Résumé
Protecting offspring from pathogen infection is an important function of parental care. In insects, a key mediator of this protection occurs during egg formation through trans-generational immune priming (TGIP), a powerful process by which parents transfer their own immunological experience to their eggs, and thus to future offspring, before oviposition. However, it is not known whether this can also be more plastic and occur after egg formation. Here, we addressed this knowledge gap using the European earwig, a Dermapteran insect in which mothers care for their eggs and juveniles for several months after oviposition. We primed 246 mothers caring for their eggs with either heat-killed solutions of the bacterium Serratia marcescens, PBS (a buffer solution) or no injection (control). We then tested whether this priming helped the mothers to be better protected against a future infection (immune priming) and whether they could transfer this improved protection to their offspring during post-oviposition care (TGIP). Our results support both predictions but, somewhat surprisingly, only for PBS-primed mothers. These mothers survived infection with live bacteria better than control mothers, whereas bacteria-primed mothers survived overall less well than control and PBS-primed mothers. Moreover, adults who had previously been cared for by PBS-primed mothers survived bacterial infection better than adults who had previously been cared for by the three other types of mothers. Overall, our results show that the effects of building up an immune defence in earwig mothers range from costly to beneficial in terms of resistance to future infection, depending on whether the priming is caused by dead bacteria or simple wounds. It also shows that these beneficial immunological changes in mothers can be transmitted to their offspring after oviposition, confirming that TGIP is a dynamic process that can occur throughout offspring development and may therefore play an important – but currently neglected - role in the evolution and maintenance of family life in insects.