Specific expression and function of the Six3 in serially homologous organs.

Organ size and pattern results from the integration of two positional information systems. One global information system, encoded by the Hox genes, links organ type with position along the main body axis. Within specific organs, local information is conveyed by signaling molecules that regulate organ growth and pattern. The mesothoracic (T2) wing and the metathoracic (T3) haltere of represent a paradigmatic example of this coordination. The Hox gene (), expressed in the developing T3, selects haltere identity by, among other processes, modulating the production and signaling efficiency of Dpp, a BMP2-like molecule that acts as a major regulator of size and pattern. However, the mechanisms of the Hox-signal integration in this well-studied system are incomplete. Here, we have investigated this issue by studying the expression and function of the Six3 transcription factor during wing and haltere development. We find that in both organs, Dpp defines the expression domain of through repression, and that the specific position of this domain in wing and haltere seems to reflect the differential signaling profile among these organs. We show that expression in wing and haltere primordia is conserved beyond in other higher diptera In , is necessary for the growth of wing and haltere. In the wing, is required for the growth of the most anterior/proximal region (the 'marginal cell') and for the correct formation of sensory structures along the proximal anterior wing margin; the halteres of mutants are also significantly reduced. In addition, in the haltere, is necessary for the suppression of sensory bristles.