EEB Student Dissertation Defense - Defining the molecular mechanisms of migration in monarch butterfly

Summary: Each year during the fall a naïve generation of monarch butterflies engages in a trans-continental migration, despite never having migrated before, which crosses over 4000km and spans 6-8 months. The fall migrant generation is in stark contrast to the preceding summer generation of butterflies who do not engage in such a feat but breed locally. Fall migration requires the coordinated expression of seasonally plastic traits such as oriented flight behavior and reproductive diapause (delayed reproduction during migration). The amalgamation of these component traits is dubbed the migratory syndrome. Yet, how these component phenotypes are integrated to form the migratory syndrome remains a largely open question. In my dissertation I studied how environmental and developmental factors interact to influence phenotypic plasticity of component migratory traits by combining field experiments with multi-omics (genomic, transcriptomic, and epigenomic) techniques. In Chapter 2 I uncover that reproductive diapause imparts a lasting transcriptional state which I hypothesize assists in the robustness of monarch butterflies during their return migration. In Chapter 3 I demonstrate that reproductive diapause and oriented flight are distinctly induced throughout the season, and each is regulated by tissue-specific transcriptional modules. Further, I also find that the unique oriented flight behavior during the seasonal transition from summer to fall is controlled by substantial cryptic genetic variation, which I hypothesize helps maintain behavioral diversity amidst seasonal unpredictability. Lastly, in Chapter 4 I present research which combines RNA- and ATAC-seq data to suggest that seasonal transcriptional plasticity is controlled by unique sets of seasonally regulated transcription factors. Overall, these works demonstrate the importance of environmental variation in defining the genetic architecture of the migratory syndrome and the regulatory mechanisms that control seasonal transcriptional plasticity. Defining these mechanisms helps us understand the evolution of migration as they define the substrates by which natural selection can operate.