Section 5— Mutagenesis 



of the spores and the segregation of the mating 

 type are normal. 



The tetrad analysis of these strains shows that 

 the resistant character segregates from the radio- 

 sensitive one with a ratio of 2: 2. In other words, 

 one genetic determinant is responsible for this 

 character. 



Radiosensitivity (S+), which is the wild type, is 

 dominant over radioresistance (S), The diploid 

 cells (S+ S") are less resistant than the resistant 

 haploid parent (S~). Their survival curve is super- 

 imposed on that of the diploid wild type (S+ S+). 



Different sites (three at least) are implicated in 

 this character; the survival curves of some S"S" 

 diploids are identical with that of the S+S+ and 

 the others are identical with that of the S ~; the 

 latter diploid cells are homoallelic and the 

 former are heteroallelic. 



The size of the target is about l/20th of the 

 whole genome. This estimation is made through 

 a dose-effect relationship for the induction of 

 this mutation by 32 P decay or by X-rays. 



When the S~ character is introduced into a 

 cross, deviation from the 2: 2 segregation ratio 

 for different biochemical markers is observed: 

 there is a large excess of prototrophs. These 

 deviations may be accounted for by a 32 P induced 

 mutation of a suppressor gene. 



5.27. Determination of Radiation-induced Mutation 

 Rates of Recessive Lethal Alleles in Saccha- 

 romyces. Wolfgang Laskowski and Klaus 

 Haefner (Berlin, Germany). 



After irradiation with small doses of u.v. and 

 X-rays diploid Saccharo?nyces-strams were iso- 

 lated, which proved by tetrad analysis to be 

 heterozygous for a single recessive lethal allele 

 ( + /1). The radiation sensitivity of some of these 

 strains was compared with the sensitivity of the 

 diploid parent strains homozygous for the cor- 

 responding wild type alleles (+/+). From these 

 data the u.v. and X-ray induced rates for the 

 mutation of wild type alleles to the recessive 

 lethal condition ( + ->1) were calculated and 

 found to be several orders of magnitude larger 

 than mutation rates from prototrophic to auxo- 

 trophic condition. 



Detailed data will be published in Z.fiir Ncitui- 

 forschung. 



5.28. A Hereditary Propensity to Untimely Death in 

 Yeast and its Temporary Reversal by Radiation. 



Allen P. James (Chalk River, Canada). 



A respiratory-deficient strain of yeast is un- 

 stable when cultured on a medium usually con- 

 sidered optimal for growth. The instability of 

 these "vegetative petites" is characterized by the 

 production of moribund cells. Analyses involving 

 the study of individual cell lines have demon- 

 strated that clones derived from a single cell 

 frequently differ in their degree of instability. In 

 general, the rate at which moribund cells are 

 produced increases in successive generations; 

 extinction of the strain is prevented only by 

 occasional and temporary reversals of this trend. 

 Complete stability can be obtained either by 

 mutation or by culturing the strain on a minimal 

 medium. The tendency to extinction can also be 

 reversed temporarily by exposure of the cells to 

 ultraviolet light but not to X-rays. The result of 

 experiments designed to uncover the underlying 

 mechanism and influence of radiation are dis- 

 cussed. 



5.29. The Mutagenic Activity of Ethyleneimino Deri- 

 vatives with Different Numbers of Reactive 

 Groups. H. LiJERS and G. Rohrborn (Berlin- 

 Dahlem, Germany). 



We compared the mutagenic activity of tri-, 

 bi-, and monofunctional ethyleneimines for 

 studying the relation between the number of 

 reactive groups and the degree of mutagenic 

 action in this class. We also tested three of their 

 basal compounds without any groups of alky- 

 lating ability. 



The tested compounds were as follows: tri- 

 functional: (A) tris-ethyleneimino-benzoquinone- 

 1,4, (B) tris-ethylene-thio-phosphor-amide; bi- 

 functional: (C) 2,5-di-n-propoxi-3,6-bis-ethyl- 

 eneimino-benzoquinone-1,4, (D) 2,5-bis-ethyl- 

 eneimino-benzoq uinone- 1 ,4 ; monofunctional : 



(E) dipiperino-phosphoric-acid-ethyleneimine, 



(F) 4-ethyleneimino-naphtoquinone-l,2, (G) 2- 

 ethyleneimino-5, 6, 7, 8-tetrahydro-naphto- 

 quinone-1,4; without ethyleneimino groups: (H) 

 p-quinone, (I) 1, 3, 5-triazine,(K)2,4,6-triamino- 

 1, 3, 5-triazine. 



Using the Berlin wild stock of Drosophila me- 

 lanogaster the rates of sex-linked recessive lethals 

 have been determined by the Base method in 

 broods of three days after having fed or injected 

 the Berlin wild males with aqueous solutions of 

 the test substances. With respect to the mutagenic 

 effects on the postmeiotic stages of the sperma- 

 togenesis there are striking differences between 

 the different compounds. Trifunctional ethyl- 

 eneimines proved to be more mutagenic than 

 bifunctional ones, and these had a higher effi- 

 ciency than the monofunctional ethyleneimines. 



64 



