THE DISTRIBUTION OF MUTATION EFFECTS ON VIABILITY IN DROSOPHILA-MELANOGASTER

Parameters of continuous distributions of effects and rates of spontan eous mutation for relative viability in Drosophila are estimated by ma ximum likelihood from data of two published experiments on accumulatio n of mutations on protected second chromosomes. A model of equal mutan t effects gives a poor fit to the data of the two experiments; higher likelihoods are obtained with leptokurtic distributions or for models in which there is more than one class of mutation effect. Minimum esti mates of mutation rates (events per generation) at polygenes affecting viability on chromosome 2 are 0.14 and 0.068, but estimates are stron gly confounded with other parameters in the model. Separate informatio n on rates of molecular divergence between Drosophila species and from rates of movement of transposable elements is used to infer the overa ll genomic mutation rate in Drosophila, and the viability data are ana lyzed with mutation rate as a known parameter. If, for example, a muta tion rate for chromosome 2 of 0.4 is assumed, maximum likelihood estim ates of mean mutant effect on relative viability are 0.4% and 1%, but the majority of mutations have very much smaller effects than these va lues as distributions are highly leptokurtic. The methodology is appli ed to estimate viability effects of single P element insertional mutat ions. The mean effect per insertion is found to be higher, and their d istribution is found to be less leptokurtic than for spontaneous mutat ions. The equilibrium genetic variance of viability predicted by a mut ation-selection balance model with parameters estimated from the mutat ion accumulation experiments is similar to laboratory estimates of gen etic variance of viability from natural populations of Drosophila.