Thesis #746

The cephalic gap gene empty spiracles (ems) encodes a homeodomain transcription factor that is essential for the regional specification of the early embryonic brain in Drosophila. This thesis presents the analysis of ems expression and function during larval and pupal development of the brain. In the late larval brain eight neuroblast lineages express ems. In seven lineages ems is only transiently expressed and expression disappears in the early pupa. In contrast, all adult-specific neurons of the medial-most lineage (EM lineage) continuously express ems throughout larval and pupal development as well as in the adult brain.
In a first study (Chapter II) we have investigated the function of ems in the EM lineage. The cell bodies of the EM lineage are located ventral to the antennal lobes from where they extend fine neurite arborizations into the suboesophageal ganglion and a prominent projection into the superior medial protocerebrum. Clonal mutant analysis of the adult-specific cells in the EM lineage has revealed three distinct functions of ems during larval development. First, the number of cells was reduced by half. This could be rescued by blocking apoptosis in ems mutant clones suggesting a function of ems in cell survival. Second, all mutant clones extended undirected misprojections into the surrounding neuropile. Third, the projection into the superior protocerebrum was missing in half of the clones. A closer examination of the projection patterns of ems mutant single-cell clones demonstrated that ems is required cell-autonomously in postmitotic neurons for the correct extension of the protocerebral projection.
In our second study (Chapter III) we have examined the role of ems in development of the olfactory projection neurons (PNs). Two of the transiently expressing ems-positive lineages in the larval brain correspond to the adult-specific anterodorsal and lateral PN lineages (adPN and lPN, respectively). Clonal mutant analysis of the GH146-positive PNs revealed different roles of ems in the two lineages. In the adPN lineage transient ems expression is required for precise dendritic targeting. In the lPN lineage ems function is necessary for the formation of the correct number of progeny during larval development. Furthermore, timely down-regulation of ems expression is necessary for the proper connectivity of PNs.
The finding that ems and its mammalian homologs Emx1/Emx2 are both expressed in second order olfactory PNs suggests conserved genetic mechanisms for the specific relay of olfactory information to higher brain centres.