Gamma-ray-induced numerical and structural chromosome anomalies in mouse immature oocytes

Young female mice were given 1, 2 or 3 Gy of chronic gamma-irradiation. Metaphase II oocytes from these mice were sampled 8 weeks after the end of the treatment and screened for numerical and structural chromosome anomalies. The proportions of hyperhaploid (n + 1) metaphase II oocytes increased after 1 and 2 Gy (significantly after the latter) but remained at the control level after 3 Gy of gamma-rays. Structural chromosome anomalies were significantly increased above control levels at all doses and also showed an increase with dose to 2 Gy and a decline at 3 Gy. The cause of this unusual dose-response pattern for induced chromosome damage is uncertain. These results show that significant chromosome damage can be induced by irradiation of immature oocytes, a cell stage previously suggested to be resistant to induced genetic damage.