Evidence of a threshold X-ray dose for sensitizing stem-cell spermatogonia of the mouse to the induction of chromosomal translocations by a second larger one.

The effect of different small conditioning doses of X-rays on the production of reciprocal translocations in stem-cell spermatogonia of the mouse (scored in spermatocytes) by a second larger dose have been examined. Fractionation regimes of 25 + 975 R, 50 + 950 R, 75 + 925 R and 100 + 900 R, all with 24 h between the fractions, were applied. The size of the first fraction strongly affected the frequency of induced translocations by the second one, and a kind of threshold dose, somewhere between 75 and 100 R existed for conditioning the spermatogonial population: the translocation yield after 25 + 975 R was 3.3%, after 50 + 975 R it was 5.0%, and after 75 + 925 R it was 5.1%; whereas 100 + 900 R resulted in 16.1% translocations. It is difficult to explain this observed threshold effect by known biological processes so far held responsible for the conditioning effect.