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

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.