GENETIC-BASIS OF THE FORMATION AND IDENTITY OF TYPE-I AND TYPE-II NEURONS IN DROSOPHILA EMBRYOS

The embryonic peripheral nervous system of Drosophila contains two mai n types of sensory neurons: type I neurons, which innervate external s ense organs and chordotonal organs, and type II multidendritic neurons , Here, we analyse the origin of the difference between type I and typ e II in the case of the neurons that depend on the proneural genes of the achaete-scute complex (ASC), We show that, in Notch(-) embryos, th e type I neurons are missing while type nr neurons are produced in exc ess, indicating that the type I/type II choice relies on Notch-mediate d cell communication, In contrast, both type I and type II neurons are absent in numb(-) embryos and after ubiquitous expression of tramtrac k, indicating that the activity of numb and the absence of tramtrack a re required to produce both external sense organ and multidendritic ne ural fates, The analysis of string(-) embryos reveals that when the pr ecursors are unable to divide they differentiate mostly into type II n eurons, indicating that the type II is the default neuronal fate, We a lso report a new mutant phenotype where the ASC-dependent neurons are converted into-type II neurons, providing evidence for the existence o f one or more genes required for maintaining the alternative (type I) fate, Our results suggest that the same mechanism of type I/type II sp ecification may operate at a late step of the ASC-dependent lineages, when multidendritic neurons arise as siblings of the external sense or gan neurons and, at an early step, when other multidendritic neurons p recursors arise as siblings of external sense organ precursors.