X-ray-induced specific-locus mutation rates in young male mice

The specific-locus mutation frequency resulting from 300 R of acute X-irradiation has been determined for the germ cells present in male mice at 2, 4, 6, 8, 10, 14, 21, 28, and 35 days of age. The sample size was large enough for each of these nine age groups to ensure that a high mutation rate would be noticed. The testis of the mouse undergoes many developmental changes between birth, when most or all germ cells are gonocytes, and 35 days, when the cell population has come to resemble that of the adult. It was important to know if the germ cells present in these developmental stages of immature male mice yield the same mutation frequency as that found earlier for spermatogonia in the adult by W. L. Russell.None of the nine age groups has a mutation rate statistically significantly higher than that of the adult. Taken together, the nine groups of males have an average mutation frequency quite to that of the adult. This does not rule out the possibility that individual age groups may have a mutation frequency somewhat different from that of the adult.The distribution of mutations among the loci seems to be similar to that found for mutations induced in spermatogonia of the adult. Clusters of specific-locus mutations were found only on day 21.This paper and that presented earlier on the newborn report the first specific-locus mutation-rate studies on male mice irradiated between birth and adulthood. If the results can be carried over to man, it can be concluded that irradiation of the immature testis, from birth to puberty, will not present any greatly increased genetic hazard over that from irradiation of the adult testis. In fact, as the data stand in the mouse, they indicate a mutation rate similar to the adult for all but the earlier stages tested and, for these stages, a probably lower rate, representing a transition from the significantly lower rate reported earlier for newborns.