New estimates of the rates and effects of mildly deleterious mutation in Drosophila melanogaster

The genomic rate and distribution of effects of deleterious mutations are i mportant parameters in evolutionary theory. The most detailed information c omes from the work of Mukai and Ohnishi, who allowed mutations to accumulat e on Drosophila melanogaster second chromosomes, shielded from selection an d recombination by being maintained heterozygous in males. Averaged over st udies, the estimated rate of nonlethal viability mutations per second chrom osome per generation under an equal-effects model, U-BM, was 0.12, suggesti ng a high genomic mutation rate. We have performed a mutation-accumulation experiment similar to those of Mukai and Ohnishi, except that three large h omozygous control populations were maintained. Egg-to-adult viability of 72 nonlethal mutation-accumulation (MA) lines and the controls was assayed af ter 27-33 generations of mutation accumulation. The rate of decline in mean viability was significantly lower than observed by Mukai, and the rate of increase in among-line variance was significantly higher, Our U-BM estimate of 0.02 is much lower than the previous estimates. Our results suggest tha t the rate of mutations that detectably reduce viability may not be much gr eater than the lethal mutation rate (0.01 in these lines), but the results also are consistent with models that include many mutations with very small effects.