Polarity in Cell Division What Frames Thy Fearful Asymmetry?

activity to make two daughter cells different from each San Francisco, California 94143 other, although its function in GMC is not clear. Prospero is a homeodomain-containing protein that controls cell fate by functioning as a transcriptional regulator (re-Asymmetric cell division is a basic means for generating viewed by Lu et al., 2000). cell fate diversity. In both unicellular and multicellular The peripheral nervous system (PNS) neural precur-organisms, asymmetric cell division often results from sors are called sensory organ precursors (SOPs). The preferential segregation of cell fate determinant(s) into body of an adult fruit fly is covered with sensory bristles, one of the two daughter cells. Among eukaryotes, sev-each one derived from a single SOP. The back of the eral biological processes have emerged as useful model thorax, called the notum, is derived from the wing imagi-systems for studying the mechanisms of asymmetric nal disc, a monolayer of epithelial cells. During pupal cell division, including the budding of the yeast Saccha-development, a subset of these epithelial cells are sin-romyces cerevisiae, the asymmetric division of the neural gled out to become SOPs. Each SOP divides asymmetri-precursors of the fruit fly Drosophila melanogaster, and cally into two secondary precursors, IIa and IIb, which the asymmetric cleavages during early embryogenesis then divide asymmetrically once and twice, respectively, of the worm Caenorhabditis elegans. Studies of those to produce a total of five progeny. One migrates away model systems have revealed a number of general fea-as a glial cell (Gho et al., 1999). The remaining four cells, tures of asymmetric cell division: a polarity cue is first a neuron, a sheath cell, a socket cell, and a hair cell, set up in the mother cell, the cell fate determinants form the sensory bristle. The asymmetric division of the (either mRNAs or proteins) are then localized with re-SOP shares many features with NB division. The SOP spect to this cue, this localization requires actin fila-division also produces two daughters of different size. ments and certain myosin motors, and the axis of divi-Some of the same cell fate determinants such as Numb sion is lined up with respect to the same cue so that and their adapter proteins, which serve to localize the the localized determinants are partitioned preferentially cell fate determinants, are employed. Those molecules into one of the two progeny cells. Additional features form a crescent at one pole of the SOP as it enters exist …