Physiological plasticity and evolution of thermal performance in zebrafish
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- Institutt for biologi 
Climate change is increasing mean and extreme temperatures globally. For aquatic ectotherms, such as fish, increasing temperatures will directly affect their thermalperformance and challenge their upper thermal limits. Whether ectotherms willcope will largely depend on whether they are able to acclimate or adapt quicklyenough to these rising temperatures and extreme warming events. In this thesis I investigated the extent to which fish can respond to these changes through both physiological plasticity and evolution of their thermal performance, specifically focusing on upper thermal tolerance. Zebrafish are a commonly used model organism and an ideal candidate species for addressing climate change related questions. However, very little is known about their natural habitat, or how representative laboratory zebrafish are of wild populations. We filled some of these knowledge gaps through fieldwork in India, where zebrafish are native. We described their habitats and measured the upper thermal tolerance of zebrafish in the wild. We showed that zebrafish can be found very close to their upper thermal limit, highlighting the relevance of using them for climate change research. By comparing wild-caught and laboratory populations we showed that lab fish have reduced physiological plasticity compared to their wild counterparts, showing they are less able to counter direct thermal effects, particularly at thermal extremes. We relate these differences to their thermal history since lab zebrafish have been held at stable and optimal temperatures, whereas wild zebrafish experience fluctuating and more extreme temperatures. These results suggest that plasticity can rapidly evolve, allowing individuals to optimise their phenotype to the environment they are in. To investigate whether upper thermal tolerance can evolve in zebrafish, we first estimated the repeatability of the trait within individuals and found that it was repeatable. Using wild-caught zebrafish, we then artificially selected for seven generations for increased and decreased upper thermal tolerance. We also conducted selection to increase tolerance after warm acclimation. Evolution to increase and decrease upper thermal tolerance occurred but was asymmetrical, with a weaker response to increase. Warm acclimation increased upper thermal tolerance, but the acclimated lines showed very little response to selection. Overall the results from this thesis suggest that whilst fish may be able to respond to climate change in the short term, through physiological plasticity, they may struggle to adapt their upper thermal limits rapidly enough to cope with climate change in the long term.