Genetic analysis of two types asymmetric divisions of sensory organ precursor cell lineage. F. Roegiers , S. Younger-Shepherd , L.Y. Jan , Y.N. Jan. HHMI/Dept Physiology, UCSF, San Francisco, CA.

   Asymmetric partitioning of cell fate determinants during development requires the coordination of positioning of these determinants with orientation of the mitotic spindle. In the Drosophila peripheral nervous system (PNS), sensory organ progenitor cells (SOPs) undergo several rounds of asymmetric division to generate five cells of distinct fates; a bristle, a socket, a sheath, a neuron and glial cell. In each division within this lineage the Numb protein, a cell fate determinant, is asymmetrically localized. We observed the asymmetric divisions that give rise to these cells in the developing pupae using GFP-fusion proteins specific to the mitotic spindle (Tau-GFP) and to determinant localization (Partner of Numb-GFP). Using these reporters we analyzed the coordination of spindle orientation and determinant crescent formation. Furthermore, we found two types of asymmetric divisions exist within the SOP lineage: the anterior-posterior pI cell-type division, where the spindle remains symmetric throughout mitosis, and the strikingly neuroblast-like apical-basal division of the pIIb cell, where the spindle exhibits a strong asymmetry at anaphase. We have shown that frizzled, a gene involved in determining planar polarity along the anterior-posterior axis, contributes to the stabilization of the Pon crescent as well as the coordination of mitotic spindle and positioning of the crescent in the first SOP division. In contrast, the correct apicobasal orientation of the spindle in the following pIIb division requires Inscuteable, a protein required for coordinating spindle orientation determinant localization in the neuroblast lineage. We are currently using the MARCM system (Mosaic Analysis with a Repressible Cell Marker: Lee, T. and Luo, L., 1999; Neuron, 22 451-461) to analyze mitotic clones of other genes involved in tissue polarity as well as genes involved in regulating the cytoskeleton to observe their effect on asymmetric cell divisions within the SOP lineage.