Abstract
1. The number of abdominal bristles of D. melanogaster was selected in both high and low directions. The response of hybrid strains (Oregon-R×Samarkand) was generally higher than that of pure strains (Oregon-R), presumably because the former had higher genetic variability at the start of the experiment.
2. Genetic variability, probably originating from mutation and recombination in polygenic systems, was induced by the irradiation, and contributed to the effectiveness of artificial selection. From the variance analysis, the grade of response of each lot was ranked as follows: both sexes treated=only males treated>only females treated> both sexes untreated. The possible difference in effectiveness between induced mutation and induced recombination is discussed.
3. The mutation rates induced by X-rays, in terms of increments of variance, have been estimated from the selection responses. They are 28.1×10-5 (both sexes treated), 10.7×10-5 (only females treated) and 20.3×10-5 (only males treated) per unit dose, respectively.
