Regulation of Drosophila Sex Combs Reduced Expression in Prothoracic Leg-Specific Sense Organ Primordia

The Drosophila adult has one pair of legs on each of its three thoracic segments, and although these structures are similar, the legs from different segments have distinct morphological features. One such feature is the patterning of the microchaete bristles (mCs), which are components of the peripheral nervous system. In the mesothoracic leg these mCs are organized into a series of longitudinal rows (L-rows) along the circumference of the tibia and tarsal segments. However, at specific positions along the circumference and proximal/distal axis of the prothoracic and metathoracic legs, the L-rows are replaced by a group of mCs organized into transverse rows (T-rows). Studies have indicated that the position of T-row bristles is established as a result of homeotic gene alteration of the L-row patterning pathway. In prothoracic prepupal legs, the homeotic gene, Sex combs reduced (Scr), is expressed at elevated levels corresponding to the T-row primordia. The complex interactions involved in regulating elevated homeotic gene expression are not fully understood. In an effort to address this question, I have investigated the genetic regulation of elevated Scr expression within the T-row primordia of Drosophila prothoracic legs. Here l present the genetic studies on the regulation of Scr by genes known to pattern the leg along its circumference and proximal/distal (Pr/Di) axis, contributing to its spatially defined expression along all three axes of the leg. Based on this data, a genetic model has been suggested that illustrates the regulatory inputs responsible for the expression of elevated Scr in the prothoracic Drosophila leg. Co-expression of the transcription factors, Dll and Dac, specify a medial leg fate and function to activate upregulated Scr expression in the distal tibia and basitarsus. Bab regulates Scr along the Pr/Di axis by inhibiting its expression in the second tarsal segment. The global regulators of leg development function to restrict elevated Scr expression to the antero-ventral T-row primordia; En represses Scr in the posterior compartment, Dpp represses in the dorsal territory, and Wg functions indirectly through its antagonistic relationship with Dpp to alleviate repression in the ventral territory. These observations provide general insight into how a gene responds to and interprets patterning information in three dimensions. In addition, our findings suggest that generation of the distinct sense organ pattern of prothoracic legs involves integration of a homeotic gene as a key component of the leg sense organ patterning pathway.