Septins regulate border cell shape and surface geometry downstream of Rho

Septins self-assemble into polymers that bind and deform membranes in vitro and regulate diverse cell behaviors in vivo. How their in vitro properties relate to their in vivo functions is under active investigation. Here we uncover requirements for septins in detachment and motility of border cell clusters in the Drosophila ovary. Septins and myosin colocalize dynamically at the cluster periphery and share phenotypes, but surprisingly do not impact each other. Instead, Rho independently regulates myosin activity and septin localization. Active Rho recruits septins to membranes while inactive Rho sequesters septins in the cytoplasm. Mathematical analyses reveal how manipulating septin expression alters cluster shape and surface geometry. This study shows that the level of septin expression regulates surface properties at different scales. This work suggests that downstream of Rho, septins tune surface deformability while myosin controls contractility, the combination of which govern cluster shape and movement.