Proboscis identity is specified by proboscipedia and Sex combs reduced via repression of appendage patterning genes. S.L. Holtzman , A. Abzhanov , A. Aplin , J.K. Cecil , T.C. Kaufman. Howard Hughes Medical Institute, Indiana University, Bloomington, IN (812)-855-7674.

   pb and Scr are required for proper mouthpart identity in Drosophila. Hypomorphs in pb display some pseudotracheal rows with aristae; pb nulls form tarsi with no pseudotracheal rows. Scr mutant mouthparts display maxillary palp identity. pb Scr double mutants lead to the development of complete antennae. We have examined the expression patterns of several appendage patterning genes (al, BarHI, dac, Dll, exd, hth, Sal and wg) in wild-type, pb, Scr, and pb Scr double mutant labial discs. We find that Pb is necessary and sufficient for the repression of Dac, Exd, and Sal in imaginal discs; Pb also down-regulates Dll and Wg in labial discs. Surprisingly, Pb activates Scr in imaginal discs. Examination of Scr in pb mutants shows low levels of Scr in the distal half of the labial disc in pb nulls and low levels at the distal tip in pb hypomorphs. Analysis of Scr's function in the labial discs reveals that Scr down-regulates Dll and Wg; Scr can also repress Sal expression in the eye-antennal disc when ectopically expressed. pb Scr double mutant labial discs express the appendage genes consistent with antennal identity. While many of the genes examined are repressed or down-regulated by pb and Scr, the distal appendage genes, al and BarHI, are still expressed in diminutive domains in the labial disc. This further demonstrates that the labial disc produces a highly modified appendage, which retains distal homology with legs and antennae, while it represses medial and proximal appendage development. We conclude that the proboscis of Drosophila is a unique appendage that does not require many of the appendage genes for its development.