No apparent cost of evolved immune response in Drosophila melanogaster

Maintenance and deployment of the immune system are costly and are hence predicted to trade-off with other resource demanding traits, such as reproduction. We subjected this long standing idea to test using laboratory experimental evolution approach. In the present study, replicate populations of Drosophila melanogaster were subjected to three selection regimes – I (Infection with Pseudomonas entomophila), S (Sham-infection with MgSO4) and U (Unhandled Control). After 30 generations of selection flies from the I-regime had evolved better survivorship upon infection with P. entomophila compared to flies from U and S regimes. However, contrary to expectations and previous reports, we did not find any evidence of trade-offs between immunity and other life-history related traits, such as longevity, fecundity, egg hatchability or development time. After 45 generations of selection, the selection was relaxed for a set of populations. Even after 15 generations, the post-infection survivorship of populations under relaxed selection regime did not decline. We speculate that either there is a negligible cost to the evolved immune response or that trade-offs occur on traits like reproductive behaviour or other immune mechanisms that we have not investigated in this study. Our research suggests that at least under certain conditions, life-history trade-offs might play little role in maintaining variation in immunity.