Modification of the Frequency of Chromosomal Re- Arrangements Induced by X-rays I N Drosophila

H E two preceding papers of this series have described the effects of near T infrared radiation (A ca. 10,000 h;) and of ultraviolet radiation (A 2,537 8) in modifying the frequency of chromosomal rearrangements induced by X-ray treatment of the spermatozoa of Drosophila naelanogaster (KAUFMANN, HOLLAENDER, and GAY 1946; KAUFMANN and HOLLAENDER 1946). In the course of the first series of experiments, females that had been inseminated by Xrayed males deposited eggs during the period of exposure to the near infrared radiation. Larvae developing from these eggs provided material for determining the effects of such exposure on chromosome recombination. In order to compare the effect of temperatures within the range customarily used in raising laboratory cultures of Drosophila, other groups of females inseminated by males receiving identical X-ray treatment were kept a t either 18°C or 28°C during the period of fertilization and early cleavage. Drosophila provides suitable material for measuring the influence of temperature and other conditions on chromosome recombination apart from any effect on the initial production of the breaks by X-rays. According to the interpretation of MULLER (1940) and KAUFMANN (1940, 1g41), based on studies of fractionation of the X-ray dose, as well as collateral lines of evidence, the potential breaks induced by irradiation of the mature spermatozoa of the adult male are not utilized in forming new combinations of chromosomes until after the sperm nucleus has penetrated the egg in fertilization. Thus, in the experiments here reported, to measure the effect of temperature on chromosome recombination, several males were treated with a 4,000-roentgen dose of X-rays, mated with virgin females of the same stock (Oregon-R), and kept a t 18°C for about 36 hours. The males were then discarded, and the inseminated females divided into three groups, one of which was put into an 18-degree incubator, another into a &degree incubator, and the third exposed to near infrared radiation during the period of oviposition. Subsequently the eggs were transferred to the 18-degree cold room for completion of the embryonic and larval development. Eggs were collected a t approximately 12-hour intervals; no effort was made to handle them individually SO as to determine accurately the termination of the period of syngamy and early cleavage during which chromosome recombination is presumably effected. Thus, varying proportions of the earlier embryonic development in individuals of two of the three groups were completed either a t 2 8 degrees or while exposed to the near infrared radiation. Determination of the fre-