The Costs and Benefits of Basal Infection Resistance vs Diverse Immune Priming Responses in an Insect

Abstract

24Insects exhibit various forms of immune responses, including basal resistance to pathogens and a form of 25 immune memory ("priming") that can act within or across generations. The evolutionary drivers of such 26 diverse immune functions remain poorly understood. Previously, we found that in the beetle Tribolium 27 castaneum, both resistance and priming evolved as mutually exclusive strategies against the pathogen 28 Bacillus thuringiensis. However, since evolved resistance improved survival far more than priming, the 29 evolution of priming in some populations was puzzling. Was resistance more costly in these populations, 30 or did priming provide added benefits? To test this, we revisited our evolved beetles and analyzed the costs 31 and benefits of evolved priming vs. resistance. Surprisingly, resistant beetles increased reproduction after 32 infection, with no measurable costs. In contrast, mounting a priming response reduced offspring early 33 survival, development rate and reproduction. Even added trans-generational survival benefits of evolved 34 priming could not tilt the balance in favor of priming. Hence, resistance is consistently more beneficial than 35 priming; and the evolution and persistence of costly priming rather than resistance remains a mystery. 36Nevertheless, our work provides the first detailed comparison of the complex fitness consequences of 37 distinct insect immune strategies, opening new questions about their evolutionary dynamics. 38 39