Effective but costly, evolved mechanisms of defense against a virulent opportunistic pathogen in Drosophila melanogaster

Abstract

Drosophila harbor substantial genetic variation for antibacterial defense, and investment in immunity is thought to involve a costly trade-off with life history traits, including development, life span, and reproduction. To understand the way in which insects invest in fighting bacterial infection, we selected for survival following systemic infection with the opportunistic pathogen Pseudomonas aeruginosa in wild-caught Drosophila melanogaster over 10 generations. We then examined genome-wide changes in expression in the selected flies relative to unselected controls, both of which had been infected with the pathogen. This powerful combination of techniques allowed us to specifically identify the genetic basis of the evolved immune response. In response to selection, population-level survivorship to infection increased from 15% to 70%. The evolved capacity for defense was costly, however, as evidenced by reduced longevity and larval viability and a rapid loss of the trait once selection pressure was removed. Counter to expectation, we observed more rapid developmental rates in the selected flies. Selection-associated changes in expression of genes with dual involvement in developmental and immune pathways suggest pleiotropy as a possible mechanism for the positive correlation. We also found that both the Toll and the Imd pathways work synergistically to limit infectivity and that cellular immunity plays a more critical role in overcoming P. aeruginosa infection than previously reported. This work reveals novel pathways by which Drosophila can survive infection with a virulent pathogen that may be rare in wild populations, however, due to their cost.

Figure 1. Average percentage survival of flies (male and female) at 48 hours post-infection of controls (open symbols) and lines selected for PA01 defense (solid symbols).

Survival was measured for every generation for selected lines and at G₆ and G₁₀ for control lines. Selection was halted at G₁₀ before defense was assessed again at G₁₅.

Figure 2. Life-history traits of control (open bars) and selected (black bars) lines measured at G₉.

Line means are plotted±sem. * P-value<0.05, ** P-value<0.01, *** P-value<0.001.

Figure 3. Venn diagram of number of transcripts that show significant expression changes (up-regulated/down-regulated) across the three lines.