Interaction of the Stubble-stubbloid locus and the Broad-Complex of Drosophila melanogaster

The 2B5 region on the X chromosome of Drosophila melanogaster forms an early ecdysone puff at the end of the third instar. The region is coextensive with a complex genetic locus, the Broad-Complex (BR-C). The BR-C is a regulatory gene that contains two major functional domains, the br domain and the l(l)2Bc domain. BR-C mutants prevent metamorphosis, including morphogenesis of imaginal discs; br mutants prevent elongation and eversion of appendages and l(l)2Bc mutants prevent fusion of the discs. The Stubble-stubbloid (SB-sbd) locus at 89B9-10 is best known for the effects of its mutants on bristle structure. Mutants of the BR-C and the SB-sbd locus interact to produce severe malformation of appendages. Viable heteroallelic and homoallelic combinations of 5B-sbd mutants, including loss-of-function mutants, affect the elongation of imaginal disc appendages. Thus, the SB-sbd+ product is essential for normal appendage elongation. SB-sbd mutants, however, do not affect eversion or fusion of discs. Correspondingly, only BR-C mutants deficient in br function interact with SB-sbd mutants. The interaction occurs in deficiency heterozygotes using single, wild-type doses of the BR-C, of the SB-sbd locus, or of both loci. These last results are formally consistent with the possibility that the BR-C acts as a positive regulator of the SB-sbd locus. The data do not exclude other possible nonregulatory interactions between the two loci, e.g., interactions between the products of both genes.