BASAL RESISTANCE AGAINST A PATHOGEN IS MORE BENEFICIAL THAN IMMUNE PRIMING RESPONSES IN FLOUR BEETLES

Insects exhibit various forms of immune responses, including basal resistance to pathogens and a form of immune memory ("priming") that can act within or across generations. The evolutionary drivers of such diverse immune functions remain poorly understood. Previously, we found that in the beetle Tribolium castaneum, both resistance and priming evolved as mutually exclusive strategies against the pathogen Bacillus thuringiensis. However, since evolved resistance improved survival far more than priming, the evolution of priming in some populations was puzzling. Was resistance more costly in these populations, or did priming provide added benefits? To test this, we revisited our evolved beetles and analyzed the costs and benefits of evolved priming vs. resistance. Surprisingly, resistant beetles increased reproduction after infection, with no measurable costs. In contrast, mounting a priming response reduced offspring early survival, development rate and reproduction. Even added trans-generational survival benefits of evolved priming could not tilt the balance in favor of priming. Hence, resistance is consistently more beneficial than priming; and the evolution and persistence of costly priming rather than resistance remains a mystery. Nevertheless, our work provides the first detailed comparison of the complex fitness consequences of distinct insect immune strategies, opening new questions about their evolutionary dynamics.