Genetic interaction between homoeotic Sex combs reduced and Regulator of bithorax (or trithorax) genes of Drosophila melanogaster

Summary Regulator of bithorax (Rg-bx)^–, or trithorax (trx)^– lethal larvae occasionally show a homoeotic transformation of the dorsal prothorax to mesothoracic structures. This transformation suggests a reduced activity of the Sex combs reduced (Scr) gene on the basis of gene dosage studies, as well as enhanced expression of the phenotypes of the weak Scr^– alleles in Rg-bx^– larvae. Morphological observations of adult flies doubly heterozygous for Rg-bx and Scr mutations also suggest the enhancement of an aspect of Scr adult phenotypes. I conclude that the Rg-bx⁺ gene function is required for the optimal expression of the Scr gene in larval and imaginai cells.