Evolution towards monomorphy leads to a loss of morph plasticity in wing polymorphic crickets

Threshold traits are widespread forms of dimorphism in which an underlying continuous liability produces discrete phenotypes once a threshold is crossed. The threshold trait model predicts that phenotypic variation will be observed only when the distribution of liability spans the threshold, and may otherwise remain latent, complicating predictions of how genetic and environmental variation shape responses to selection. We tested this prediction in the wing-polymorphic cricket Gryllus lineaticeps, in which wing length is a heritable, plastic threshold trait with an endocrine basis. We confirmed the final juvenile instar as the sensitive period for wing-morph determination and validated general esterase activity as a predictor of adult wing morph. We manipulated genetic variation in liability through directional selection on wing morph and then measured plasticity in wing morph and esterase activity. Selected populations, predicted to have extreme liability distributions not spanning the threshold, were non-plastic for wing-morph frequency despite maintaining plasticity in esterase activity. Thus, physiological plasticity persisted but became cryptic at the morphological level. These results support predictions of the threshold trait model and demonstrate that shifts in the liability distribution can mask environmental sensitivity, providing a framework for understanding the dynamic evolution and role of phenotypic plasticity in threshold traits.