The insect upper lip (labrum) is a nonsegmental appendage-like structure

Abstract

The insect upper lip--the labrum--is a lobe-like structure anterior to the mouth opening. Whether the labrum represents a fused pair of segmental appendages or evolved independently is heavily debated. Here, we identify additional similarities of the regulatory gene network active in labrum and trunk appendages. However, we do not find a labral parasegment boundary and we show that labral Tc-Dll expression is independent of Tc-wg and Tc-hh signals. In contrast, Tc-Dll expression in all trunk appendages does require these signals. Finally, we identify crucial differences between the location of the labrum and trunk appendages: the labrum develops in median rather than lateral tissues and is part of an anterior nonsegmental tissue marked by and dependent on Tc-six3 activity. To reconcile these seeming contradictory results, we propose that the genetic network evolved in either labrum or trunk appendages and became redeployed at a novel location to form the other structure.