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 i n long-maintained inbred lines from two experiments with Drosophila an d mice. The rate of decline in fitness and its sampling distribution w ere predicted for both experiments using Monte Carlo simulation with a range of mutational parameters and models. The predicted rates of cha nge in fitness were compared to the empirical observed rates, which we re close to zero. The classical hypothesis of many deleterious mutatio ns (about one event per genome per generation) of small effect (1-2%) resulting in a mutation pressure for fitness of about 1% per generatio n is incompatible with the data. Recent estimates suggesting an overal l mutation pressure for fitness traits of about 0.1% are, however, com patible with the observed load.