Further evidence demonstrating germinal selection in early premeiotic germ cells of Drosophila males.

ULLER (1954) in discussing the errors associated with estimating the frequency of lethals induced on the X chromosome in gonial or primordial germ cells of the male gives perhaps the most generally applied definition of germinal selection. “This term refers to the fact that some of the lethals, especially those involving deficiencies, kill or retard the proliferation of the very cells in which they occurred, through their effects on cell metabolism. Since the cells of the male contain but one X chromosome there is no normal allele present to dominate over these lethals.” HARRIS (1929), working under MULLER’S direction, was the first worker to call attention to germinal selection as one of the two possible causes of differences in induced mutation frequency for different germ cell stages. The second and more common cause, differential mutability for different germ cell stages, has since been amply demonstrated by scores of Drosophila workers. Addressing themselves to the question of germinal selection, a number of the pioneer workers obtained evidence supporting this view. In a series of experiments, SHAPIRO and SEREBROVSKAYA (1934) first demonstrated that the ratio of induced X chromosome to autosome lethals in postmeiotic male germ cells was approximately 1 :2.5, and SHAPIRO ( 1936) demonstrated that this ratio was more extreme, 1:5.6, in premeiotic germ cells sampled 20 or more days after treatment. The last mentioned work is probably the most important single evidence for germinal selection. KOSSIKOV (1937) demonstrated that the male germ cells showed a much more drastic decrease in X-linked recessive lethals than female germ cells when followed over a number of broods, suggesting that selection was operating against mutations in the hemizygous X. More recently AUERBACH (1954) using X rays, and PURWM (1957) using chemicals, obtained very similar results to those of SHAPIRO. The premeiotic gonial cells fall into two distinct groups as summarized by COOPER (1950). There exists a small population of asynchronously dividing cells referred to as primary or pre-definitive spermatogonia. These are believed to function as a reservoir for the other group, the secondary or definitive gonial cells. This other population of gonial cells, the secondary or definitive cells, occurs