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

ABSTRACT

The germline mutation rate μ is a fundamental evolutionary parameter, yet its plasticity in response to environmental factors, particularly temperature, remains poorly understood. While often modelled as a species‐specific constant, we tested whether μ 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 canalised, 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 optimise 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.