Section 5 — Mutagenesis 



expression were distinguished: Minute - slight, 

 M(s), with only the macrochaetes reduced in 

 length and thickness; Minute- intermediate, M(i) 

 with more extreme bristle effect, accompanied 

 by some abnormality of wings, eyes or body; 

 Minute - extrem, M(e), highly pleiotropic with 

 several external features grossly affected. Radi- 

 ation induced Minutes showed all grades of 

 expression with a preponderance of the M(i) 

 class, whereas those of macromolecules were 

 either M(s) or M(e), with the M(i) class virtually 

 absent. 



A correlation was established between the 

 phenotypic expression of the Minute and the 

 genetic position of the chromosome deletion 

 causing it. In a sample of 165 M(e) mutations 

 induced by various macromolecules all but 3 

 (which werenearer the M(i) class) were on the 

 IVth chromosome. The deletion involved the 

 proximal 4/5th of the salivary gland IVth 

 chromosome, and covered the majority of its 

 marker genes, including the most proximal: 

 sparkling-poliert (spaP° l .) and eyeless (ey). In 

 contrast, of the 34 M(s) mutations studied, 33 

 were on the large autosomes (Ilnd or Illrd). 

 The proportion of the M (e) class among the 

 Minutes induced by various macromolecules 

 was different, reaching a maximum of 95 per 

 cent with DNP and a minimum of 29 per cent 

 with polymethacrylic acid. This indicates a 

 differential distribution of the chromosome 

 deletions with the various macromolecules. 



The frequency of induction of IVth chromo- 

 some deletions with DNA was compared with 

 that occuring with radiation. Among 47 fertile 

 Minutes induced by DNA, 38 were on the IVth 

 chromosome (81 per cent) and all covered 

 spaP°K ; whereas with 46 X-ray Minutes recovered 

 in a comparable experiment only 13 occurred on 

 the IVth (28 per cent), 12 of which covered 

 spa? 01 . These two distributions are very sig- 

 nificantly different (x 2 [i] =26.0, P<10" 6 ) 

 indicating a high degree of selective gene elimi- 

 nation with DNA. 



5.33. The Genetic Effects of Labeled DNA Precursors. 



W. D. Kaplan, H. D. Gugler and K. K. Kidd 

 (Duarte, U.S.A.). 



Sex-linked recessive lethals have been induced 

 in Drosophila males by feeding tritiated thymi- 

 dine to larvae. In three experiments feeding was 

 restricted to 8 hr to limit the incorporation of 

 the isotope. The most heavily labeled germ 

 cells appeared in the brood sperm utilized for 

 the first of three 3-days broods tested for induced 

 lethals. In one experiment, during which larvae 



were permitted to feed on labeled food through- 

 out larval life a higher frequency of labeled 

 sperm bundles was obtained and also heavier 

 labeling of individual sperm. 



Induced mutation rates reflected the degree 

 of incorporation, and mutations were restricted 

 to those broods with labeled sperm. 



The physical characteristics of the beta e- 

 missions of tritium - - low energy and short 

 mean path — led us to test the distribution of 

 the induced mutations. The pattern is non- 

 random and differs significantly from those 

 obtained by the use of X-irradiation. The region 

 of the X-chromosome from 1 to 20 is relatively 

 free of mutations, whereas the regions between 

 30-35, 50-55, and 60-65 have a higher number 

 of induced lethals than would be expected at 

 random. 



Mutations have also been induced by tritiated 

 deoxycytidine. These mutants will be localized 

 and the distribution compared to the one 

 observed with H 3 thymidine. If the pattern of 

 induced mutations reflects the varying fre- 

 quencies of the pyrimidine bases along the 

 length of the chromosome, the pattern induced 

 by H 3 deoxycytidine should differ from that 

 observed with the use of H 3 thymidine. 



5.34. Incidence of Mutations in Drosophila melano- 

 gaster raised from Flies fed on Irradiated 

 Medium. S. Nirula, M. S. Swaminathan, 

 A. T. Natarajan and R. P. Sharma (New 

 Delhi, India). 



Earlier studies at the Indian Agricultural 

 Research Institute on the effects of culture 

 media, potato mash and fruit juices irradiated 

 with ionizing radiations on mitosis in root 

 meristems of barley, Vicia faba and onion 

 grown on them have revealed that such irradiated 

 products may have radio-mimetic effects. In 

 view of the obvious bearing of these data on 

 assessing the wholesomeness of food material 

 sterilised through ionizing radiations, a study 

 was initiated in Drosophila melanogaster to 

 ascertain whether the mutational load is en- 

 hanced in flies fed with irradiated food. 



The basic medium consisting of glucose agar 

 and yeast is irradiated with 150 k rad of gamma- 

 rays (sterilizing dosage) at a 160 curie Co 60 

 source. Young Oregon-K flies (soon after 

 emergence) are fed on this irradiated medium. 

 The flies are allowed to breed and parent flies 

 are killed about 7 days later. The male flies 

 which emerge out of the larvae fed exclusively 

 with irradiated food are then used for conducting 

 M-5 tests on normal culture medium for further 



66 



