Nature of strain differences in spermatogonial responses to radiation

In the course of an earlier study, the authors observed a clear-cut strain difference in embryonic mortality following exposure of the father's spermatogonial stem cells to 4 x 500 R acute X rays. The overall frequency of embryonic mortality was about 23% for irradiated (C3H x 101)F1 or (101 x C3H)F1 males, and about 7% for irradiated (SEC x C57BL)F1. The strains differed in the same direction, but to a lesser degree, with regard to length of the sterile period resulting from killing of stem-cell spermatogonia. To obtain some clues as to the nature of the strain differences and check further the possible relationships between cytotoxic and cytogenetic responses, they initiated experiments in which the condition of X-ray exposures was varied. In the earlier study, the four fractions of 500 R were separated by four-week intervals. In the new study, males from the two strains were exposed simultaneously to either of the following regimens: 500 R, single dose; 2 x 500 R, 24 h between doses; 4 x 500 R, 8 weeks between doses; and 100 R + 900 R, 24 h between doses. The corresponding incidences of embryonic mortality among offspring resulting from irradiated spermatogonia (males were mated within 3 months after the recovery of fertility) are 12% and 8%, 30% and 17%, and 17% and 13%. These results show that the length of the sterile period for the two strains may differ in opposite directions depending upon the exposure regimen used, but the corresponding incidence of embryonic mortality is either similar in the two strains or clearly higher when fathers are (C3H x 101)F1. Furthermore, within each of the two strains, the two endpoints exhibit no correlation whatsoever