The within‐host dynamics of infection in trans‐generationally primed flour beetles

Abstract

Many taxa exhibit plastic immune responses initiated after primary microbial exposure that provide increased protection against disease-induced mortality and the fitness costs of infection. In several arthropod species, this protection can even be passed from parents to offspring through a phenomenon called trans-generational immune priming. Here, we first demonstrate that trans-generational priming is a repeatable phenomenon in flour beetles (Tribolium castaneum) primed and infected with Bacillus thuringiensis (Bt). We then quantify the within-host dynamics of microbes and host physiological responses in infected offspring from primed and unprimed mothers by monitoring bacterial density and using mRNA-seq to profile host gene expression, respectively, over the acute infection period. We find that priming increases inducible resistance against Bt around a critical temporal juncture where host septicemic trajectories, and consequently survival, may be determined in unprimed individuals. Our results identify a highly differentially expressed biomarker of priming, containing an EIF4-e domain, in uninfected individuals, as well as several other candidate genes. Moreover, the induction and decay dynamics of gene expression over time suggest a metabolic shift in primed individuals. The identified bacterial and gene expression dynamics are likely to influence patterns of bacterial fitness and disease transmission in natural populations.

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

“…Thus, priming perhaps rescues post‐infection by reducing the pathogen load. Prior studies also show that priming alters bacterial load, though the timing (Tate, Andolfatto, Demuth, & Graham, ) and direction of the impact varies (compare Pham, Dionne, Shirasu‐hiza, & Schneider, ; Kutzer, Kurtz, & Armitage, ). Together, these results indicate that pathogen‐mediated reduction in life span may impose strong selection for a priming response in natural populations, as predicted by theoretical models (Best et al, ) and observed in laboratory‐evolved populations (Khan et al, ).…”

Natural variation in the contribution of microbial density to inducible immune dynamics

“…Within each experiment, we performed qPCR for each transcript template using PowerUp Sybr Green Master Mix (Life Technologies) on the QuantStudio 6 machine (Applied Biosystems) by running all individuals on the same 384 well plate in duplicate, where possible, using default settings unless indicated in Table . We confirmed equivalent Ct thresholds and ran a subset of individuals on both plates to minimize plate effects, as in (Tate et al, ).…”

“…Bacillus thuringiensis is a spore‐producing entomopathogen that relies on host killing for transmission (Raymond, Johnston, Nielsen‐LeRoux, Lereclus, & Crickmore, ). Whether it naturally enters the host through the gut or is septically injected, it grows rapidly in the haemolymph of infected beetles and kills them within 12 hr (Tate et al, ). To infect beetles with Bt (ATCC 55177, a berliner strain that contains a cry protein gene active against Coleoptera), we prepared a culture from a glycerol stock kept at −80°C and grown overnight in 10 ml of Nutrient Broth #3 (Sigma Aldrich).…”

“…Primers that performed with high efficiency and equivalently in both species are listed in Table . We used previously published primers (Tate et al, ; Tate & Graham, ) to quantify Bt density relative to host tissue (Table ), where the amplified product has been previously shown to correlate strongly with bacterial CFU as determined by samples plated on agar (Tate & Graham, ).…”

“…To test the hypothesis that the evolutionary costs and benefits of microbe‐independent and –dependent inducible immune responses depend on the context of the specific infection and are thus likely to vary among populations, we infected seven wild‐caught flour beetle ( Tribolium castaneum and T. confusum ) populations with one of two entomopathogenic bacteria, the rapidly growing and virulent Bacillus thuringiensis (Bt) and the slower‐growing but immunomodulatory Photorhabdus luminescens (P.lum). To quantify inducible immune gene expression, we infected individuals from each population with a gradient of initial bacterial doses or a sterile wound and then sacrificed them during a point in the acute infection phase late enough to avoid lag in the induction of immunity but prior to the onset of host mortality (Tate, Andolfatto, Demuth, & Graham, ). Using RT‐qPCR, we quantifed the impact of natural variation and humoral (IMD) immune pathway signalling efficiency on the intercept (microbe‐independent) and slope (microbe‐sensitive) of the relationship between microbe density and immune gene transcription.…”