ASYMMETRIC DISTRIBUTION OF NUMB PROTEIN DURING DIVISION OF THE SENSORY ORGAN PRECURSOR CELL CONFERS DISTINCT FATES TO DAUGHTER CELLS

The four cells of an external sense organ in the Drosophila peripheral nervous system, the neuron, its sheath cell, and two ''outer support cells'' that form the hair and socket, are derived from a common precu rsor, the sensory organ precursor (SOP), after two rounds of division. We determined by immunocytochemistry that numb is a membrane-associat ed protein which localizes asymmetrically to one-half of the predivisi onal SOP cell. Upon division, numb segregates differentially to one da ughter. Loss of numb function causes the descendants of the SOP to dif ferentiate inappropriately, producing four outer support cells and no neuron or sheath. Ectopic expression of numb during the time of SOP di vision results in a transformation that is opposite to the null mutant transformation. Thus, numb functions to determine the fates of the se condary precursors; the differential distribution of numb as the SOP d ivides generates an asymmetric division in which the daughter cells ac quire distinct identities.