Step-wise evolution of temperature-mediated phenotypic plasticity in eyespot size across nymphalid butterflies

There are two disparate views regarding phenotypic plasticity. One regards plasticity as a derived adaptation to help organisms survive in variable environments 1, 2 while the other views plasticity as the outcome of flexible, non-canalized, developmental processes, ancestrally present in most organisms, that helps them colonize or adapt to novel environments 3–5 e.g., a pre-adaptation. Both views of plasticity currently lack a rigorous, mechanistic examination of ancestral and derived states and direction of change 2 . Here we show that the origin of phenotypic plasticity in eyespot size in response to environmental temperature observed in Bicyclus anynana butterflies is a derived adaptation of this lineage. Eyespot size is regulated by temperature-mediated changes in levels of a steroid hormone, 20E, that affects proliferation of eyespot central cells expressing the 20E receptor (EcR) 6, 7 . By estimating the origin of the known physiological and molecular components of eyespot size plasticity in a comparative framework, we showed that 20E titer plasticity in response to temperature is a pre-adaptation shared by all butterfly species examined, whereas the origin of expression of EcR in eyespot centers, and the origin of eyespot sensitivity to the hormone-receptor complex are both derived traits found only in a subset of species with eyespots. The presence of all three molecular components required to produce a plastic response is only observed in B. anynana . This gradual, step-wise, physiological/molecular response to temperature is a likely adaptation to temperature variation experienced across wet and dry seasons in the habitat of this species. This work supports, thus, the first view of plasticity as a derived adaptation.