Frequency of Dominant Lethals Induced by Radiation in Sperms of Drosophila Melanogaster.

HE proportion of the eggs laid by Drosophila females that fail to develop T into adult flies is increased when the sperms used in fertilizing the females have been previously treated with ionizing radiation. It is generally believed that these induced failures of development are due to “dominant-lethal” changes in the genetic material of the treated sperms. These changes have been visualized alternatively as being closely related either to gene mutations or to chromosomal aberrations. Early investigators inclined toward the first view, but now the latter view is becoming firmly.established. Most of the individuals carrying dominant lethals die in the egg stage, so that the proportion of eggs that fail to hatch is usually a good index of the frequency of dominant lethals. Determination of the proportion of organisms that attain, or fail to attain, some more advanced stage of development (such as pupation or emergence of adult fly) offers a better chance of scoring all the induced dominant lethals, but may more frequently be interfered with by external, accidental causes of death. X-ray-induced dominant-lethal effects in Drosophila were first noticed by MULLER (1927). Quantitative experiments to investigate the general characteristics of the phenomenon were made soon afterwards by HANSON (1928) and then by TIMOF~EFF-RESSOVSKY (1931). It was realized that the relationship between the dosage of radiation treatments and the frequency of dominant lethals is fundamental for the interpretation of the nature of these changes. GOWEN and GAY (1933) published graphs indicating that this relationship is of the type expected in the case of “single-event,” or “single-hit,” phenomena. Accordingly, these authors considered dominant lethals as special types of gene mutations. Their evidence is not adequate, however, for the solution of this problem. In the first place, the experiments did not yield details in the most critical range of the dosage-effect relationship-that is, a t low dosage. In the second place, these authors did not measure the actual proportion of organisms developing in any single culture, but simply compared the total numbers of flies emerging from experimental and control cultures raised under parallel conditions. Finally, comparison of their results on recessive lethals with similar results by other authors suggests that their dosage measurements were in error by as much as a factor of 2 : s (SCHULTZ 1936). SONNENBLICK (1940) in a more recent experiment was mainly concerned with the direct observation of anomalies in the embryonic development accompanying dominant lethals. Within the sections of single eggs, he found distortions of various types in the mitotic