The Relation of Neutron Dose to Chromosome Changes and Point Mutations in Drosophila. I. Translocations

Abstract

1. The frequency of translocations between the Y, the second and the third chromosomes, induced by neutron irradiation of mature Drosophila spermatozoa (ejaculated within six days after irradiation), varies in linear manner with the dose of radiation over the range studied, extending from a frequency of 2 per cent to about 10 per cent for translocations connecting the second and third chromosomes. That this is not a mere appearance of linearity, resulting from the differential production of dominant lethals in sperm of heterogeneous mutability, is made clear by the third series of experiments, in which the curve did not rise above the linear, over a four-fold range of dose, when sperm ejaculated within about one day after the irradiation of very young males, and therefore of comparatively high homogeneity, were used. 2. Since there is evidence that at the higher doses used a large proportion of the spermatozoan nuclei was traversed by more than one proton track, the linear frequency-dosage relation must mean that in this material (as in Giles' studies on Tradescantia microspores) the chromosome breaks are produced in the immediate vicinity-within a distance of the order of a micron or less-of the activations which instigate them, and that the breakage-ends of the chromosomes unite preferentially with those breakage-ends which had been produced within this distance. It follows as a corollary that the chromosomes overlap in their positions along the length of the sperm head, as has also been indicated on cytological grounds in a parallel study, by Herskowitz and Muller. 3. Translocations of the sc8.Y chromosome which allow fertility in the male when the X has the short arm of a Y attached to it are produced with a frequency about .8 as high as that of translocations between the second and third chromosomes. The frequency of the former is probably slightly less than that of all translocations (including those resulting in male sterility) which would be undergone by an ordinary Y chromosome, and it is about twice the frequency observed in our first two series of experiments for translocations of an ordinary Y which are fertile in the male and which at the same time do not give rise to viable aneuploid recombinants. Translocations connecting the sc8.Y with the third chromosome were found nearly twice as frequently as those connecting it with the second chromosome. 4. Translocations simultaneously connecting all three of the chromosomes studied (Y, II, and III) are produced with higher frequency than is explicable by the accidental concatenation of two or more separately produced translocations. The excess is probably composed in large measure of multiple, rotational exchanges. This excessive "coincidence" indicates that the overlapping of chromosome threads along the length of the sperm head is of multiple nature. 5. No significant differences were found between the frequencies of translocations obtained, per n unit of dose, with neutrons from the Oak Ridge National Laboratory pile and with those from the ORNL cyclotron. In both cases, there was a consistent production of translocations between the second and third chromosomes of 68 (±3) x 10-6 per rep. The value as calculated from the data published by Dempster would be approximately 27 x 10-6/rep, while the data of Catsch et al. would give a value of 8 x 10-6/rep. At a level of doses yielding a 10 per cent frequency of these translocations, neutrons as observed in the present experiments are 21/2 times as efficient as X-rays in their production of translocations; with lower doses their efficiency in this respect, relative to that of X-rays, would rise, until that very low level of dosage was reached below which, according to theory, the production of translocations by X-rays also became linear.