Revisiting the Drosophila microchaete lineage: a novel intrinsically asymmetric cell division generates a glial cell

The bristle mechanosensory organs of the adult fly are composed of four dif ferent cells that originate from a single precursor cell, pI, via two round s of asymmetric cell division. Here, we have examined the pattern of cell d ivisions in this lineage by time-lapse confocal microscopy using GFP imagin g and by immunostaining analysis. pi divided within the plane of the epithe lium and along the anteroposterior axis to give rise to an anterior cell, p IIb, and a posterior cell, pIIa, pIIb divided prior to pIIa to generate a s mall subepithelial cell and a larger daughter cell, named pIIIb. This unequ al division, oriented perpendicularly to the epithelium plane, has not been described previously, pIIa divided after pIIb, within the plane of the epi thelium and along the AP axis, to produce a posterior socket cell and an an terior shaft cell. Then pIIIb divided perpendicularly to the epithelium pla ne to generate a basal neurone and an apical sheath cell. The small subepit helial pIIb daughter cell was identified as a sense organ glial cell: it ex pressed glial cell missing, a selector gene for the glial fate and migrated away from the sensory cluster along extending axons. We propose that mecha nosensory organ glial cells, the origin of which was until now unknown, are generated by the asymmetric division of pIIb cells, Both Numb and Prospero segregated specifically into the basal glial and neuronal cells during the pIIb and pIIIb divisions, respectively. This revised description of the se nse organ lineage provides the basis for future studies on how polarity and fate are regulated in asymmetrically dividing cells.