Section 5 — Mutagenesis 



while the author was a Postdoctoral Fellow of 

 The National Foundation. 



1. Adv. Virus Res., 1958. 



2. For the effects of such mutants in the wild 

 type Oregon-R strain of Drosophila see 

 Baumiller, Genetics, 1963. 



5.55. Effects of Irradiated Chromosomes on Viability 

 in Heterozygotes and Hemizygotes. Raphael 

 Falk, Nehama Ben-Zeev, and Shula Baker 

 (Jerusalem, Israel). 



Drosophila males were irradiated with an 

 X-ray dose of 2000 r and mated according to a 

 design that permitted the determination of 

 viability effects in both the heterozygotes and 

 hemizygotes carrying the irradiated X-chromo- 

 some. Apart from the induced lethals and semi- 

 lethals, the milder quasi-normal viability mu- 

 tations could also be detected in the hemizygotes 

 for irradiated chromosomes. 



In crowded cultures, when deleterious mu- 

 tations caused a decline in the number of males 

 hemizygous for the irradiated chromosome, the 

 remaining genotypes competed for the available 

 space. If the efficient competitors were heter- 

 ozygous for an irradiated chromosome as well, 

 the induced viability effect was offset in the final 

 outcome by their advantage in competition. The 

 heterozygotes for irradiated chromosomes thus 

 exhibited a spurious increase in viability as 

 compared with their controls which carried 

 unirradiated chromosomes. When intra-culture 

 competition was kept at a minimum it could be 

 demonstrated that the induced mutations had an 

 adverse effect on the viability of heterozygotes. 



The heterozygotes carrying semi-lethal mu- 

 tations were the most seriously affected. The 

 heterozygotes for quasi-normal chromosomes 

 were considerably less viable than the controls, 

 their viability being reduced at least as much as 

 that of heterozygotes for lethals. 



This work was supported by grant GM 8258 

 from the U.S. Public Health Service. 



5.56. Further Data on Overdomi nance. 



Wallace (Ithaca, U.S.A.). 



Bruce 



in heterozygous condition in flies (1) otherwise 

 homozygous for their second chromosome, 

 (2) heterozygous for two different chromosomes 

 obtained from the same locality, and (3) hetero- 

 zygous for chromosomes obtained from widely 

 separated localities. Two chromosomes from 

 each of three localities were used for these 

 studies; radiation levels were r, 250 r, 750 r, 

 and 2250 r. 



At the time this is written statistical analyses 

 are incomplete. Nevertheless, the data suggest 

 that the mean viability of homozygotes is 

 improved by the new heterozygosity (confirming 

 the report made at the Xth Int. Cong, of Ge- 

 netics) while that of heterozygous individuals i& 

 lowered. The implications of these observations 

 will be discussed. 



Studies have been completed on the average 

 viability effects of radiation-induced mutations 

 (second chromosome; Drosophila melanogaster) 



5.57. Transitory Increase in Genetic Load in Ir- 

 radiated Laboratory Populations of Drosophila 

 melanogaster. H. L. Carson (St. Louis, 

 U.S.A.). 



Four replicate populations (two control and 

 two experimental) of se ss e s k ro stock were 

 maintained for three years in vial populations 

 in which food, space and change schedule were 

 rigidly controlled. Populations produced by 

 this design are small (not more than 200 adults) 

 and are equilibrated under strong natural 

 selection. Population size and production were 

 measured weekly. The experimental populations 

 received radiation treatment; the dose was 

 given over a period of two years and totalled 

 65,000 r units of X-ray. Rate of administration 

 was 1000 r/week with three periods of 10, 10, 

 and 20 successive weeks during which radiation 

 was suspended. No radiation was given in the 

 final year. Genetic loads carried by each popu- 

 lation were measured three times during the 

 final year. Random lots of eggs from each popu- 

 lation were collected, counted and then adults 

 were reared from these eggs under ideal con- 

 ditions. Yields of adults from the irradiated 

 populations were significantly below those of 

 the controls at the first measurement. This 

 indicates that these populations were carrying 

 genetic loads due to the history of radiation. 

 These differences, however, were absent after 

 one year, indicating that the loads due to 

 radiation had disappeared. This effect is ascribed 

 to the efficiency with which natural selection 

 removes new deleterious mutants from small 

 populations. At the termination of the experi- 

 ment, population sizes of the experimentals did 

 not differ significantly from the controls. 



74 



