Bristle Formation Research Articles

Bristle formation controlled by the achaete locus in genetic mosaics of Drosophila melanogaster.

IN D. melanogaster, the genetic control of differentiation at specific sites is conveniently studied by the use of genetic mosaics. In wild-type flies, a definite number of macrochaetae or bristles is formed at specific sites of the thorax. Certain mutant genes are quite specific in their effects on bristle formation. The effect of achaete (ac) , for example, is primarily restricted to removal of two bristles, the anterior and posterior dorsocentrals. Earlier studies (STERN 1954a,b) have shown that bristle development is cell autonomous in mosaics. That is, in gynandromorphs in which the site of a posterior dorsocentral bristle is in ac tissue, a bristle will usually not differentiate even when most of the surrounding tissue is ac+. Conversely, when the site of the bristle is ac+, differentiation is always initiated regardless of the amount and distribution of surrounding ac tissue. This was interpreted by STERN as indicating that the hypodermal layer of the imaginal disc possesses regions in which the site of bristle formation is predetermined. A cell with the wild allele is competent to respond to this “prepattern,” but a cell with the ac allele lacks this competence. The present study of bristles affected by the ac locus uses a technique of mosaic production different from the one employed by STERN, one which produces a comparatively high proportion of mosaics thus permitting a quantitative breakdown of the various types of mosaics recovered.

The genetic control of developmental competence and morphogenetic tissue interactions in genetic mosaics.

1. InDrosophila melanogaster the genotype Theta causes the development of an interalar bristle-organ which is not formed in the wild type. The genotype Minute-n causes, in Theta flies, a strong reduction in the frequency of the occurrence of the interalar bristle. 2. By means of genetic mosaics, produced by somatic crossing over and segregation, a developmental analysis was applied to the differences in interalar differentiation between the wild-type, the Theta and the Minute-n genotype. 3. Loss of Minute-n in segregated tissues of Minute-n Theta flies leads to differentiation of the interalar bristle. Loss of Minute-n and Theta in segregates of such flies may also lead to differentiation of the bristle, particularly in spots of small area. 4. In the neighborhood of the border between the genetically different tissues of mosaics extra bristles may be formed. 5. The data furnish evidence for interactions between the genetically different tissues of mosaics: inhibition of bristle formation in the surroundings of differentiating bristles, and spread of gene-dependent substances concerned with differentiation. 6. The results can be interpreted in terms of a constant prepattern for the interalar bristle in wild-type, Theta and Minute Theta genotypes, but of different competence of cells with the three genotypes to respond to the prepattern by differentiation of the interalar bristle.