Factors and processes shaping the temperature dependence of the point mutation rate

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Mutations introduce variation into the genomes of organisms and are thus the process without which evolution would not be possible. The mutation rate of a species has long been considered an environmentally independent constant. However, in a recent study, we were able to comprehensively show for the first time that environmental temperature actually strongly determines the mutation rate. There is an optimum temperature at which the mutation rate is lowest. Deviations in either direction cause the rate to increase. These results immediately raise several important questions: What are the processes behind the observed optimum curve? What processes drive the variation in mutation rate? Why does the mutation rate increase toward both extreme temperatures? What determines the position of the optimum of the mutation rate on the temperature scale? Is it a species-specific constant or does it evolve in response to local temperature conditions? With this project, I aim to understand the causal processes for the observed temperature dependence of the mutation rate and explore its evolutionary causes and consequences. Specifically, two different hypotheses will be tested: The observed increase in mutation rate to lower than optimal temperatures is due to generation length. Oxidative cold stress is responsible for the increase in mutation rate at lower temperatures. These hypotheses are tested by experimentally decoupling generation time from temperature. The thermal optimum of mutation rate differs between populations depending on the climate experienced in their evolution, thus it is locally adapted. Alternative: the thermal optimum of mutation rate is a species-specific constant. This hypothesis is tested in a common-garden design by determining the temperature dependence of the mutation rate of cold-adapted and warm-adapted populations.