Early life exposure to diel thermal variation alters microRNA expression and performance in zebrafish.

Abstract

Freshwater ecosystems are characterized by large thermal variations, especially in the summer months. However, this variability is rarely accounted for in laboratory studies examining physiological processes of fish inhabiting these environments, thereby producing results which lack ecological realism. Understanding how animals respond to and cope with rapid changes in temperature is more crucial than ever as the range of thermal fluctuations in these habitats is expected to increase dramatically due to increases in the frequency and severity of heat waves. Recently, it was posited that epigenetic mechanisms could buffer fish against such thermal fluctuations as they act on a more rapid timescale than genetic adaptation. Thus, the aim of this thesis was to understand how chronic exposure to thermal variability impacts the physiology of freshwater fish, and if these effects are associated with changes in epigenetic modulators called microRNAs (miRNAs) which act by repressing the translation of target genes. This was achieved by subjecting zebrafish (Danio rerio) to realistic diel thermal fluctuations (FLUX; 28 ± 5°C) throughout the embryonic and larval stages to evaluate the effects on physiological processes like metabolism and survival, and on the expression of seven thermosensitive miRNAs and three heat shock proteins (HSPs). These fish were compared to those kept at constant control (CTRL; 28°C) and constant elevated (HEAT; 33°C) conditions to allow for comparisons with fish reared under optimal lab conditions and in traditional climate change experiments, respectively. After the developmental stages, the fish from CTRL and FLUX treatments were reared under common control conditions until adulthood to understand if developmental exposure to fluctuations altered miRNA expression profiles in the brain and the upper thermal tolerance (CTmax). This study revealed that while the HEAT conditions reduced survival throughout development, the FLUX thermal regime had no impact on this. Additionally, both FLUX and HEAT conditions significantly altered body weight as well as the miRNA expression profiles during early life stages. Although juveniles from both FLUX and HEAT conditions were able to metabolically compensate to their respective thermal regimes, the degree of compensation was greater in FLUX fish. I found that miR-181a-5p, which regulates pathways associated with mitochondrial biogenesis and respiration, was significantly upregulated in the juveniles from these groups, suggesting that changes in this miRNA could modulate the observed metabolic impacts. Notably, this miRNA remained elevated in the brains of longitudinal adults with FLUX developmental histories, though their CTmax was unaffected. Besides its role in modulating mitochondrial function, insights from mammalian models suggest that this miRNA can also enhance neuronal injury/damage in vertebrate brains. This could indicate that FLUX fish have an altered capacity for recovery following acute thermal stress. Collectively, the findings presented in this thesis demonstrate that developmental plasticity may be regulated by changes in epigenetic processes and underscore the need to incorporate variability into our experiments, as it produces a robust and long-lasting impact on the physiology of fish that is distinct from static temperature exposures. Ultimately, ecologically realistic conditions and the plasticity potential of populations need to be accounted for in investigations to accurately predict how fish will respond to the multiple stressors caused by climate change.