The differentiation of a secondary sex comb under the influence of the gene engrailed in Drosophila melanogaster.

HE pattern of distribution of bristles in Drosophila melanogaster is under 'genic control. Thus, the first tarsal segment of a foreleg of female genotype does not differentiate the row of heavy bristles which constitutes a sex comb while this structure is formed on a foreleg of male genotype. The determination of the sex comb was studied by STERN and HANNAH (1950) by means of analysis of gynandric mosaic forelegs. It was found that the differentiation of sex comb teeth occurred whenever cells of male genotype were present in the region of the forelegs where sex combs normally develop and that the occurrence of this differentiation was independent of the relative sizes of the female and male areas of the mosaic tarsal segment. It was concluded that during development of a foreleg a certain region is singled out from the rest, whose property it is to evoke differentiation of a sex comb if the cells of the region are male but which is not able to cause such differentiation if the cells are female. STERN (1954a, 1957) called the invisible pattern of regional differentiation which precedes that of morphogenetic differentiation a prepattern. Female cells, then, do not respond to the prepattern of a sex comb, while male cells do so fully. Cells of triploid intersexual genotype seem to be intermediate in their sensitivity so that fewer cells respond by formation of sex comb teeth (STERN 1956a, HANNAH-ALAVA and STERN 1957). In a series of papers the presence of invariant prepatterns, but of differential response of different genotypes, was also demonstrated for the differences between the bristle patterns of achaete and nonachaete, hairy and nonhairy, Theta and non-Theta, scute and nonscute (STERN 1954a,b, 1956b; STERN and SWANSON 1957). KROEGER (1959a,b) has applied and extended the concept of prepattern to the interpretation of his interesting experiments on mosaics between the fore and hind wings of the moth Ephestia, and between the male and female genital discs of Drosophila. In both cases invariant prepatterns seem to be involved. Recently, HANNAH-ALAVA ( 1958a) has described a case which may constitute genic determination of a prepattern itself. The present study concerns the action of the recessive autosomal gene engrailed (en) which leads to the appearance of a secondary sex comb on male forelegs which forms approximately a mirror image of the normal, primary sex comb. By the use of gynandric mosaics it had been shown that the foreleg of a homozy-