Frizzled regulates localization of cell-fate determinants and mitotic spindle rotation during asymmetric cell division

Cell-fate diversity is generated in part by the unequal segregation of cell -fate determinants during asymmetric cell divisions. In the Drosophila pupa , the pI sense organ precursor cell is polarized along the anterior-posteri or axis of the fly and divides asymmetrically to generate a posterior pIIa cell and an anterior pIIb cell. The anterior pIIb cell specifically inherit s the determinant Numb and the adaptor protein Partner of Numb (Pon). By la belling both the Pon crescent and the microtubules in living pupae, we show that determinants localize at the anterior cortex before mitotic-spindle f ormation; and that the spindle forms with random orientation and rotates to line up with the Pon crescent. By imaging living frizzled (fz) mutant pupa e we show that Fz regulates the orientation of the polarity axis of pI, the initiation of spindle rotation and the unequal partitioning of determinant s. We conclude that Fz participates in establishing the polarity of pI.