Dose Rate Effects in the Developing Germ Cells of Male Drosophila.

ENETIC damage was not modified by variations in the dose rate of radiation when mature sperm of Drosophila were treated (MULLER 1940), nor were changes in the rate or in the fractionation of radiation dose effective in influencing biological damage in mature sperm of Drosophila virilis when, treatments were given in atmospheres of oxygen or nitrogen (ALEXANDER and BERGENDAHL 1962). However, LUNING (1958) reported deviations in the proportion of genetic damage expected when mature sperm of Drosophila mlanogaster were treated, and explained the results as due to genetic recovery mechanisms. When one radiation dose was given in nitrogen followed by radiation treatment in air, the genetic damage was equal to that observed when both doses were given in air-if the second dose was given within 30 minutes of the first treatment. Intermediate values between the two were obtained when the second dose followed the first by a period of 30 minutes or longer. WOLFF and LINDSLEY (1960) did not observe deviations from the expected mean values in similar types of tests when the oxygen pressure was increased by substituting oxygen for air. The authors explain these differences by assuming either that the limiting factor is oxygen rather than cellular components or that two oxygen sensitive systems are present, affecting breakage and reunion. In immature germ cells, more consistent and definite modifications have been observed. Although modifications are more definite, they are not always predictable. In the silkworm, TAZIMA, KONDO and SADO (1961) observed differences in genetic damage in germ cells of spermatogenesis and oogenesis with acute and chronic doses of radiation. An increase or reduction in the mutation rate with chronic doses depended upon the type and proportion of germ cells present when the radiation doses were administered. A reduction in mutation rate has been reported with chronic radiation doses in the spermatogonial cells of the mouse (RUSSELL, RUSSELL and KELLY 1958) and in oocytes of Drosophila by OSTER, ZIMMERING and MULLER (1959). However, RUSSELL (1962) found an increase in specific gene mutations in spermatogonia of the mouse when the radiation