Cross-regulation of Hox genes in the Drosophila melanogaster embryo.

Cross-regulation of Homeotic Complex (Hox) genes by ectopic Hox proteins during the embryonic development of Drosophila melanogaster was examined using Gal4 directed transcriptional regulation. The expression patterns of the endogenous Hox genes were analyzed to identify cross-regulation while ectopic expression patterns and timing were altered using different Gal4 drivers. We provide evidence for tissue specific interactions between various Hox genes and demonstrate the induction of endodermal labial (lab) by ectopically expressed Ultrabithorax outside the visceral mesoderm (VMS). Similarly, activation and repression of Hox genes in the VMS from outside tissues seems to be mediated by decapentaplegic (dpp) gene activation. Additionally, we find that proboscipedia (pb) is activated in the epidermis by ectopically driven Sex combs reduced (Scr) and Deformed (Dfd); however, mesodermal pb expression is repressed by ectopic Scr in this tissue. Mutant analyses demonstrate that Scr and Dfd regulate pb in their normal domains of expression during embryogenesis. Ectopic Ultrabithorax and Abdominal-A repress only lab and Scr in the central nervous system (CNS) in a timing dependent manner; otherwise, overlapping expression in the CNS in tolerated. A summary of Hox gene cross-regulation by ectopically driven Hox proteins is tabulated for embryogenesis.