Lineage, cell polarity and inscuteable function in the peripheral nervous system of the Drosophila embryo

The stereotyped pattern of the Drosophila embryonic peripheral nervous syst em (PNS) makes it an ideal system to use to identify mutations affecting ce ll polarity during asymmetric cell division. However, the characterisation of such mutations requires a detailed description of the polarity of the as ymmetric divisions in the sensory organ lineages. We describe the pattern o f cell divisions generating the vp1-vp4a mono-innervated external sense (es ) organs. Each sensory organ precursor (SOP) cell follows a series of four asymmetric cell divisions that generate the four es organs cells (the socke t, shaft, sheath cells and the es neurone) together with one multidendritic (md) neurone. This lineage is distinct from any of the previously proposed es lineages. Strikingly, the stereotyped pattern of cell divisions in this lineage is identical to those described for the embryonic chordotonal orga n lineage and for the adult thoracic bristle lineage. Our analysis reveals that the vp2-vp4a SOP cells divide with a planar polarity to generate a dor sal pIIa cell and a ventral pIIb cell. The pm, cell next divides with an ap ical-basal polarity to generate a basal daughter cell that differentiates a s an md neurone. We found that Inscuteable specifically accumulated at the apical pole of the dividing pIIb cell and regulated the polarity of the pII b division. This study establishes for the first time the function of Inscu teable in the PNS, and provides the basis for studying the mechanisms contr olling planar and apical-basal cell polarities in the embryonic sensory org an lineages.