Section 5 — Mutagenesis 



lifetime reproductive function when treated mice 

 were mated to untreated mice. Comparison of 

 the two general types of progeny, inbreds and 

 hybrids, revealed significant differences among 

 these genotypes. Following treatment with 160r 

 at 10£ days gestation 64 per cent of the inbred 

 embryos were born with external malformations 

 compared to 29 per cent of the hybrid progeny. 

 In addition, the incidence of stillborn births 

 among inbred progeny after 160r at 10| days was 

 greater than that of the hybrids (100 per cent 

 vs. 64 per cent). The general response of the 

 inbreds and hybrids also was observed in those 

 treatments which produced the most noticeable 

 effects on growth and reproductive performance. 

 For example, after 320 r at 17^ days gestation ir- 

 radiated mice later produced a mean number of 24 

 progeny in a one year period compared to 63 

 progeny in the controls. Of particular significance 

 is the fact that all of the inbred embryos that had 

 received this treatment later were sterile. Half of 

 the hybrid progeny produced litters however. 

 Results indicate that heterotic effects may exist 

 during prenatal stages of development. 



This work has received assistance from Con- 

 tract No. AT (11-1) 107 from the United States 

 Atomic Energy Commission. 



5.70. An Attempt at Genetic Extinction of a Small 

 Hybrid Mouse Population by Gonadal Irra- 

 diation. Earl L. Green (Bar Harbor, U.S.A.). 



Genetic extinction of a population may be 

 claimed only if a non-irradiated generation, one 

 or more generations after the last irradiated 

 generation, fails to reproduce. This experiment 

 was an attempt to extinguish a small population 

 of hybrid mice (8 mated pairs in each generation) 

 by exposure of the gonads of the male parents, 

 when six weeks old, to 900 r of X-rays in each 

 generation. The gonadal dose of 900 r was select- 

 ed because it neither killed nor permanently 

 sterilized these hybrid mice. Seven weeks after 

 exposure, each male was mated to a randomly 

 selected non-sister female of the same generation. 

 All mice of this population (DX-GE) were des- 

 cended from a four-way cross of strains C3HeB/ 

 FeJ, C57BL/6J, DBA/2J, and BALB/cJ. 



After six generations of irradiation, genetic 

 extinction had not been achieved. Furthermore, 

 reproductive performance (fertility, number 

 of litters, number of offspring born and wea- 

 ned, average litter size) had not been greatly 

 affected. 



5.71. Influence of Cobalt-60 continuing Irradiation 

 on Fertilities and Life Spans of Different 

 Strains of Mice W. Janice Stadler and John 

 W. Gowen (Ames, U.S.A.). 



Continuous 22 hour a day irradiations have 

 now been received by up to 18 successive 

 generations of different strains of our mice. 

 Full lifetime fertilities, life spans and other 

 characteristics of the successive generations of 

 progenies are being taken. Five generations now 

 have completed life spans. Dosages varying 

 from 0.03 r to 0.10 r per hour allow reproduction 

 to continue. Both parental sexes as well as in 

 uterine development are under irradiation. 

 Irradiation to the progenitors prior to con- 

 ception, ancestral irradiation, may be separated 

 from that received by the mouse itself, direct 

 irradiation. The results showed that within 

 these dosage ranges mice maintain numbers in 

 first litters directly comparable to those observed 

 for unirradiated mice of like strains at corre- 

 sponding periods in the colony. Sex ratios were 

 comparable within the different strains. Strain 

 differences in reproductive performance under 

 irradiation were evident. The factors most 

 important to reproductive worth of both 

 irradiated and unirradiated mice were similar; 

 time when fertility was initiated, well balanced 

 reproductive sequence and litters adjusted to 

 physiological capacities of the parents. Search 

 for visible gene mutations showed none in the 

 untreated mice and one recessive, brachypod, 

 in the irradiated group. This mutation appearing 

 in the 9th B X S generation is allelic and phenoty- 

 pically comparable to a similar mutation from 

 mice receiving high dosage acute irradiation 

 some years earlier. Cumulative irradiation 

 dosages over 10 generations exceeded 1500ror 

 27 times the 50 per cent sterilizing acute dose for 

 the females, 4 times that for the males and more 

 than 2 times the acute lethal dose for either sex. 



1. Journal Paper of the Iowa Agri- 

 cultural and Home Economics Experiment 

 Station, Ames. Project Nos. 1180 and 1187. 

 Assistance has been received from Contract 

 AT(ll-l) 107 Atomic Energy Commission, 

 U.S.A. 



5.72. X-irradiation Effects on Lifetime Repro- 

 ductivities of Different Strains of Mice. 



John W. Gowen and Janice St ADLER^CAmes, 

 U.S.A.). 



Effects of single dose X-ray irradiations to 



79 



