Alternating polarity integrates chemical and mechanical cues to drive tissue morphogenesis

The spatial patterning of molecules within a sheet of cells directs morphogenesis in many epithelial tissues. In Scaptodrosophila follicle cells, Par3/Bazooka and aPKC were previously shown to localize to a set of cell edges destined to elongate and form the base of each eggshell dorsal appendage. This study establishes that the mechanism underlying this localization pattern is an alternating, in-plane polarity of the cells. A candidate screen identified several potential molecular players, whose epistatic relationships were then examined using a custom culture assay. These experiments demonstrated that positive feedback between actin polymerization and PI4P production leads to polarization of these cells individually, while mechanical force coordinates polarization among these cells. This work adds to the growing evidence for a role of mechanics in cell polarity, and also provides an example where morphological differences between species can be understood at the level of changes in fundamental cell biological processes. Summary In Scaptodrosophila , the alternating (left, right, left) polarization in a row of cells drives the formation of up to eight eggshell respiratory filaments. This study uncovers the underlying pathway, which integrates chemical signaling through actin and PI4P with mechanical force.