Ecology and Evolution of Melanin Pigmentation Plasticity in a Complex World

Abstract

Phenotypic plasticity is the ability of a single genotype to produce multiple phenotypes in response to different environmental stimuli. Plasticity is a widespread phenomenon that takes many forms and is often thought to be adaptive, helping organisms optimally match phenotypes to a heterogenous environment. Organisms experience and respond to many dimensions of environmental variation at once. Many plastic traits are induced by multiple environmental cues and are under more than one selective pressure. Together, these environments determine the functional roles of a trait and the constraints on a trait. To determine if plasticity is adaptive, it is necessary to understand the balance of these functions versus constraints. In the following studies I investigate melanin pigmentation plasticity in the white-lined sphinx moth, Hyles lineata (Sphingidae). During the late larval instars this species displays melanin plasticity (plasticity in the degree of melanin pigmentation), which is induced by multiple environmental cues. I investigated two potential functional roles of melanin: thermoregulation and desiccation prevention. While I did not find support for a role of melanin in desiccation prevention, I did find evidence that melanin is adaptive in cold environments. More melanic larvae outperform less melanic larvae in cold environments, while suffering no costs of melanization in a warm environment. I then considered two potential constraints on melanin: resource limitation and resource allocation trade-offs. I found that tyrosine, the amino acid precursor of melanogenesis can constrain the production of melanin, although it is still prioritized in certain contexts. Furthermore, larval melanin pigmentation trades off with other important traits, including immunity and adult pigmentation, which may be costly in certain environments. Finally, I tested whether patterns of melanin plasticity have diverged adaptively between populations from different thermal environments. I found that reaction norms differed between populations, although the patterns were only partially consistent with a role of climate in driving differences in plasticity. Overall, I show that to determine whether melanin plasticity is adaptive in this species, it is important to consider not just the thermal environment but also the nutritional environment and the costs of producing melanin. Understanding both the functional roles of and the constraints on a plastic trait are necessary to determine the adaptive value of its plasticity.