Prospero distinguishes sibling cell fate without asymmetric localization in the Drosophila adult external sense organ lineage

The adult external sense organ precursor (SOP) lineage is a model system fo r studying asymmetric cell division. Adult SOPs divide asymmetrically to pr oduce IIa and IIb daughter cells; IIa generates the external socket (tormog en) and hair (trichogen) cells, while IIb generates the internal neuron and sheath (thecogen) cells. Here we investigate the expression and function o f prospero in the adult SOP lineage. Although Prospero is asymmetrically lo calized in embryonic SOP lineage, this is not observed in the adult SOP lin eage: Prospero is first detected in the IIb nucleus and, during IIb divisio n, it is cytoplasmic and inherited by both neuron and sheath cells. Subsequ ently, Prospero is downregulated in the neuron but maintained in the sheath cell, Loss of prospero function leads to 'double bristle' sense organs (re flecting a IIb-to-IIa transformation) or 'single bristle' sense organs with abnormal neuronal differentiation (reflecting defective IIb development). Conversely, ectopic prospero expression results in duplicate neurons and sh eath cells and a complete absence of hair/socket cells (reflecting a IIa-to -IIb transformation). We conclude that (1) despite the absence of asymmetri c protein localization,prospero expression is restricted to the IIb cell bu t not its IIa sibling, (2) prospero promotes IIb cell fate and inhibits IIa cell fate, and (3) prospero is required for proper axon and dendrite morph ology of the neuron derived from the IIb cell. Thus, prospero plays a funda mental role in establishing binary IIa/IIb sibling cell fates without being asymmetrically localized during SOP division. Finally, in contrast to prev ious studies, we find that the IIb cell divides prior to the IIa cell in th e SOP lineage.