Mutation frequency as conditioned by the manner of application of radiation

Abstract

(I) THE INEFFECTIVENESS OF TEMPERATURE IN INFLUENCING THE PRODUCTION OF MUTATIONS BY X -RAYS. (with 2 Tables and 2 Diagrams)

A study has been made of the effect of temperature during irradiation on the frequency of X -ray induced lethal mutations, and of trans - locations between chromosomes X, II and III. The flies were X -rayed simultaneously and under otherwise identical conditions, at the temperatures 4° to 6 °C and 36° to 38° C, resp ectively. No significant difference was obtained in the two groups. This is regarded as proving that the temperature coefficient for lethal mutations and translocations during X- raying is very nearly 1. Hence the completion of the secondary chemical processes, leading from the initial ionisations to the observed mutations, must be very little affected by temperature.

(II) TRANSLOCATION FREQUENCY IN RELATION TO TIMING OF IRRADIATION (With 5 TABLES):

Experiments on Drosophila melanogaster planned with a view to determining the possible effect of differences in the time factor on the production of translocations by X -rays in spermatozoa clearly show that even long intervals between and after the exposures do not alter the frequency of chromosome re-arrangements, provided the total dose of X -rays, as measured either by ionisation or by the production of sex-linked lethal mutations, remains the same.

Three groups were compared in the tests of the frequency of translocations. In one, the exposure of the inseminated females was continuous an the flies, after treatment, were allowed to deposit their eggs at once. The second group was irradiated similarly and simultaneously, but the deposition of eggs was postponed for one month, by keeping the flies at 8°C on syrup food. In the third series the dosage was divided into four equal fractions, administered at weekly intervals, and the flies were bred a week after the last irradiation, i.e., one month after the first irradiation. The amount of dosage applied in the three cases was the same, viz. ca. 1500 r. The results showed no significant differences in the frequencies of translocations.

Series to test translocations at a very low dose (ca. 375 r), which was one-fourth of the above, were run simultaneously with those mentioned, above. The results of these tests, combined with those of a parallel experiment by Raychaudhuri, showed that, for the range of dosage between 375 r and 1500 r, the number of translocations is proportional to a power o the dosage which is certainly well above 1, and is probably nearer to 2 than. to 1.

Sex-linked lethals also were tested simultaneously in all. these cases, and the results obtained showed that the differences in timing of the treatment and breeding caused no significant differences in their frequencies. As this was expected on the basis of previous work, these results also served as a check on the dosage given for translocations.

On the basis of the frequency-dosage relations here found, especially when these are taken in connection with the results of other experiments, it is concluded that radiation produces gross rearrangements by first breaking the chromosomes in two or more places by means of separate ionisations, and that the broken ends later undergo recombination.

On the basis of the finding that the results are independent of timing it is further concluded that the broken ends cannot rejoin so long as they are included within the free spermatozoon, but that after fertilisation some change in conditions make union of the pieces possible.