OBER, KA; JOCKUSCH, EL*; College of the Holy Cross; University of Connecticut: Conserved appendage morphology results from divergence in appendage developmental networks in insects

Appendage development has been well studied in Drosophila. However, Drosophila limbs are formed from imaginal discs, which are patterned during larval life, rather than developing directly during embryogenesis as in most other insects. A key step in appendage development is expression of Distal-less (Dll) in the distal appendage region. Although Dll expression and function promoting appendage outgrowth is highly conserved, its regulation is not understood in insects outside of Drosophila. We explored the expression and function of two genes important in Dll regulation and dorsoventral appendage patterning, wingless (wg) and decapentaplegic (dpp), in the red flour beetle, Tribolium castaneum. Here we show that the upstream components in the Dll regulatory pathway have diverged significantly between Tribolium and Drosophila and that there is significant divergence in regulation even within Tribolium castaneum. RNAi shows that wg is required for appendage formation and activation of Dll, except in the antennae and labrum, which are morphologically normal and express Dll in the absence of wg. Reduction of dpp does not affect appendage allocation nor is it required to activate Dll for limb outgrowth. Our results suggest differences in the appendage developmental pathway, specifically appendage allocation, proximodistal, and dorsoventral patterning between Tribolium and Drosophila. Much of comparative evolutionary development has focused on conservation of developmental networks despite divergence in morphology. Our data suggest the opposite: appendage morphology is conserved within insects; the underlying developmental networks however, have diverged significantly.