Expression of Drosophila BarH1-H2 homeoproteins in developing dopaminergic cells and segmental nerve a (SNa) motoneurons.

Barh1/h2 genes encode two related homeobox transcription factors (B-H1 and B-H2) previously shown to play essential roles in the formation and specification of the distal leg segments and in retinal neurogenesis. Here we describe the restricted expression pattern of the B-H1/-H2 homeoprotein within the embryonic ventral nerve cord of Drosophila. We show that B-H1/-H2 are specifically expressed in a subset of dopaminergic neurons, namely the unpaired ventral midline dopaminergic neuron, and in a subpopulation of laterally projecting motoneurons, i.e. the five motoneurons forming the segmental nerve a (SNa) branch. Using the GAL4-UAS system we show that B-H1/-H2(Gal4) in combination with a membrane-targeted enhanced green fluorescent protein reporter line provides a powerful genetic tool reproducibly to label SNa motoneuron projections and terminals at the periphery, and their dendritic tree in the ventral nerve cord. Thus, the highly restricted expression pattern of the B-H1/-H2 homeoproteins and notably the related Gal4 driver represent powerful genetic tools to identify and study genes that control axon guidance, synaptogenesis or dendritic arborization within a small subpopulation of motoneurons identifiable from embryogenesis to late larval stages.