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.