Pan-leg developmental regulators control pro-thoracic leg specific Scr expression. Christopher L McCallough, Ece Eksi, Emily R Wyskiel, Teresa V Orenic. Biology, University of Illinois at Chicago, Chicago, IL.

   The Drosophila adult has one pair of legs on each of its three thoracic segments (T1-T3). Although these structures exhibit serial homology, the legs from different segments have distinct morphological features. One such feature is the patterning of the peripheral nervous system in the form of small mechanosensory bristles (mCs). In the T2 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 T1 leg, the L-rows are replaced by a group of mCs organized into transverse rows (T-rows) [1,2]. Studies have indicated that the position of T-row bristles on the tibia and basitarsus of T1 legs is established as a result of Hox gene modification of the L-row patterning pathway [3,4]. In T1 prepupal legs, Sex combs reduced (Scr) is expressed at elevated levels within the T-row primordia. We have found that Scr modifies the mC pattern on T1 legs via repression of Delta, a key regulator of leg mC patterning [4]. Our model for T-row patterning suggests that a central step in this process is establishment of spatially defined Scr expression within defined domains of the leg primordium in response to the global regulators of leg development. The mechanisms that generate morphological diversity among the legs will therefore require an understanding of the regulation of Scr in the T-row primordium. Here we will present our genetic studies on the regulation of Scr by genes known to pattern the leg along its circumference and P/D axis.