Section 5 — Mutagenesis 



F 1 and F 2 breeding. The experiments have been 

 repeated thrice. 



Sex-linked recessive lethals have been found 

 only in the families derived from the irradiated 

 medium-series (6.44 per cent). A wide range of 

 phenotypic changes was a striking feature of 

 the F2 families from the irradiated medium 

 cultures (their frequency ranging from 0.13- 

 0.37 per cent against 0.018-0.095 per cent in 

 control on population basis). Some of the 

 mutant flies, viz. curly wings, yellow body, and 

 another with dominant wing mutation were 

 found to breed true. A few changes like half 

 thorax, rotated abdomen and absence of neck 

 belong to the non-inherited group of abnormali- 

 ties of Morgan (Bridges and Sturtevant 1925) they 

 were observed only in the irradiated medium 

 series. The implications of these studies will be 

 discussed. 



5.35. Experiments with Chemical Compounds which 

 Reduce or Increase Mutagenic Effects of 

 Ionizing Radiations. H. A. Kunkel and 

 A. Rosener (Hamburg, Germany). 



This paper deals with investigations on three 

 chemical compounds well known as radiopro- 

 tecting agents with regard to somatic radiation 

 damage: cysteine, aminoethylisothiuronium 

 (AET) and 5-hydroxytryptamine (serotonin). 

 Experiments were carried out on Drosop/iila and 

 on E. coli. In Drosophila the rate of ray-induced 

 sex-linked recessive lethals was tested after X- 

 irradiation by means of the Muller-5-Method. 

 In the experiments on E. coli the rate of back- 

 mutations was observed of a leucine-dependent 

 and of a streptomycine-dependent strain to 

 prototrophy. Cysteine considerably reduced the 

 X-ray induced back-mutations in E. coli. No 

 influence, however, was observed in Drosophila. 

 Serotonin reduced the rate of mutations in 

 Drosophila as well as in E. coli. Protection, 

 however, was only small. After AET the ray- 

 induced rate of mutations significantly increased 

 although this compound per se has no mutagenic 

 effect. Lastly experiments are reported with 

 bromodeoxyuridine which is incorporated into 

 DNA instead of thymidine and provides in- 

 creased radiosensitivity in some cases. 



mutation process and in the investigation of the 

 essence of the natural biochemical protection 

 of the cell. 



The class of antimutagenic substances consists 

 so far of very few compounds. The need for 

 new antimutagens is great. For the solution of 

 this problem we have founded our investigations 

 on the following principles: 



1. The search for new antimutagens in the 

 group of substances of universal protection, 

 i.e. protecting the cell from possibly many various 

 effects. Kihlman has shown that streptomycin 

 protects against ultraviolet and y-rays and 

 a-particles. Our experiments with streptomycin 

 have shown that it is an antimutagen. 



2. Searches in the group of substances which, 

 being protective against radiation, are at the 

 same time a natural and an active cellular 

 metabolite. The aminoacid arginine, responding 

 to these requirements, was also found to be an 

 antimutagenic compound. 



The antimutagenic effect of streptomycin, 

 cysteamine and arginine has been proven on 

 onion roots and broad beans, which had been 

 protected by these substances from the occurrence 

 of spontaneous chromosome rearrangements. 

 The cytological analysis of the antimutagenic 

 effect of the compounds studied has shown that 

 it is connected with a decrease in the frequency 

 of primary breaks and not with the processes 

 of the re-union of fragments. In experiments 

 with recessive sex-linked lethal mutations we 

 have shown that streptomycin produces an 

 antimutagenic effect upon the occurrence of 

 spontaneous gene mutations. 



Depending on the concentration of the anti- 

 mutagenic compound it may, in optimum 

 concentrations, produce an antimutagenic effect, 

 in high doses it may prove to be mutagenic, in 

 average and minimum concentrations it may be 

 neutral. 



As a rule, in certain concentrations the antimu- 

 tagens acquire properties of substances which 

 are able to protect the chromosomes against the 

 effect of ionizing radiations. It is shown that the 

 character of this protection is different for 

 different phases of the cellular cycle. 



Of great importance in the problem of muta- 

 gensis is the analysis of their differential effect 

 on various genes. 



5.36. The Problem of Antimutagens in Connection 

 with Mutagenesis and Chemical Protection. 



N. P. Dubinin (Moscow, U.S.S.R.). 



The problem of antimutagens plays a 

 paramount role in the control of the spontaneous 



5.37. Methods for Estimating Differential Radio- 

 sensitivity. 1. 1. Oster and E. Pooley (Philadel- 

 phia, U.S.A.). 



Although marked differences in the sensitivity 

 of the various stages of germ cell development 



67 



