Physiological and Cellular Responses Used By Lake Whitfish (Coregonus Clupeaformis) in Response to Thermal and Hypoxic Stress at Different Points in Development

Abstract

Early life stages of fish are particularly susceptible to environmental change and even brief exposures to thermal or hypoxic stress can have detrimental effects. Fish may be able to respond to stress, using physiological and cellular mechanisms, to mitigate some of the damaging effects of stressor exposure and maintain homeostasis. I investigated the development of the hypothalamus-pituitary-interrenal axis (HPI-axis) and two key cellular stress responses, the heat shock response (HSR) and hypoxia response, in lake whitefish (Coregonus clupeaformis) at several life-stages, to understand when these responses could be activated in response to stress. The HSR and HR are mediated by heat shock proteins (Hsps) and hypoxia-inducible factor 1α (Hif-1α), respectively. The results from this thesis show that lake whitefish can respond to environmental fluctuations with the activation of the HPI-axis and both cellular stress responses. Increases in the mRNA levels of multiple hsps were observed in response to both thermal and hypoxic stress from the earliest ages studied, 15 and 21 days post fertilisation (dpf), respectively, and was the only response I observed at these ages. Further, increases in hsp70 were observed at all embryonic ages studied in response to both stressors, suggesting that the HSR plays a fundamental role in the response to stress in lake whitefish embryos. At 38 dpf, the HPI-axis could be activated in response to hypoxia, resulting in increased whole-embryo cortisol levels. This is the first time that this has been observed during early embryogenesis in a teleost fish and was not observed at any other embryonic ages studied. It was also at 38 dpf that the activation of the hypoxia-inducible factor pathway was first observed, with an increase in the mRNA levels of multiple Hif-1 responsive genes. In response to hypoxia, the ability to induce a cellular response persisted through to 2 weeks post hatch (wph), although this was age and gene specific. In contrast, by 3 and 4 wph, mRNA levels of several genes, including hsp70, and whole body cortisol levels were below those in controls, suggesting that at these ages larval lake whitefish were either unable to initiate a stress response or the response could not be sustained for the duration of the 6-hour treatment. Taken together, these data suggest that sensitive windows exist in lake whitefish development and that at these ages developing fish may be more vulnerable to stress exposure. Lastly, I show that young of the year (YOY) juvenile lake whitefish at 18 wph, can respond to acute thermal stress, but not hypoxia, with strong increases in hsp mRNA. Hypoxia exposure resulted in the up-regulation of multiple genes that are commonly induced by hypoxia in other organisms. Exposure to thermal and hypoxic stress simultaneously caused increases in hsps and hypoxia inducible genes. The more severe multi-stressor treatments resulted in some mortality which suggests that these stressors act synergistically or additively, highlighting the value in studying multi-stressor scenarios which are more indicative of what is occurring in natural systems. Overall, this thesis demonstrates that the physiological and cellular response of developing lake whitefish to temperature and hypoxia vary with age, severity and type of stressor and contributes to our understanding of the nature of the stress responses developing fish use when exposed to changes in their environment.