Historical contingency is revealed by trade-offs upon sequential adaptation to different infection routes

In Drosophila melanogaster, prior systemic adaptation to Pseudomonas entomophila predicts sex-dependent differences in how populations evolve increased survival under oral infection.

• Why it matters: Host–pathogen interactions differ across ecological and physiological contexts, so historical contingency can be tested by tracking how one infection route reshapes responses to another.
• What they did: Replicate fly populations previously adapted to systemic infection were evolved under oral exposure to the same pathogen, then survival, fecundity costs, and bacterial clearance were measured across sexes and generations.
• The result: Oral adaptation increased survival in both histories but followed different trajectories: females with systemic history started lower yet adapted faster, while males showed weaker differences; oral adaptation reduced systemic survival and involved distinct, sex- and generation-specific clearance mechanisms alongside shared genetic components.

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