Pfenninger, M.; Nieto Blazquez, M. E.; Bulut, B.

The germline mutation rate is a fundamental evolutionary parameter, yet its plasticity in response to environmental factors, particularly temperature, remains poorly understood. While often modeled as a species-specific constant, we tested whether evolves in response to local climatic conditions. Using whole-genome sequencing of mutation accumulation lines in the non-biting midge Chironomus riparius, we demonstrate divergent thermal reaction norms between populations from climatically distinct regions: Central Europe (Germany) and the Mediterranean (Spain). The Central European population displays a highly plastic, U-shaped reaction norm, whereas the Mediterranean population exhibits a canalized, temperature-insensitive response. This divergence conforms to theoretical expectations: the higher thermal variance of high-latitude habitats selects for plasticity, while thermally more stable Mediterranean habitats favour robustness. Mechanistically, this is mirrored by Reactive Oxygen Species (ROS) dynamics, where Mediterranean larvae maintain lower ROS levels and a buffered response to thermal extremes. Furthermore, population-specific mutational spectra (Ts/Tv ratios) indicated evolved differences in DNA repair machinery. These findings provide evidence for local adaptation of the mutation rate itself, challenging the assumption of constancy in molecular dating and demographic inference. Consequently, evolutionary models must integrate environmental context and population-specific reaction norms, particularly when forecasting responses to climate change.