Abstract
Arctic Grayling (Thymallus arcticus), a cold-water Holarctic salmonid, particularly vulnerable to warming freshwater environments. This dissertation examines how chronic, acute, and early-life thermal stress shape physiological performance and DNA methylation patterns in Arctic Grayling, integrating growth experiments, acute and chronic stress challenges with whole-genome epigenetic profiling. The main goal is to identify the biological processes that underpin thermal stress responses and to determine whether temperature-driven epigenetic variation may contribute to phenotypic plasticity.
The first chapter provides a literature review of Arctic Grayling biology and conservation status, synthesizing current knowledge on population status, thermal vulnerability, and potential management strategies. It also examines emerging trends in ecological and evolutionary epigenomics, highlighting methodological advances and the expanding use of DNA methylation in both model and non-model organisms.
The second chapter presents a 145-day growth trial across ten constant temperatures, which generated a thermal performance curve (TPC) for juvenile Arctic Grayling. Growth increased until an optimal range of ~16–18 °C, followed by a sharp decline at higher temperatures. These results helped us estimate the thermal optima, upper thermal limits, and thermal safety margins for the species.
The third chapter examines epigenetic responses to chronic heat exposure using liver tissue from the second chapter. Enzymatic methylation sequencing (EM-seq) revealed widespread but localized differentially methylated regions (DMRs) among low, optimal, and high-temperature groups. Although global methylation remained stable, chronic warming produced distinct epigenomic responses.
The fourth chapter evaluates whether a brief early-life heat exposure (20 °C for 96 h) generates persistent phenotypic or epigenetic effects. After a year of standard rearing at 12 °C, no significant differences in growth, or acute heat-challenge responses were detected between thermally imprinted and control juveniles. However, EM-seq identified subtle but consistent DMRs between groups, indicating that even short exposures can leave detectable epigenetic marks despite limited downstream physiological divergence.
Together, these chapters demonstrate that temperature shapes Arctic Grayling biology across developmental scales: influencing growth trajectories, inducing chronic heat-specific epigenetic signatures, and leaving modest but measurable early-life methylation marks. By integrating physiology, ecology, and epigenomics, this dissertation advances our understanding of thermal tolerance and plasticity in a cold-water salmonid of conservation concern under accelerating climate change.