A Re-Examination of the Problem of Manifold Effects of Genes in Drosophila Melanogaster.

LMOST two decades ago, one of us (DOBZHANSKY 1924) found that some A mutant strains of Drosophila melanogaster differed from each other and from normal flies in certain details of the structure of their genitalia. The mutants concerned were known to affect the eye color (white, ivory), body color (ebony, yellow), eye size (Bar), and bristle shape (forked), but the existence of differences in the genitalic armatures had neither been known nor suspected. The shape of the spermatheca was chosen as a test character for an examination of the genetic causation of the genitalic differences (DOBZHANSKY 1927). Mutant strains kept in laboratories often differ not only in the mutant genes which produce the obvious external traits for which these mutants are named but also in minor modifying genes which affect various structural and physiological characters of the flies. The observed differences in the shape of the spermatheca might, therefore, be produced either by the same mutant genes which are responsible for the alteration of the eye and body colors, bristle shape, etc., or by independent genes which affect the spermatheca. To discriminate between these possibilities, the mutants were crossed to a wild type strain, the segregation products obtained in Fz generation were intercrossed again, and the series of crosses were carried for from three to 31 generations. From time to time samples of wild type flies and of flies showing the external characteristics of the mutants were taken, and the shapes of their spermathecae were examined. The differences in the spermatheca shape between the wild type and the mutants ebony, sooty, and tan2 had not diminished after respectively 31, 16, and three generations of crossing; the differences between the wild type and the mutants cinnabar, white, yellow, and ivory did diminish but did not disappear after the crossing. These results were interpreted to mean that the mutant genes named above affect the spermatheca shape as well as the eye and body colors, but that some independent modifiers of the spermatheca shape are also present. SCHWAB (1940) has correctly pointed out that the system of crosses used by DOBZHANSKY is not sufficient to exclude the possibility that the apparent effects of the mutant genes on the spermatheca shape are produced by modifiers borne in the same chromosomes as the respective mutants. The system used by SCHWAB himself consisted in crossing the mutants for 40 generations to an especially prepared inbred wild type strain; in this way strains should be obtained which have identical genetic backgrounds except for more or less short chromosome segments carrying the mutant genes under study. Of the eleven