ENHANCED SPECIFIC-LOCUS MUTATION RESPONSE OF 101 H MALE-MICE TO SINGLE, ACUTE X-IRRADIATION/

The specific-locus mutation yield from 101/H mice following single, ac ute 6 Gy spermatogonial X-irradiation was significantly higher than bo th a concurrent C3H/HeH x 101/H F-1 hybrid control and historical data obtained with mice of equivalent genotype. There is therefore discord with the reduced translocation response to single X-ray doses previou sly identified in this strain. By contrast, the mutation yield followi ng a 24-h interval 3 + 3 Gy fractionated X-ray dose was not significan tly different from that of its concurrent hybrid control, nor from res ults obtained by others with mice of the equivalent or different genot ypes. Here there is no discord with the translocation response obtaine d with a 1 + 5 Gy 24-h interval fractionated regime. Appraisal of comp arable specific-locus and translocation data indicates that the differ ing results obtained with the two genetic end-points are not without p recedence. This, together with the observation that the responses for cytologically visible deletions and specific-locus mutations are simil ar, suggests that the latter two events predominantly derive from one- hit events, with translocations deriving from two-hit events and that the probabilities of induction of each type of lesion vary according t o the organisation of the nucleus in different phases of the cell cycl e.