Differential Delta expression underlies the diversity of sensory organ patterns among the legs of the Drosophila adult.

Many studies have shown that morphological diversity among homologous animal structures is generated by the homeotic (Hox) genes. However, the mechanisms through which Hox genes specify particular morphological features are not fully understood. We have addressed this issue by investigating how diverse sensory organ patterns are formed among the legs of the Drosophila melanogaster adult. The Drosophila adult has one pair of legs on each of its three thoracic segments (the T1-T3 segments). Although homologous, legs from different segments have distinct morphological features. Our focus is on the formation of diverse patterns of small mechanosensory bristles or microchaetae (mCs) among the legs. On T2 legs, the mCs are organized into a series of longitudinal rows (L-rows) precisely positioned along the leg circumference. The L-rows are observed on all three pairs of legs, but additional and novel pattern elements are found on T1 and T3 legs. For example, at specific positions on T1 and T3 legs, some mCs are organized into transverse rows (T-rows). Our studies indicate that the T-rows on T1 and T3 legs are established as a result of Hox gene modulation of the pathway for patterning the L-row mC bristles. Our findings suggest that the Hox genes, Sex combs reduced (Scr) and Ultrabithorax (Ubx), establish differential expression of the proneural gene achaete (ac) by modifying expression of the ac prepattern regulator, Delta (Dl), in T1 and T3 legs, respectively. This study identifies Dl as a potential link between Hox genes and the sensory organ patterning hierarchy, providing insight into the connection between Hox gene function and the formation of specific morphological features.

Delta and Hairy establish a periodic prepattern that positions sensory bristles in Drosophila legs.

In vertebrates and invertebrates, spatially defined proneural gene expression is an early and essential event in neuronal patterning. In this study, we investigate the mechanisms involved in establishing proneural gene expression in the primordia of a group of small mechanosensory bristles (microchaetae), which on the legs of the Drosophila adult are arranged in a series of longitudinal rows along the leg circumference. In prepupal legs, the proneural gene achaete (ac) is expressed in longitudinal stripes, which comprise the leg microchaete primordia. We have previously shown that periodic ac expression is partially established by the prepattern gene, hairy, which represses ac expression in four of eight interstripe domains. Here, we identify Delta (Dl), which encodes a Notch (N) ligand, as a second leg prepattern gene. We show that Hairy and Dl function concertedly and nonredundantly to define periodic ac expression. We also explore the regulation of periodic hairy expression. In prior studies, we have found that expression of two hairy stripes along the D/V axis is induced in response to the Hedgehog (Hh), Decapentaplegic (Dpp) and Wingless (Wg) morphogens. Here, we show that expression of two other hairy stripes along the orthogonal A/P axis is established through a distinct mechanism which involves uniform activation combined with repressive influences from Dpp and Wg. Our findings allow us to formulate a general model for generation of periodic pattern in the adult leg. This process involves broad and late activation of ac expression combined with refinement in response to a prepattern of repression, established by Hairy and Dl, which unfolds progressively during larval and early prepupal stages.