Heterotrimeric G proteins regulate daughter cell size asymmetry in Drosophila neuroblast divisions

division often generates unequally sized daughter cells by off-center cleavages, which are due to either displacement of mitotic spindles or their asymmetry. Drosophila neuroblasts predominantly use the latter mechanism to divide into a large apical neuroblast and a small basal ganglion mother cell (GMC), where the neural fate determinants segregate. Apically localized components regulate both the spindle asymmetry and the localization of the determinants. Here, we show that asymmetric spindle formation depends on signaling mediated by the GP subunit of heterotrimeric G proteins. Gbeta13F distributes throughout the neuroblast cortex. Its lack induces a large symmetric spindle and causes division into nearly equal-sized cells with normal segregation of the determinants. In contrast, elevated Gbeta13F activity generates a small spindle, suggesting that this factor suppresses spindle development. Depletion of the apical components also results in the formation of a small symmetric spindle at metaphase. Therefore, the apical components and Gbeta13F affect the mitotic spindle shape oppositely. We propose that differential activation of GP signaling biases spindle development within neuroblasts and thereby causes asymmetric spindles. Furthermore, the multiple equal cleavages of GP mutant neuroblasts accompany neural defects; this finding suggests indispensable roles of eccentric division in assuring the stem cell properties of neuroblasts.