Multigenerational copper sulphate exposure alters the allometric scaling of the condition-dependent eyespan trait in the stalk-eyed fly Teleopsis dalmanni

Sexual selected traits are often sensitive to environmental stress, reflecting individual condition and the costs of their expression. Yet almost all research on condition dependence in sexually selected traits examines only a single generation, and we still know little about how trait sizes evolve over multiple generations in response to selection and adaptation. Here, we exposed stalk-eyed flies (Teleopsis dalmanni) to three generations of chronic copper sulfate stress. In this species, eyespan is an exaggerated, sexually selected trait. Copper exposure led to an immediate surge in mortality, reduced mean body size and eyespan. Larger males and females had disproportionately smaller reductions in eyespan, which could allow more intense sexual selection favoring individuals better adapted to this novel stress. In successive generations exposed to copper, mortality, eyespan and body size and allometry recovered partially. When copper was removed in the 4th generation, mortality returned close to baseline, whereas eyespan and allometric slopes remained significantly different. These findings reveal that persistent environmental stress can leave lasting imprints on the genetic architecture of sexual selection.