The role of centrosomes and astral microtubules during asymmetric divisionof Drosophila neuroblasts

Drosophila neuroblasts are stem cells that divide asymmetrically to produce another large neuroblast and a smaller ganglion mother cell (GMC). During neuroblast division, several cell fate determinants, such as Miranda, Prosp ero and Numb, are preferentially segregated into the GMC, ensuring its corr ect developmental fate. The accurate segregation of these determinants reli es on proper orientation of the mitotic spindle within the dividing neurobl ast, and on the correct positioning of the cleavage plane, In this study we have analyzed the role of centrosomes and astral microtubules in neuroblas t spindle orientation and cytokinesis. We examined neuroblast division in a sterless (asl) mutants, which, although devoid of functional centrosomes an d astral microtubules, form well-focused anastral spindles that undergo ana phase and telophase, We show that asl neuroblasts assemble a normal cytokin etic ring around the central spindle midzone and undergo unequal cytokinesi s. Thus, astral microtubules are not required for either signaling or posit ioning cytokinesis in Drosophila neuroblasts. Our results indicate that the cleavage plane is dictated by the positioning of the central spindle midzo ne within the cell, and suggest a model on how the central spindle attains an asymmetric position during neuroblast mitosis, We have also analyzed the localization of Miranda during mitotic division of asl neuroblasts. This p rotein accumulates in morphologically regular cortical crescents but these crescents are mislocalized with respect to the spindle orientation. This su ggests that astral microtubules mediate proper spindle rotation during neur oblast division.