Neuroblast lineage identification and Hox gene action during postembryonic development of the tritocerebrum and subesophageal ganglion in the "Drosophila" brain

In this thesis, the neuroblast lineages which give rise to the adult-specific interneurons of two Drosophila brain regions, the tritocerebrum and the subesophageal ganglion, were described, and an analysis of the expression and function of the developmental control genes of the Hox family in these regions was performed. In the first study (chapter 2), the Hox gene and tritocerebral maker labial was used together with clonal genetic labeling to identify the 4 postembryonic neuroblast lineages of the tritocerebrum. Subsequently, clonal mutation was used to show that expression of labial is required for timely-precise apoptosis of 2 neuroblasts, which otherwise give rise to outgrowing ectopic lineages in the tritocerebrum. In the second study (Chapter 3), the postembryonic subesophageal ganglion was investigated. First, markers for cell adhesion molecules and synapses were used to characterize the neuronanatomical structures of the subesophageal ganglion in respect to thoracic neuromeres. In this analysis, a small number of secondary axon tracts were found. Second, genetic labeling was used to identify the clonal basis for this reduction and a number of only 14 subesophageal neuroblast lineages were identified in the late larval subesophageal ganglion. Third, the subesophageal neuroblasts were traced through development, and a severe reduction in neuroblast number was found in this region to take place between middle embryonic and late larval stages. Fourth, the Hox genes Deformed, Sex combs reduced and Antennapedia were shown to be expressed in discrete domains in the subesophageal ganglion, and most of the subesophageal lineages in the late larva were found to be positive for one of these Hox proteins. Fifth, clonal genetic knockout was used to show that the Dfd, Scr and Antp genes are required for three lineage-specific functions in the subesophageal ganglion: proper axonal targeting of 2 lineages, correct cell number in 2 lineages and termination of 5 neuroblasts via programmed cell death. In chapter 4, the expression and function of Hox genes in the Drosophila nervous system was reviewed in the context of genetic interactions and evolutionary conservation. In summary, postembryonic generation of adult-specific neurons in the tritocerebral and subesophageal brain regions of Drosophila was shown to be mediated by a small number of neuroblast lineages and to involve three lineage-specific functions of Hox proteins.