recently been described. Since invertebrates do not possess antibodies, the mechanism of this transmission remains unknown. If the offspring is exposed to the maternal infection, an elevated immunocompetence can help it cope better with it. Nonetheless, apart from this benefit, several cues indicate that the TGIP bears some fitness costs for individuals. The evolution of TGIP will be favoured when its benefits outweigh its fitness costs. Thus, studying its costs and benefits can lead us to a better understanding of the selection pressures that lead to its evolution in invertebrates. During my thesis, I associated the occurrence of TGIP in an insect, the mealworm beetle Tenebrio molitor, to several fitness costs for the females transmitting it as well as for the offspring receiving it.In the first chapter, we stimulated the adult female's immune system with a non pathogenic immunogene, and studied several aspects of the subsequent transfer of antibacterial activity to the eggs. We saw that the transmission of antibacterial activity inside the eggs started two days after the immune challenge, and stopped at ten. Then, we highlighted a cost for the females on their fecundity to this transmission.In the second chapter, we stimulated the immune system of the females with three different heat-killed microorganisms, and exposed their larval progeny to living microorganisms. We did not see any benefit of the TGIP on the young larvae of T. molitor. However, we saw that the exposure of young larvae to an entomopathogenic fungus decreased the time-lap between the two first larval moults.In the third chapter, we stimulated the immune system of either the adult females or the males of T. molitor, and we observed several immune parameters in their adult offspring. This allowed us to see that maternally and paternally-derived TGIP affected different immune effectors in the adult offspring, and that maternally-derived TGIP bear a cost on the developmental time of the offspring.These fitness costs to the TGIP suggest that it is not just a side-effect of the immune reaction of the females, but rather an investment that has been selected because of the benefits it represents for the offspring in certain ecological conditions
