THE EVOLUTION OF PHENOTYPIC PLASTICITY IN BUTTERFLIES

Abstract

Phenotypic plasticity is the ability of an organism to have distinct phenotypes in response to environmental cues. Early work in insects implicated changes to hormone titers as key molecular mediators of plasticity, but details on how the environment alters the development of traits via these hormones and how such complex plastic systems originated are not known. I investigated proximate mechanisms of temperature-induced eyespot size plasticity across homologous eyespots in one species of Nymphalid butterfly, Bicyclus anynana. Investigations in this thesis conclude that sex- specific differences in 20-hydroxyecdysone (20E) hormone titers lead to sexually dimorphic and seasonally plastic sexual ornaments. A comparative study across 13 butterfly species showed eyespot size plasticity in response to rearing temperature is an ancestral trait in butterflies. By mapping onto a phylogeny of Nymphalid butterflies, how 20E titers respond to rearing temperature across species, as well as the presence of 20E receptor in eyespots, and the sensitivity of eyespots to 20E signaling, I inferred that the derived plastic response of B. anynana eyespots to temperature was gradually acquired via multiple separate evolutionary steps. Phenotypic plasticity in eyespot size in Satyrid butterflies evolved in an adaptive manner incorporating environmental cues, one trait at a time.