Pathogen susceptibility and fitness costs explain variation in immune priming across natural populations of flour beetles

1. In many insects, individuals primed with low doses of pathogens early in life have higher survival after exposure to the same pathogen later in life. Yet, our understanding of the evolutionary and ecological history of priming of immune response in natural insect populations is limited. Previous work demonstrated population-, sex- and stage- specific variation in the survival benefit of priming response in flour beetles (Tribolium castaneum) infected with their natural pathogen Bacillus thuringiensis. However, the evolutionary forces responsible for this natural variation remained unclear. 2. In the present work, we tested whether the strength of the priming response (measured as the survival benefit after priming and subsequent infection, relative to unprimed controls) was associated with multiple fitness parameters and immune components across 10 flour beetle populations collected from different locations in India). 3. Our results suggest two major selective pressures that may expl...