Facultative but persistent trans-generational immunity via the mother's eggs in bumblebees.

It is widely acknowledged that a mother, and the maternal environment, can have a profound effect on the phenotype of her offspring. For example, immunocompetent vertebrate mothers can pass on immune factors to their otherwise immune naïve offspring. Recently, it has also been demonstrated in invertebrates that maternal contact with pathogens and immune elicitors can have a positive impact on offspring resistance and immune system components. Here, using a cross-fostering design, we show that trans-generational immune priming in a social insect, similar to birds, takes place through factors in the eggs, with effects persisting into adult worker offspring. Additionally, eggs from immune-challenged mothers are imbued with increased internal antibacterial activity when they are laid.