Section 5 — Mutagenesis 



be explained by mixed populations of resistant 

 and sensitive spermatid cells. 



Work supported, in part, by Contract No. AT- 

 (40-l)-3014 with the United States Atomic 

 Energy Commission. 



5 .46. Effect of Penicillin Feeding on the Reduction of 

 Radiation Induced Mutation Rate in Drosophila 

 melanogaster. Toshio Shiomi (Chiba, Japan). 



It has been reported by Burdette (1961) that 

 D. melanogaster, grown on medium containing 

 penicillin, exhibits a diminished X-ray induced 

 mutation rate. The present work reports the 

 following results: a wild type D. melanogaster, 

 Canton-S, was grown in a culture medium con- 

 taining penicillin G (20,000 units per ml); 24 hr 

 old virgin males hatching from these cultures 

 were irradiated with X-rays. The induced sex- 

 linked recessive lethal mutations in mature 

 sperms were tested by the Muller-5 method. 

 X-ray doses used were 500, 1000, 2000, and 3000 

 r. 



(a) The radiation induced mutation rates in 

 penicillin-fed Drosophila are found to be about 

 one-half of those for flies grown in normal 

 culture conditions. 



(b) The radiation dose-effect relationship re- 

 mains linear. 



(c) Penicillin containing medium lengthens the 

 growth period of flies by nearly one day as com- 

 pared to the normal culture, and the rate of 

 emergence is higher than in the control. 



(d) The feeding of penicillin during the first 

 half of the larval stage is more effective in re- 

 ducing the induced mutation rate than when fed 

 during the second half. 



The interpretation of this action of penicillin 

 on the radiation induced mutations, especially 

 its effect in the early stage of feeding, is difficult. 

 One might assume that the metabolic patterns of 

 the larvae are affected by penicillin or its deriv- 

 atives, and the genie material itself or its sur- 

 roundings are changed to more radioresistant or 

 easily recoverable conditions. Whether the effect 

 is caused by penicillin itself or by its degraded 

 forms remains for future work. 



5.47. X-ray Induced "Dominant Lethals" in Insemi- 

 nated Eggs of Drosophila. (a) Experiments in the 

 stage between completion of meiosis and be- 

 ginning of cleavage. H. Ulrich (Zurich, Swit- 

 zerland). 



In insects, pre-adult mortality of the progeny 

 of irradiated parents is thought to result from 

 radiation induced "dominant lethals" in the 

 genome of the maternal or paternal gametes. 

 These "dominant lethals" can be either gene or 

 chromosome mutations. Similar radiation effects 

 (e.g. embryonic mortality) are found if instead of 

 the gametes the progeny (e.g. young embryos) is 

 irradiated. This raises the question whether in 

 both types of experiments pre-adult mortality is 

 the result of "dominant lethals" (i.e. alterations 

 in the genome) and/or "physiological effects" 

 (i.e. effects in cell components others than 

 chromosomes). 



During the last 10 years we have accumulated 

 quite a number of experimental results on in- 

 seminated Drosophila eggs. X-rayed after depo- 

 sition in the stage between completion of meiosis 

 and beginning of first cleavage. Most data (e.g. 

 differential radiosensitivity of cell parts with or 

 without a nucleus, the one hit dose action curves 

 for mortality and recessive lethals, influence of 

 oxygen during irradiation, etc.) were consistent 

 with the assumption that the most important 

 radiation effects are alterations in the genome 

 which behave like dominant lethals (probably of 

 a one hit type predominantly). However, new 

 results (experiments with cold post treatment and 

 the analysis of radiosensitivity of eggs and em- 

 bryos with different chromosomal constitutions) 

 indicate that, at least some of the radiation effects 

 have to be considered as "physiological effects". 



5.48. X-ray Induced "Dominant Lethals" in Insemi- 

 nated Eggs of Drosophila. (b) Experiments with 

 different stages between insemination and end of 

 second cleavage division. F. E. Wurgler 

 (Zurich, Switzerland). 



The nature of X-ray induced mortality in in- 

 seminated Drosophila eggs has been studied by 

 analysing the radiosensitivity in different stages 

 of meiosis and early cleavage. The highest sen- 

 sitivity was found during late anaphase/early 

 telophase of meiosis II and each cleavage division 

 analysed so far. Sensitivity was lowest when the 

 nuclei were in interphase. 



Dose action curves for the different stages vary 

 greatly in shape and slope. The one hit curve 

 found for egg samples containing all stages 

 between completion of meiosis and beginning of 

 cleavage (see Ulrich, preceding report) could be 

 shown to result from superposition of different 

 non linear dose effect curves. This result invali- 

 dates the strongest argument supporting the 

 hypothesis that radiation induced mortality is 

 of a one hit type. 



71 



