Evolution and mechanisms of thermal tolerance in fish

Evolution and mechanisms of thermal tolerance Thermal biology is the study of how temperature shapes organisms and ecosystems. It has successfully explained many features of nature, however, two of the most central questions of thermal biology have remained unresolved: 1. What limits the tolerance to high temperatures in cold blooded animals such as fish? 2. How rapidly can tolerance to high temperatures adapt in fish through evolution? These issues are important for our fundamental understanding of physiological mechanisms, as well as biogeography. Furthermore, our limited understanding of thermal biology is hampering predictions of how animals will respond to climate change. The aim of this project is to identify the limiting physiological mechanisms that underlie tolerance to high temperatures, and to estimate the evolutionary rate of thermal tolerance. This will be accomplished through artificial selection experiments in zebrafish (Danio rerio). We will expose zebrafish to a range of different thermal treatments, and select based on their thermal tolerance. This will be repeated over several generations, allowing us to measure the rate of evolutionary adaptation to high temperature. The physiological and genetic changes that result from the selective breeding will be measured by continuously monitoring the fish. This will show us how rapidly fish can adapt to a warming climate. We will also be able to investigate which critical physiological and genetic changes are needed for evolution of thermal tolerance to occur. These insights will be useful for understanding why cold blooded animals are found where they are in nature (i.e., their biogeography), as well as for predicting if and how evolution might rescue animals from some of the impacts of climate change. We have now selected fish for seven generations and we see a clear evolutionary response where upper thermal tolerance is increasing. We show that the response is slow and asymmetrical. A major milestone on the project is the publication of the evolution experiment manuscript in the prestigious journal PNAS. The maximum increase in thermal tolerance through evolution appears to be slower than the warming predicted under a business-as-usual climate change scenario. In addition, there appears to be a ceiling to the combined effects of acclimation and adaptation of warming tolerance. This means evolution is unlikely to rescue many fish populations from the harmful effects of warming. The project also revealed physiological mechanism limiting heat tolerance in fish. The brain, and specifically oxygenation of the brain, was found to be the most thermally sensitive function. The project has been extremely successful and has led to many important papers on the impacts of warming on fish.

From this project, we have learned that a major mechanism limiting thermal tolerance in fish is brain oxygenation. This is already having a major impact on research in the field. We also discovered that there is limited potential for evolutionary rescue to climate change impacts in fish, and this will in addition to scientific impact also influence how we manage fish stocks facing climate change. It also means that is is even more urgent to reduce carbon emissions and fossil fuel extraction.

Thermal biology is the study of how temperature shapes organisms and ecosystems. It has successfully explained many features of nature. However, two of the most central questions of thermal biology remain unresolved: 1. What limits the thermal tolerance of ectothermic animals? 2. How rapid is the evolution of thermal tolerance in a population? These issues are important for our fundamental understanding of physiological mechanisms as well as biogeography. Furthermore, our poor understanding of thermal biology is hampering predictions of how animals may respond to climate change through physiological plasticity, adaptation, relocation or reduced performance. Aims: - Identify the limiting physiological mechanisms underlying thermal tolerance - Estimate the heritability and evolutionary rate of upper thermal tolerance This will be done through artificial selection experiments in zebrafish (Danio rerio), and subsequent investigations of the thermal physiology of the adapted populations.