Strain-specific priming of resistance in the red flour beetle, Tribolium castaneum

Abstract

As invertebrates lack the molecular machinery employed by the vertebrate adaptive immune system, it was thought that they consequently lack the ability to produce lasting and specific immunity. However, in recent years, it has been demonstrated that the immune defence of invertebrates is by far more complicated and specific than previously envisioned. Lasting immunity following an initial exposure that proves protection on a secondary exposure has been shown in several species of invertebrates. This phenomenon has become known as immune priming. In the cases where it is explicitly tested, this priming can also be highly specific. In this study, we used survival assays to test for specific priming of resistance in the red flour beetle, Tribolium castaneum, using bacteria of different degrees of relatedness. Our results suggest an unexpected degree of specificity that even allows for differentiation between different strains of the same bacterium. However, our findings also demonstrate that specific priming of resistance in insects may not be ubiquitous across all bacteria.

Figure 1

The proportion of individuals surviving following a challenge when they had been previously primed with either a homologous or heterologous bacteria. Homologous: filled circles, Bt1–Bt1, Bs–Bs, Ec–Ec, Bt2–Bt2. Heterologous: open circles, Bt2–Bt1, Bt1–Bt2; Bt2–Bs, Bs–Bt2, Bt1–Bs, Bs–Bt1; Ec–Bt2, Bt2–Ec, Bt1–Ec, Ec–Bt1, Bs–Ec, Ec–Bs; Rin–Ec, Rin–Bt1, Rin–Bt2, Rin–Bs; naive–Bt1, naive–Ec, naive–Bs, naive–Bt2. Controls: filled diamonds, naive–naive, Rin–Rin, naive–Rin. Bt, B. thuringiensis; Bs, B. subtilis; Ec, E. coli; Rin, Ringer.

Figure 2

The proportion of individuals surviving following a challenge ((a) challenge with Bt1, (b) challenge with Bt2, (c) challenge with Bs, (d) challenge with Ec) when they had been previously primed with either a homologous (filled circles: (a) Bt1–Bt1, (b) Bt2–Bt2, (c) Bs–Bs, (d) Ec–Ec) or different levels of heterologous bacteria (open circles: (a) Bs–Bt1, (b) Ec–Bt2, (c) Bt1–Bs, (d) Bt1–Ec; filled down triangles: (a) Bt2–Bt1, (b) Bs–Bt2, (c) Bt2–Bs, (d) Bt2–Ec; open up triangles: (a) Ec–Bt1, (b) Bt1–Bt2, (c) Ec–Bs, (d) Bs–Ec). Filled squares represent animals that were left naive at the priming; open squares represent animals that were treated with Ringer's solution at the priming.