Pins is not Required for Spindle Orientation in the $\textit{Drosophila}$ Wing Disc

Abstract

In animal cells, mitotic spindles are oriented by the dynein/dynactin motor complex, which exerts a pulling force on astral microtubules. Dynein/dynactin localization depends on Mud/NUMA, which is typically recruited to the cortex by Pins/LGN. In $\textit{Drosophila}$ neuroblasts, the Inscuteable/Baz/Par-6/aPKC complex recruits Pins apically to induce vertical spindle orientation, whereas in epithelial cells, Dlg recruits Pins laterally to orient the spindle horizontally. Here we investigate division orientation in the $\textit{Drosophila}$ imaginal wing disc epithelium. Live imaging reveals that spindle angles vary widely during prometaphase and metaphase, and therefore do not reliably predict division orientation. This finding prompted us to re-examine mutants that have been reported to disrupt division orientation in this tissue. Loss of Mud/NUMA misorients divisions, but Inscuteable expression and $\textit{aPKC}$, $\textit{dlg}$ and $\textit{pins}$ mutants have no effect. Furthermore, Mud localizes to the apical-lateral cortex of the wing epithelium independently of both Pins and cell cycle stage. Thus, Pins is not required in the wing disc because there are parallel mechanisms for Mud localization and hence spindle orientation, making it a more robust system than other epithelia.