Notch signaling induces cell proliferation in the labrum in a regulatory network different from the thoracic legs

The insect head is composed of several segments and an anterior non-segmental region. While patterning of the segmental region relies - at least in part - on the known trunk patterning mechanisms, development of the anterior most region remains poorly understood. The labrum is an enigmatic structure of the anterior median region (AMR) of the insect head. Based on similar development and gene expression patterns it has been suggested to be a serial homolog of trunk appendages. However, its position in the non-segmental region indicated an independent origin. In order to learn more about development of the AMR including the labrum, we screened the results of the large scale RNAi screen iBeetle to identify novel genes. We found the Notch ligand Tc-Serrate and the ubiquitin ligase Tc-mind bomb1 to be required for labrum formation. Further studies showed that Notch signaling is acting upstream of the genetic hierarchy and is required for regulating cell proliferation. We combined our work with previous data to compare the regulatory gene networks of labrum and trunk appendage formation. This reveals that despite the involvement of a similar set of genes, the genetic interactions are quite different.