Effects of x rays on genetic loads in a aage population of Drosophila melanogaster.

N assessing the hereditary effects of radiation on human populations, it is very I important to know the population dynamics of mutant genes induced by radiation and their effect on population fitness. It is still controversial whether radiation-induced mutations are deleterious in the heterozygous condition, though they appear to be mostly deleterious in the homozygous condition. A pioneering experiment on this problem was conducted by WALLACE (1 956; see also WALLACE and KING 1951) with Drosophila melanogaster. He examined the changes in frequency of lethal second chromosomes in populations, either acutely or chronically irradiated with gamma rays, and found that the frequency of lethal chromosomes increases in successive generations when the populations are chronically irradiated. Examining the viability of lethal heterozygotes, he later (1963) concluded that shortly after irradiation started, lethal genes showed a deleterious effect on the heterozygote viability but later developed an enhancing effect on viability, i.e., overdominance. CARSON (1963) also noted a transitory increase in population size and egg-to-adult viability in Drosophila populations which had received 1,000r of X rays per week over a period of two years. On the other hand, PROUT ( 1954), TORROJA ( 1964) and SANKARANARAYANAN ( 1964) reported that radiation-induced mutations are more or less deleterious in the heterozygous condition. Mouse data obtained by LUNING and SHERIDAN (1964) indicate that radiation does not necessarily decrease the population fitness. In view of this circumstance, a new type of experiment was initiated with Drosophila melanogaster in order to obtain more penetrating information on this problem. This experiment is featured in examining how the so-called random and inbreeding loads change under continuous irradiation. From the rate of increase in these genetic loads, we can estimate the heterozygous effect of mutant genes. As a special type of inbreeding load, the frequency of lethal second chromosomes has also been determined. As will be seen in the following, the results obtained show that mutations are on the average slightly deleterious in the heterozygous condition.