A Trade-off Between Developmental Speed and Replication Fidelity Governs the Spontaneous Mutation Rate

The spontaneous mutation rate (μ) is shaped by two potentially opposing forces: the passage of chronological time, which is inevitably associated with mutation accumulation, and the speed of development, which may compromise replication fidelity. Disentangling these forces has been a major challenge, particularly in ectotherms. Testing the competing predictions of the classic time-dependent Generation Length Hypothesis and the replication-dependent speed-fidelity trade-off model, we experimentally assessed individuals with short (mean 15 days) and long (mean 37.7 days) generation time (GT) in the midge Chironomus riparius under constant temperature and estimated the de novo mutation rates by whole genome sequencing. We found that long-GT lines accumulated 1.3-fold more mutations per generation (μ/gen), consistent with time-dependent mutagenic processes. Conversely, short-GT lines exhibited a nearly two-fold higher mutation rate per day (μ/day) and a trend towards a transition-biased mutational spectrum (Ts/Tv ratio = 1.28 vs. 0.95), a pattern consistent with a speed-fidelity trade-off in DNA replication. These results suggest that two distinct processes shaped the overall mutation rate. Integrating our data with previous studies and life-history data, we show that the daily mutation rate followed a non-linear relationship with respect to generation time, and that the species' generation time mode coincides with its minimum. This suggests that the generation time is, amongst other factors, selected to optimise the mutational load by balancing between replication accuracy and developmental speed.