Interaction between spineless-aristapedia gene and genes from Antennapedia and bithorax complexes of Drosophila melanogaster.

Mutations in the spineless-aristapedia (ssa) gene of Drosophila melanogaster are pleiotropic, and their classical manifestations include a reduction in size of all bristles (spineless phenotype), transformation of distal parts of antennae into tarsal segments of the mesothoracic leg (aristapedia phenotype), and, in extreme alleles, fusion of tarsal segments on all six legs and the transformed aristaes. We isolated a new allele, which is a severe loss-of-function mutation and, in addition to the above-mentioned features, is characterized by amplification of sex combs on the first leg. This phenotype can be caused by a change in the expression of the Sex combs reduced (Scr) gene of the ANTP-C. Identification of this phenotype, together with observed variations in the extent of the fusion of tarsal segments in the legs of different segments, raised the possibility that ssa interacts with homeotic genes controlling the identity of segments. This possibility was tested in genetical experiments using flies with loss-of-function mutations in several homeotic genes and flies transformed by heat shock-driven homeotic genes. Analysis of adult phenotypes of different ssa alleles in the background of under-, over-, or ectopic expression of some genes of BX-C and ANT-C suggests that the ssa product is required to prevent the effect of the homeotic gene products in the distal segments of the appendages.