Abstract

Virtually all organisms face some level of heterogeneity in the quality of their environment. When such fluctuations are seasonal and thus predictable, there is scope for anticipating them and adjusting life history strategies well in time. Such plasticity is a common feature of animal life, and may include such diverse adaptations as migration, diapause or plumage molt.

As part of my PhD, I study mechanisms by which the afrotropical butterfly Bicyclus anynana copes with its habitat’s contrasting seasonal environments. In the warm wet season, when food is abundant, butterflies reproduce fast and live relatively short lives. In contrast, in the cold dry season food is limited and butterflies bridge this harsh period as cryptic adults, postponing reproduction until the start of the next wet season.

We studied how conditions during larval development induce these two alternate phenotypes. In particular, we found how Ecdysteroid hormones play an important role in interpreting the environment during development by responding in a threshold-like manner to an environmental gradient, acting like a switch mechanism between alternate developmental trajectories. Subsequently, these hormones induce the changes in the adult traits involved in the seasonal adaptation, such as wing pattern and fecundity, in a time- and temperature dependent manner. This suggests that rather than constraining environmental responses, the hormone system allows for both flexibility and integration of traits underlying adaptations to divergent environments.

Environmentally induced phenotypic differences between seasonal morphs ultimately depend on transcriptional regulation, since both morphs develop from the same genetic background. Transcripts of innate immunite genes were expressed at much higher level in the wet season, possibly related to the higher reproductive activity. In contrast, lipid metabolic and transport genes showed higher activity in dry season conditions, reflecting increased storage of acquired resources.