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

@entity24007 @entity70839
The adult external sense organ precursor (SOP) lineage is a model system for studying asymmetric cell division. Adult SOPs divide asymmetrically to produce IIa and @entity70839 daughter cells; IIa generates the external socket (tormogen) and hair (trichogen) cells, while @entity70839 generates the internal neuron and sheath (thecogen) cells. Here we investigate the expression and function of @entity24007 in the adult SOP lineage. Although @entity24007 is asymmetrically localized in embryonic SOP lineage, this is not observed in the adult SOP lineage: @entity24007 is first detected in the @entity70839 nucleus and, during @entity70839 division, it is cytoplasmic and inherited by both neuron and sheath cells. Subsequently, @entity24007 is downregulated in the neuron but maintained in the sheath cell. Loss of @entity24007 function leads to 'double bristle' sense organs (reflecting a @entity70839 -to-IIa transformation) or 'single bristle' sense organs with abnormal neuronal differentiation (reflecting defective @entity70839 development). Conversely, ectopic @entity24007 expression results in duplicate neurons and sheath cells and a complete absence of hair/socket cells (reflecting a IIa-to- @entity70839 transformation). We conclude that (1) despite the absence of asymmetric protein localization, @entity24007 expression is restricted to the @entity70839 cell but not its IIa sibling, (2) @entity24007 promotes @entity70839 cell fate and inhibits IIa cell fate, and (3) @entity24007 is required for proper axon and dendrite morphology of the neuron derived from the @entity70839 cell. Thus, @entity24007 plays a fundamental role in establishing binary IIa/ @entity70839 sibling cell fates without being asymmetrically localized during SOP division. Finally, in contrast to previous studies, we find that the @entity70839 cell divides prior to the IIa cell in the SOP lineage.
