Inferences on genome-wide deleterious mutation rates in inbred populations of Drosophila and mice

A theoretical analysis was carried out on the mutation load observed in long-maintained inbred lines from two experiments with Drosophila and mice. The rate of decline in fitness and its sampling distribution were predicted for both experiments using Monte Carlo simulation with a range of mutational parameters and models. The predicted rates of change in fitness were compared to the empirical observed rates, which were close to zero. The classical hypothesis of many deleterious mutations (about one event per genome per generation) of small effect (1-2%) resulting in a mutation pressure for fitness of about 1% per generation is incompatible with the data. Recent estimates suggesting an overall mutation pressure for fitness traits of about 0.1% are, however, compatible with the observed load.