Divergent Thermal Reaction Norms of the Spontaneous Mutation Rate Among Populations of Chironomus riparius

The germline mutation rate {micro} 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 {micro} evolves in response to local thermal regimes. 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 more 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 and optimises the mutational load in the respective thermal regimes. Furthermore, population-specific mutational spectra (Ts/Tv ratios) indicated evolved differences in DNA repair machinery. However, this is only partially mirrored by Reactive Oxygen Species (ROS) dynamics, where Mediterranean larvae maintain lower ROS levels and a buffered response to thermal extremes. These findings provide evidence for evolution of the mutation rate itself, challenging the assumption of constancy.