Experimental evolution of insect immune memory versus pathogen resistance

Abstract

Under strong pathogen pressure, insects often evolve resistance to infection. Many insects are also protected via immune memory (immune priming), whereby sublethal exposure to a pathogen enhances survival after secondary infection. Theory predicts that immune memory should evolve when the pathogen is highly virulent, or when pathogen exposure is relatively rare. However, there are no empirical tests of these hypotheses, and the adaptive benefits of immune memory relative to direct resistance against a pathogen are poorly understood. To determine the selective pressures and ecological conditions that shape immune evolution, we imposed strong pathogen selection on flour beetle (Tribolium castaneum) populations, infecting them with Bacillus thuringiensis (Bt) for 11 generations. Populations injected first with heat-killed and then live Bt evolved high basal resistance against multiple Bt strains. By contrast, populations injected only with a high dose of live Bt evolved a less effective but strain-specific priming response. Control populations injected with heat-killed Bt did not evolve priming; and in the ancestor, priming was effective only against a low Bt dose. Intriguingly, one replicate population first evolved priming and subsequently evolved basal resistance, suggesting the potential for dynamic evolution of different immune strategies. Our work is the first report showing that pathogens can select for rapid modulation of insect priming ability, allowing hosts to evolve divergent immune strategies (generalized resistance versus specific immune memory) with potentially distinct mechanisms.

Keywords: Bacillus thuringiensis; Tribolium castaneum; immune priming; pathogen selection; specific immunity.

Figure 1.

(a) Design of experimental evolution and selection regimes. (b) Generating standardized beetles to measure evolved priming and resistance.

Figure 2.

Adult survival during the first 48 h after infection with live Bt cells (day 16–18; figure 1a), during the course of experimental evolution. Only PI and I beetles were infected, and C and P beetles were injected with buffer.

Figure 3.

Survival curves and estimated hazard ratios for priming and resistance to Bt (a) in the ancestral population (n = 22–24 females per treatment) (b–g) after eight generations of selection (n = 16–24 females per treatment population) and (h,i) after 11 generations of selection (n = 16–26 females per treatment per population). (f,g) Hazard ratios calculated from survival curves shown in (b–e). Survival curves for panels (h) and (i) are given in electronic supplementary material, figure S3. (f,h) The survival benefit of priming (a greater hazard ratio indicates higher benefit of priming); (g,i) the resistance to infection (a greater hazard ratio indicates higher susceptibility to infection, or lower resistance). (f–i) Horizontal dark grey lines denote group mean hazard ratios and dashed lines indicate a hazard ratio of 1. Asterisks denote hazard ratios significantly different from 1 (p ≤ 0.05). (Online version in colour.)

Figure 4.

Estimated hazard ratios for (a) immune priming response and (b) resistance to Bt1 infection across replicate populations of each regime (n = 16–24 females per treatment per population). Horizontal grey lines denote group mean hazard ratios and dashed lines indicate a hazard ratio of 1. Asterisks denote hazard ratios significantly different from 1 (p ≤ 0.05).