The correlation between dorsal and ventral segmental units in diplopod myriapods is complex and disputed. Recent results with engrailed (en), hedgehog (hh), wingless (wg), and cubitus-interruptus (ci) have shown that the dorsal segments are patterned differently from the ventral segments. Ventrally, gene expression is compatible with the classical autoregulatory loop known from Drosophila to specify the parasegment boundary. In the dorsal segments, however, this Wg/Hh autoregulatory loop cannot be present because the observed gene expression patterns argue against the involvement of Wg signalling. In this paper, we present further evidence against an involvement of Wg signalling in dorsal segmentation and propose a hypothesis about how dorsal segmental boundaries may be controlled in a wg-independent way. We find that (1) the Notum gene, a modulator of the Wg gradient in Drosophila, is not expressed in the dorsal segments. (2) The H15/midline gene, a repressor of Wg action in Drosophila, is not expressed in the dorsal segments, except for future heart tissue. (3) The patched (ptc) gene, which encodes a Hh receptor, is strongly expressed in the dorsal segments, which is incompatible with Wg-Hh autoregulation. The available data suggest that anterior–posterior (AP) boundary formation in dorsal segments could instead rely on Dpp signalling rather than Wg signalling. We present a hypothesis that relies on Hh-mediated activation of Dpp signalling and optomotor-blind (omb) expression to establish the dorsal AP boundary (the future tergite boundary). The proposed mechanism is similar to the mechanism used to establish the AP boundary in Drosophila wings and ventral pleura.
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We would like to thank Diethard Tautz for continuous support and Wolfgang Dohle, Alessandro Minelli and Bjarne Meidell for stimulating discussions on Glomeris segmentation. This work was mainly supported by the European Union via the Marie Curie Research and Training Network ZOONET (MRTN-CT-2004-005624). The work of G.E.B. was supported by the Swedish Research Council (VR) and the Swedish Royal Academy of Sciences (KVA). The work of W.G.M.D. was supported by the German Research Foundation (DFG) via SFB 572 of the University of Cologne. The work of N.M.P. was supported by a grant from the German Research Foundation (DFG grant PR 1109/1-1).
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