Responses of disparate phenotypically-plastic, melanin-based traits to common cues: limits to the benefits of adaptive plasticity?

Abstract

The evolution of perfect adaptive phenotypic plasticity of a given trait may be influenced by, among other things, phenotypic costs associated with the expression of a given trait value, relative to alternative trait values. One potential cause of such phenotypic costs is the allocation of limited resources to multiple traits. When multiple traits rely on the same resource, trait values for one adaptively plastic trait might be unavoidably associated with maladaptive trait values for other traits. I address this problem in three traits of Pieris rapae L. (the small cabbage white butterfly) that all rely on the pigment melanin and are adaptively plastic, but have very different functions: wing pattern, immune defense, and pupal color. Cool, short-day rearing conditions simultaneously increased total wing melanization and decreased a melanin-based immune response in females, consistent with predictions. However, cool, short days also reduced the melanin-based immune response in males, despite little effect on male wing melanization. Furthermore, contrary to predictions, these patterns were not altered by differences in dietary resources. Finally, dark-colored rearing backgrounds during pupation substantially increased pupal melanization in both sexes, but was not associated with differences in wing melanization. These results offer only mixed support for the hypothesis of melanin-based trade offs as a source of phenotypic costs to adaptive plasticity in these traits. However, patterns of sexual dimorphism for these traits suggest trade offs might be at work at another level: relative to males, females have consistently more heavily melanized wings but less heavily melanized pupae and immune responses. The reduced immune response under cool, short-day conditions may also have implications for the evolutionary ecology of these butterflies.