Paling in Comparison: The Role of Sex and Temperature in Melanin-Based Immune Function

Abstract

Sex differences in disease susceptibility have been observed across a wide range of species, yet we lack a strong understanding of how environmental context influences these patterns. In this thesis, I take a life-history approach to investigating the plasticity of immunological sex differences in insects. Insects rely on the pigment melanin for both immune function and the structure and coloration of the cuticle (i.e. integument). Although many studies have shown evidence of correlations between immunity and cuticle pigmentation, most do not take into account the many potential modes of selection acting on cuticle melanism. Given that the biochemical precursors of melanin are a common currency in thermoregulation, sexual ornamentation, and immune function in insects, we hypothesized that sex differences in melanin-based resource allocation contribute to sex differences in immunity, and that the thermal environment will moderate this influence. My thesis work focused on the interactive effects of sexual selection and thermal selection on cuticle melanism and immunity in the Pacific field cricket (Teleogryllus oceanicus). After rearing crickets under multiple temperatures, we found, as predicted, that females invested more in immunity, males invested more in cuticle melanism, and both immune function and cuticle melanism were reduced in individuals that developed under warmer temperatures. Our results suggest that sex-specific investment in melanin corresponds with sex differences in immunity and that thermoregulation may act as an immune constraint under high temperatures, thus regulating the extent to which males and females diverge in disease susceptibility. By quantifying melanin-based traits underlying strategies for both reproduction and survival in insects, this study sheds light onto the selective forces shaping sex differences in immunity and insect life-history in general.