Section 5 — Mutagenesis 



to ionizing radiation have been recognized, 

 conflicting estimates of the relative amounts of 

 damage have been reported by some investi- 

 gators. Fortunately, it is becoming in- 

 creasingly apparent that such discrepancies 

 are in large measure, if not entirely, due to the 

 siphoning off of cell samples representing 

 different stages of development (and therefore 

 different intrinsic radiosensitivities) at the time 

 of treatment. Not only would such mixtures 

 result in false estimates of the relative radio- 

 sensitivity of the various stages but it is known 

 that many external and/or internal factors 1 are 

 capable of altering the degree of heterogeneity 

 of any particular sample of cells which is being 

 analyzed. The extent to which this occurs is 

 often exceedingly variable even within the same 

 experiment. 



By utilizing a combination of techniques we 

 have been able to avoid these difficulties and 

 have made cytogenetical analyses of cells repre- 

 senting homogeneous stages of development at 

 the time of treatment. These methods include 

 the use of our so-called multi-purpose stocks 

 (i.e. strains which allow for the detection of 

 several different types of genetical damage in the 

 offspring of the same treated individual), his- 

 tological examination of the germ tract of the 

 treated individuals, and a sampling technique 

 that involves the treatment of immature stages 

 which contain a preponderance of germ cells in 

 a similar stage of development. The clear-cut 

 pattern of germ cell radiosensitivity as regards 

 various types of genetical damage which has 

 emerged for Drosophila is in general agreement 

 with that obtained more recently for the mouse 

 by Dr. L. B. Russell. 



This parallelism suggests that the mecha- 

 nism(s) underlying differential radiosensitivity 

 may be quite widespread. 



In addition to a comprehensive report of the 

 results obtained with radiation, comparisons 

 will be made with the data obtained by treating 

 the reproductive cells with chemical mutagens. 



This work has been supported by grant 

 AT (30-l)-2618 from the U.S. Atomic Energy 

 Commission. 



1. E.g. C. Auerbach, Sensitivity of the Droso- 

 phila testis to the mutagenic action of X-rays. 

 Z. indakt. Abstamm.- u. Vererb.-Lehre 86, 

 113 (1954). 



5.38. The Rates of Visible Mutations in Sequential 

 Broods of Irradiated Drosophila melanogaster 

 Males. Aloha Hannah-Alava (Turku, Fin- 

 land). 



The brood pattern of recovered mutations was 

 determined for three different types of visibles- 

 dominants and Minutes in all chromosomes 

 and recessives at specific third-chromosome 

 loci --in the progeny of D. melanogaster males 

 of two genotypes: wild-type, Oregon-RS, males 

 (Exp. I) and males heterozygous for 12 third- 

 chromosome recessive markers (Exp. II), mated 

 singly and sequentially to females of an appropri- 

 ate genotype for detecting mutations at specific 

 loci and/or crossovers. The males in Exp. I 

 were 2-24 hr of age and in Exp. II 24-48 hr of 

 age at the time of treatment (3000r hard X-rays) 

 and initial mating. The sequential broods 

 derived from these males, mated up to 24 days, 

 totaled 94,081 and 42,951 Fi offspring for the 

 irradiated and control series respectively. 



Except for the first brood (1-3 days) in which 

 there was a lower incidence of mutations in the 

 progeny of the older males than the younger 

 males, the curves of induced mutational response 

 were not significantly different in the two 

 experiments. In the broods following the period 

 of excessive sterility (6-8 days) 113 mutations 

 were recovered from 46,922 offspring of the 

 irradiated males. The slopes of the curves in the 

 premeiotic broods (11-24 days) suggest that one 

 of the consequences of radiation of the early 

 gonial stages is elimination of the germ cells 

 with potential visible mutations to a greater 

 extent than elimination of germ cells with either 

 induced crossovers or recessive lethals with no 

 visible phenotype. 



Research supported by IAEA contract No. 31 

 and USAEC contract No. AT (30-1) 2690. 



5.39. The Induction of XO Males, by Irradiation 

 of Drosophila. melanogaster Males. B. Leigh 



(Leyden, the Netherlands). 



It has been found that, after the irradiation of 

 adult males, there is a difference in the sperm 

 samples which show the highest frequencies of 

 lethals and translocations, and those which 

 show the highest frequency of XO males. A set 

 of experiments were carried out to investigate 

 this discrepancy. Twenty-four-hour old males, of 

 the genetic constitution X c2 yB; sc 8 Y, were 

 treated with doses from O-3000 r. A brood 

 pattern of five 2-day broods was used, with six 

 females per male per brood. The Fi was scored 

 for XO males, non-disjunction females, and 



68 



