BMP signaling likely had an ancestral role in providing global embryonic dorsal-ventral polarity in insects. Jeremy A. Lynch^1,2, Orhan Özüak², Thomas Buchta², Siegfried Roth². 1) Molecular, Cell, and Developmental Biology, University of Illinois at Chicago, Chicago, IL; 2) Institute for Developmental Biology, University of Cologne, Cologne, Germany.

   In Drosophila, the Toll signaling pathway plays the dominant role in patterning the embryo along the dorsal-ventral axis. The use of Toll signaling is not typical of most animals, and in fact a role for Toll in early axis formation has only been demonstrated in holometabolous insects. Most other animals depend heavily on BMP signaling for establishment of DV polarity and for patterning the germ layers. While this pathway plays an important role in Drosophila embryonic patterning, it is subordinate to Toll, and its function is restricted to the most dorsal regions of the embryo. Since the dependence on Toll appears to be a state derived within the insects, it is of interest to know when this trait arose in evolution. To address this, we examined the embryonic patterning system of the wasp Nasonia, which represents the most basally branching lineage of holometabolous insects, but undergoes an independently derived mode of embryogenesis that is highly similar to that of Drosophila. We have found that while Toll signaling plays a role in establishing ventral cell fates, BMP signaling (revealed by dpp RNAi) is crucial to provide global DV polarity to the embryo. We also have found evidence that a major function of BMP signaling is to repress an autoregulatory loop on the ventral side that is initiated by Toll signaling.