Effects of unequally fractionated X-ray exposures on the induction of chromosomal rearrangements in mouse spermatogonia

The induction of reciprocal translocations in mouse stem-cell spermatogonia was determined following different unequally fractionated X-ray exposures with 24 h between the fractions. The results indicate that quite variable levels of chromosomal damage can be induced using the same exposure schemes. This variation does not seem to be correlated with radiation set-up or mouse strain. In general, with higher conditioning exposures from 12.5 to 300 rad (or R), higher frequencies of translocations were produced using a larger second challenging exposure of 700-900 rad (or R). The increase in yields of aberrations was more or less paralleled by a shift from a strong deviation from a Poisson fit of the number of translocations per spermatocyte, to a good fit, suggesting a transition from the original heterogeneity of the stem cell population to a radiation induced, more homogeneous stage. Earlier observations concerning a threshold dose for sensitization (van Buul and Léonard, Mutation Res., 70 (1980) 95-101) could not be confirmed.