Section 5 — Mutagenesis 



sex ratio. The highest frequency of XO males 

 was found in sperm sampled on the 7-8th days 

 after irradiation, presumably corresponding to 

 young spermatocytes, and this was not accompa- 

 nied by any comparable increase in the number of 

 non-disjunction females. From the structure of 

 the ring-X it is expected that most breaks will 

 result in loss of the chromosome. This is sup- 

 ported by the fact that the same dose of irradia- 

 tion produces more XO males from the ring-X 

 than from a rod-X. The dose effect curve for the 

 production of XO males is a straight line, one- 

 hit event. A comparison of this dose effect 

 curve with that of the change in sex ratio gives 

 further evidence that XO males are mainly 

 produced by loss of the X chromosome. A 

 hypothesis to explain the different peaks for 

 the two types of breakage-involving effects, XO 

 males and translocations, will be discussed. 



Sponsored by the Institute for Radiopathology 

 and Radiation Protection. 



5.40. The Brood Pattern of X-ray Induced Crossing- 

 overs in Drosophila melanogaster Males. 



Jaakko Puro (Turku, Finland). 



sperm production to the control level by the 

 12th day. Furthermore it appears that the high 

 incidence of crossovers, on the 10th day, is the 

 result of induction of crossovers in definitive 

 (secondary) gonia. 



Research supported by IAEA contract No. 31 

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



5.41. An Analysis of X-ray induced Non-disjunction in 

 Drosophila. A. J. Bateman and Ann C. Chand- 

 ley (Manchester, Great Britain). 



It is common to distinguish between two forms 

 of meiotic non-disjunction: at first and second 

 division respectively. This is a gross simplifi- 

 cation. There are many possible mechanisms re- 

 sulting in non-disjunction. Some of them are 

 recognisable by their genetic effects. An analysis 

 of matroclinous females in Drosophila, produced 

 by X-rays, has been made in an attempt to deter- 

 mine which modes of non-disjunction are, in fact, 

 operating in their production. Though the 

 results are ambiguous, some definite conclusions 

 can be drawn. 



The pattern of induced crossing-overs in 

 Drosophila melanogaster males was studied using 

 a hybrid stock of 12 recessive markers in the 

 third-chromosome. Adult males, after treatment 

 with 3000 r X-rays, were mated singly and 

 sequentially (at 2- or 3- day intervals for the 

 first three broods and daily from the 7th to the 

 24th day) to females of an appropriate genotype 

 for detecting crossovers. Fertility and fecundity 

 of the treated males, after decreasing to the 

 lowest level on day 8, had a marked recovery 

 on day 10 followed by a small depression on 

 days 11 and 12; no differences between the 

 experimentals and controls were found for any 

 of the rest of the broods. The number of tested 

 Fi offspring totaled 51,964 from 46 treated and 

 25,778 from 1 1 control males. 



With rare exceptions, proven crossovers were 

 first detected on the 9th day but the highest 

 incidence was on the 10th day after treatment. 

 Clusters of crossovers, many of which continued 

 in several broods, began on the 11th or 12th day. 



The evidence from the continuing crossover- 

 clusters substantiates the hypothesis proposed 

 by the author (1962) from clusters of recessive 

 third-chromosome lethals, that the treated pre- 

 definitive gonia — on an average five in number 

 per male, basing the estimation upon the size of 

 clusters — are responsible for the recovery of 



5.42. Dose-dependence of X-ray Induced Non-dis- 

 junction in Drosophila melanogaster Females. 



Ann C. Chandley and A. J. Bateman (Man- 

 chester, Great Britain). 



The frequency of induced non-disjunction in 

 female germ cells has been estimated on eleven 

 successive days following irradiation with X- 

 rays. Doses of 1 , 4, 6 and 8 k rad have been used 

 and the results indicate a non-linear dose-de- 

 pendence both for female (XXY) and male (XO) 

 exceptions. 



Over the first six days, the frequency appears 

 to increase with the square of the dose. In spite 

 of the frequency of male exceptions being 4-5 

 times as great as that of the female exceptions, 

 the shapes of the dose-dependence curves are 

 closely similar. 



After day 6, when the female and male ex- 

 ceptions have equal incidences, the dose-depend- 

 ence departs even further from linearity. 



5.43. Induction of Mutations and Cell Killing in 

 Irradiated Drosophila Spermatogonia. Per Of- 



tedal (Montebello, Norway). 



Treatments of Drosophila spermatogonia with 



69 



