The Artificial Production of Mutations by Ultra-Violet Light

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Abstract
Ultra-violet light produced mutations in Drosophila, especially evident if the developing eggs were treated at the polar-cap stage, at which time the germ tract cells are close to the surface of the egg and readily accessible to the u.-v. light. Mutations induced at the polar-cap stage can be identified as having been produced by the agent from the fact that they are multiplied or "reduplicated" in the course of development. They therefore appear in "bundles" among the mature reproductive cells. A polar cap at a medium stage of its development contains about 10 cells, so that each one gives rise to about 10% of the total reproductive cells of the mature fly. The fully developed polar cap contains about 20 cells, and mutations taking place at this stage appear in about 5% of the mature germ cells. Only mutations reduplicated enough to form at least 5% of the total mature germ cells were regarded as having arisen during the polar cap stage. The mutations looked for were lethals in the x-chromosome, and they were detected by the Cl B method. To detect "reduplications," at least 50 F1 daughters per treated [male] were reared as a rule (the [male][male] having been treated when in the polar-cap stage). The composition of the F1 [female][female] was [image]. The source of the u.-v. light was a quartz mercury arc lamp, used as a rule at a distance of 1 meter and at 45-50 volts. Two series of egg experiments were performed. The earlier series control eggs were not protected against possible stray u.-v. light; in the 2nd series precautions were taken to protect the control eggs against stray reflected light. In the 1st series 122 eggs were treated, and 6 reduplicated lethals produced. In the controls, there were 108 eggs and 1 reduplicated lethal. In the 2nd series, 117 eggs were treated and 7 reduplicated lethals produced. In the controls, there were 114 eggs and no reduplicated lethals. Using the 2nd series only, as the more accurate, the increase in the mutation rate induced by the u.-v. is arrived at as follows: Since 117 eggs were treated and since there were about 10 cells per polar cap on the average at the time of treatment, there were about 1170 cells under treatment (in the 2nd series). Among these 1170 cells, 7 mutations occurred, or 1 in 167 cells contained a mutation. Since the polar cap stage lasts for only 1 hour, this gives a mutation rate of 1 cell per 167 per hour. The "natural" mutation rate in the present experiments was about 1 mutation in 800 cells over the 15-day reproductive life cycle (360 hours), or 1 mutation per 288,000 cells per hour. In the dosage used, u.-v. therefore increased the mutation rate 1700 times that of the natural rate.

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