X-ray-induced reciprocal translocations in stem-cell spermatogonia of the rhesus monkey: dose and fractionation responses.

Rhesus monkeys received total body or local testes X-irradiation with unfractionated (50, 300, 400, 800 and 850 rad) or fractionated (200 + 200 rad with 24-h interval) exposures. At different times after irradiation, chromosomal analysis was made of C-banded dividing spermatocytes. The observed frequencies of translocation configurations confirmed earlier results about the low induction rate of reciprocal translocations in stem-cell spermatogonia of the rhesus monkey. The absence of any translocation induction at doses of 400 rad and higher indicates an extreme insensitivity of surviving radiation-resistant stem cells for the induction of this type of genetic damage. The frequency of translocations following a fractionated exposure to 400 rad, which is above the peak yield for single exposures, was clearly higher than that obtained when the same dose was applied as a single exposure (0.71 versus 0%), but significantly lower than expected on the basis of additivity of the two fractions (0.71% versus 1.98%).