The response of spermatogonia and spermatocytes of the Northern vole Microtus oeconomus to the induction of sex-chromosome nondisjunction, diploidy and chromosome breakage by X-rays and fast fission neutrons.

Microtus males were exposed to different doses of 250 kV X-rays or fast fission neutrons of 1 MeV mean energy. Early (= round) spermatids were analyzed for the presence of extra sex chromosomes, diploidy and micronuclei at different time intervals corresponding with treated differentiating spermatogonia and spermatocytes. Induction of nondisjunction of sex chromosomes could not be detected. In contrast, induction of diploids by both types of radiation was statistically significant at all sampling times. Dose-effect relationships for most of the sampling times were linear and sometimes linear-quadratic concave upward or downward. There were pronounced stage-specific differences in sensitivity as reflected by differences in doubling doses that ranged from 4 to 22 cGy for X-rays and from 0.4 to 4 cGy for neutrons. Spermatocytes at pachytene were the most sensitive cells and proliferating spermatogonia the least sensitive ones. The relative biological effectiveness (RBE) of neutrons depended on the cell stage treated and fluctuated between 1.4 and 9.2. Evidence for radiation-induced chromosomal breakage events was obtained via detection of micronuclei. Induction of micronuclei by X-rays or neutrons was statistically significant at all spermatocyte stages tested. There was no effect in spermatogonia. With a few exceptions dose-effect relationships were linear. Differences in stage sensitivity were clearly present as evidenced by doubling dose which ranged from 5 to 29 cGy for X-rays and from 1 to 3 cGy for neutrons. RBE values varied from 5.2 to 12.7. Maximum sensitivity was detected in spermatocytes at diakinesis, MI and MII. Resting primary spermatocytes (G1 and S phase) were somewhat less sensitive and actively proliferating spermatogonia were the least sensitive cells. The pattern of stage sensitivity for induction of diploids was distinctly different from that for induction of chromosomal breakage.