Effect of abx, bx and pbx mutations on expression of homeotic genes in Drosophila larvae.

We have examined the patterns of expression of the homeotic gene Ubx in imaginal discs of Drosophila larvae carrying mutations in the abx, bx and pbx regulatory domains. In haltere discs, all five bx insertion mutations examined led to a general reduction in Ubx expression in the anterior compartment; for a given allele, the strength of the adult cuticle phenotype correlated with the degree of Ubx reduction. Deletions mapping near or overlapping the sites of bx insertions, including three abx alleles and the bx34e-prv(bx-prv) allele, showed greatly reduced Ubx expression in parts of the anterior compartment of the haltere disc; however, anterior patches of strong Ubx expression often remained, in highly variable patterns. As expected, the pbx1 mutation led to reduced Ubx expression in the posterior compartment of the haltere disc; surprisingly, pbx1 also led to altered expression of the en protein near the compartment border in the central region of the disc. In the metathoracic leg, all the bx alleles caused extreme reduction in Ubx expression in the anterior regions, with no allele-specific differences. In contrast, abx and bx-prv alleles resulted in patchy anterior reductions in third leg discs. In the larval central nervous system, abx but not bx alleles affected Ubx expression; the bx-prv deletion gave a wild-type phenotype, but it could not fully complement abx mutations. In the posterior wing disc, the bx-prv allele, and to a much lesser extent the bx34e chromosome from which it arose, led to ectopic expression of Ubx. Unlike other grain-of-function mutations in the BX-C, this phenotype appeared to be partially recessive to wild type. Finally, we asked whether the ppx transformation, which results from early lack of Ubx+ function in the mesothorax and is seen in abx animals, is due to ectopic Scr expression. Some mesothoracic leg and wing discs from abx2 larvae displayed ectopic expression of Scr, which was variable in extent but always confined to the posterior compartment.