Population differences in melanin pigmentation plasticity in response to a seasonal cue

Adaptive plasticity allows organisms to interact with heterogenous environments and respond to environmental change. Population-level comparisons of plasticity provide insights into the selective factors driving plasticity evolution and properties of reaction norms likely to evolve. We test how thermal environments shape melanin plasticity in response to a seasonal cue in the white-lined sphinx moth, Hyles lineata. We compare how photoperiod affects melanization in two populations that experience different thermal environments: Colorado and Arizona. If thermal environment drives differences in melanin plasticity in response to photoperiod, then the reaction norms should differ in intercept (higher melanization in Colorado larvae across photoperiods, due to colder temperatures), slope (steeper in Arizona larvae, due to a larger range of temperatures across relevant photoperiods), and shape (linear in Arizona larvae and quadratic in Colorado larvae, due to the relationship between photoperiod and temperature). Results are partially consistent with these predictions: the Arizona population had a steeper slope, but a higher intercept. The Colorado population likely relies more heavily on temperature cues to inform melanization, requiring lower temperatures to increase melanin. Populations did not differ in reaction norm shape, suggesting that while slope and intercept are labile, there may be constraints on the evolution of shape. Because only two populations were compared in this study, replication at the population level is needed to corroborate the generality of these results. This study highlights the complexity of plasticity evolution and the need to consider multiple cues and selective pressures, as well as potential constraints on the evolution reaction norms.