THE RELATION BETWEEN X‐RAY DOSAGE AND THE FREQUENCY OF CHROMOSOMAL ABERRATIONS

THE RELATION between X-ray dosage and the frequencies of various types of chromosomal aberrations has provided evidence regarding the mechanism involved in the production of these induced structural changes in the chromosomes. Chromosomal aberrations which involve two chromosomes or two loci in the same chromosome tend to increase in frequency as the square of the X-ray dose, while simple deletions resulting from a single break tend to increase in direct proportion to the X-ray dose. But this relation between X-ray dose and the frequency of the more complex aberrations is dependent upon the radiation time-intensity factor (Sax, 1941a; Marinelli et al., 1942), and it does not seem to apply to the meiotic and root tip chromosomes of Allium and Vicia (Marshak and Malloch, 1942). Certain discrepancies have also been reported in analyses of the chromosomes of Tradescantia microspores (Faberge, 1940; Newcombe, 1942; Thoday, 1942). Dosage experiments have been continued with Tradescantia microspores and with Vicia root tips in order to determine the basis for some of the apparent discrepancies in the results of various investigators. Certainly one of the factors involved is measurements of X-ray dosage, although this factor is of no great importance in the study of dosage curves so long as the dosimetry is consistent. Some of our earlier measures of dosage were based upon time of exposure and distance from the X-ray target, and due either to errors in original calibrations when the X-ray machine was installed, or to decreased output of the tube with age, the estimated doses were too low by a factor of about 10 per cent. Later a Victoreen dosimeter was used for most of the experiments, and during the past eighteen months, we have used a dosimeter built by Mr. Cloud of The Massachusetts Institute of Technology. This dosimeter is calibrated in r-units and can be read in the control chamber during irradiation. It has been checked with a Victoreen dosimeter at Massachusetts Institute of Technology and by another Victoreen dosimeter from the Harvard physics department, with identical results. Dosage curves derived from microspore chromosomes of Tradescantia have been obtained intermittently for about a year to determine possible seasonal effects and the range of uncontrolled variation. All of this work has been done with a single clone of Tradescantia paludosa, a diploid species. The types of aberrations have been described in earlier papers (Sax, 1940, 1941b). Of the types of aberrations induced in the resting stage, onlv the 1 Received for publication April 9, 1943. 2 National Research Fellow. This work was supported, in part, by a grant from the International Cancer Research Foundation. dicentric chromosomes and centric ring chromosomes have been included. Both types are attributed to the breaks and illegitimate fusion of broken ends of different chromosomes or chromosome arms, and are referred to as two-hit aberrations. Interstitial deletions cannot be scored with the same degree of accuracy, and terminal deletions are too infrequent to be of much statistical value. Irradiation of prophase nuclei results in chromatid dicentrics, and reciprocal translocations involving two chromosomes, and occasional chromatid rings, all of which are classed as two-hit aberrations. Deletion of both chromatids, followed by the fusion of adjacent broken ends, produces a chromatid bridge and a U-shaped fragment which are referred to as onehit chromatid aberrations. Deletions of only one of the chromatids do occur, but their frequency is low in our material, contrary to the experience of Lea and Catcheside (1942), and we have not scored them. Both the absolute frequency of X-ray-induced two-hit chromosomal aberrations, and the relative frequency at various doses, are known to depend to a considerable extent on the time-intensity factor. If the irradiation is given at low intensities, not only are the number of aberrations fewer than at high intensities, but the shape of the dosage-aberration curve is changed. At high intensities the frequency of two-hit aberrations tends to increase as the square of the dose, but at very low intensities the relation becomes nearly liinear (Sax, 1941a). This X-ray intensity factor is of minor importance when the intensity is greater than 50 roentgens per minute. A series of dosage curves was obtained at 70 r/m and is compared with the results of other investigators as shown in table 1. The first six experiments were done at Harvard University over a period of about a year. The microspores were fixed five days after irradiation. Five or six slides were made for each X-ray treatment, and 100 metaphase or anaphase figures were examined for the lowest dosage treatment, 50 for the next, and 25 for the highest dose. Such a method is a compromise between practicable cytological technique and statistical adequacv, but it has given reasonably consistent results (cf. Marinelli et al, 1942). The exponents for the dosage curve equations were obtained by the method used by Newcombe (1942), log effect 2/log effect 1)/(log dose 2/log dose 1). The dosage curve equations in our own experiments vary from (D/K) ' 6 to (D/K) 20 5, and the value of the constant K also varies considerably in different experiments. The average exponent for the six experiments is 1.87 where radiation intensity is 70 r/m. Nebel (Marinelli et al, 1942) gets an exponent of 1.65 at an intensity of 160 r/m and