Proneural and abdominal Hox inputs synergize to promote sensory organ formation in the Drosophila abdomen

The atonal (ato) proneural gene specifies a stereotypic number of sensory organ precursors (SOP) within each body segment of the Drosophila ectoderm. Surprisingly, the broad expression of Ato within the ectoderm results in only a modest increase in SOP formation, suggesting many cells are incompetent to become SOPs. Here, we show that the SOP promoting activity of Ato can be greatly enhanced by three factors: the Senseless (Sens) zinc finger protein, the Abdominal-A (Abd-A) Hox factor, and the epidermal growth factor (EGF) pathway. First, we show that expression of either Ato alone or with Sens induces twice as many SOPs in the abdomen as in the thorax, and do so at the expense of an abdomen-specific cell fate: the larval oenocytes. Second, we demonstrate that Ato stimulates abdominal SOP formation by synergizing with Abd-A to promote EGF ligand (Spitz) secretion and secondary SOP recruitment. However, we also found that Ato and Sens selectively enhance abdominal SOP development in a Spitz-independent manner, suggesting additional genetic interactions between this proneural pathway and Abd-A. Altogether, these experiments reveal that genetic interactions between EGF-signaling, Abd-A, and Sens enhance the SOP-promoting activity of Ato to stimulate region-specific neurogenesis in the Drosophila abdomen.     

Direct regulation of the muscle-identity gene apterous by a Hox protein in the somatic mesoderm

Hox genes control segment identity in the mesoderm as well as in other tissues. Most evidence indicates that Hox genes act cell-autonomously in muscle development, although this remains a controversial issue. We show that apterous expression in the somatic mesoderm is under direct Hox control. We have identified a small enhancer element of apterous (apME680) that regulates reporter gene expression in the LT1-4 muscle progenitors. We show that the product of the Hox gene Antennapedia is present in the somatic mesoderm of the second and third thoracic segments. Through complementary alterations in the Antennapedia protein and in its binding sites on apME680, we show that Antennapedia positively regulates apterous in a direct manner, demonstrating unambiguously its cell-autonomous role in muscle development. Finally, we determine that LT1-4 muscles contain more nuclei in the thorax than in the abdomen and we propose that one of the segmental differences under Hox control is the number of myoblasts allocated to the formation of specific muscles in different segments.