ventral veins lacking is required for specification of the tritocerebrum in embryonic brain development of Drosophila

The homeotic or Hox genes encode a network of conserved transcription factors which provide axial positional information and control segment morphology in development and evolution. During embryonic brain development of Drosophila, the Hox gene labial (lab) is essential for tritocerebral neuromere specification; lab loss of function results in tritocerebral cells that fail to adopt a neuronal identity, causing axonal pathfinding defects. Here we present evidence that the POU-homeodomain DNA-binding protein ventral veins lacking (vvl) acts genetically downstream of lab in the specification of the tritocerebral neuromere. In the embryonic brain, vvl expression is seen in all brain neuromeres, including the tritocerebral lab domain. Lab mutant analysis shows that vvl expression in the tritocerebrum is dependent on lab activity. Loss-of-function analysis focussed on the tritocerebrum reveals that inactivation of vvl results in patterning defects which are comparable to the brain phenotype caused by null mutation of lab. In the absence of vvl, mutant tritocerebral cells are generated and positioned correctly, but these cells fail to express neuronal markers indicating defects in neuronal differentiation. Moreover, longitudinal axon pathways in the tritocerebrum are severely reduced or absent and the tritocerebral commissure is missing in the vvl mutant brain. Genetic rescue experiments show that vvl is able to partially replace lab in the specification of the tritocerebral neuromere. Our results indicate that vvl acts downstream of the Hox gene lab and regulates specific aspects of neuronal differentiation within the tritocerebral neuromere during embryonic brain development of Drosophila.