The Genetic Effects of Radiation on the Longevity of Progeny in Drosophila melanogaster

1. In order to estimate the genetic effects of radiation on longevity, the life-span of the offspring from irradiated feameles and from irradiated males were measured . Controls were made at the same time. 2. The lives of the male offspring of these irradiated parents were significantly shorter than those of the controls. 3. The female offspring of treated parents lived as long as those from the control when they were kept together with males, while they lived significantly shorter periods than the controls when kept alone, without males. 4. The male offspring seemed to be more strongly affected by irradiation than the female offspring. 5. These results suggest that mutations which have an effect on longevity may be induced by X-rays, but no linear relationship between dose and effect was found . Many investigations have been done in connection with radiation effects on the longe vity of the irradiated animals themselves. However, there had been no study regarding the genetic effects of irradiation on the life-span of the progeny, before Russell (1957) first report ed the result obtained with mice. According to him, shortening of life was found in the offspring of male mice exposed to neutron radiation from an atomic bomb, with a linear relationship between dose and effect. In 1958, preliminary experiments with two strains of Drosophila melanogaster were carried out in our laboratory, giving the result that the off spring of females treated with X-rays of 900r were clearly shoter-lived than those of non treated females. The purposes of the present study are (1) to ascertain whether radiation would shorten the life-spans of offspring descended from irradiated parents, (2) to test for the existence of sexual differences with respect to the offspring affected as well as the parent treated, and (3) to see if there could be some differential effect on the longevity of flies depending on whether the two sexes were cultured together or separately. MATERIALS AND METHODS To study the problem of effects of radiation on the life-span of male and female off spring from irradiated parents, the Tokyo strain of D. melanogaster was used. Bilewicz (1953) found that in D. me-lanogaster, virgin females lived approximately twice as long as ma ted ones, and that unmated males also lived slightly longer than mated ones. These results seem to suggest that the mating behavior of the flies, especially the females, may bring about shortening of their own life-span. In D. subobscura, Smith (1958) also showed another in teresting fact: that ovariless females and virgin females lived significantly longer than nor mal mated females. This he interpreted as indicating that the shortening of life might be caused by egg-laying; that is, the life-span of females could be increased by reducing the rate of egg-laying by keeping them unmated. Taking account of these results, the present study was planned to consist of two series, Experiment I and Experiment II. Experiment I consisted of a control and two experimental series with five sets of expo sures (300r, 600r, 900r, 1200r and 1500r) in each. In the control, ten non-treated flies of each sex were mated and allowed to deposit eggs for 24 hours in each culture bottle with 20% yeast medium. Soon after emergence, the offspring were separated into 20 vials (3 cm X 10cm), in each of which 10 females and 10 males were placed together on a common corn meal-agar medium. The flies were transferred to vials with fresh medium every 2 or 3 days, and daily counts were made of the number of dead flies. In the experimental series, 2-3 day-old males were treated and immediately crossed with 4-5 day-old virgin females, without etherization; or 4-5 day-old virgin females were irradia ted and on the same day mated with 2-3 day-old males, without etherization. Ten pairs of flies per vial were mated in both sets of 300r and 600r as well as in the control, while 15 pairs were used in 900r and 25 pairs in 1200r and 1500r. Experiment II consisted of another control and two experimental series with three sets (900r, 1500r and 3000r) in each. The experimental procedures were almost the same as those of Experiment I, but the males and females to be examined for longevity were kept separate; that is, twenty males and twenty females were placed separately in viales with the common cornmeal-agar medium. The temperature was kept at 25°C throughout the experiments.