Epigenetic modification of the hypothalamic-pituitary-adrenal axis during early life of the house sparrow (Passer domesticus)

The early environment impacts many aspects of an individual’s developing phenotype. In particular, early environmental conditions are important for shaping the hypothalamic-pituitary-adrenal (HPA) axis, which coordinates an individual’s stress response. These developmental changes are likely mediated by epigenetic modifications, functional changes to the genome that can alter gene expression in response to environmental variation, resulting in significant phenotypic differences (Kundakovic and Champagne 2015; Richards 2006). Determining how early life variation alters epigenetic modifications (such as DNA methylation) of genes throughout the HPA axis, and how these marks change over time, in wild organisms is important for understanding their potential long-term fitness consequences. Here, I examine DNA methylation modifications in the HPA axis in relation to early environmental variation in free-living house sparrows (Passer domesticus). In Chapter 1, I show a relationship between natural variation in the early environment and DNA methylation marks of numerous genes related to HPA axis function, which in turn predict growth trajectories. In Chapter 2, I show that early life stress in particular impacts DNA methylation in genes critical to HPA axis function, but does so differently depending on the life history stage in which stress is encountered. Finally, in Chapter 3, I find that these early life marks have long-term effects past the developmental period, predicting longevity as well as lifetime reproductive output in a sex-specific manner. Overall, my dissertation adds to a growing understanding of the dynamic role of epigenetic modifications in mediating phenotypic responses to the early life environment in wild birds, and demonstrates the potential long-term fitness outcomes of these changes.