Highlights d Asymmetric expression of a single Toll receptor leads to Myo-II polarization d The adhesion GPCR Cirl binds to Toll-8 mediating Toll-8induced Myo-II polarization d Toll-8 boundaries generate a Cirl interfacial asymmetry that can polarize Myo-II d Differences in Toll-8 levels lead to interdependent Toll-8 and Cirl planar polarity

Tissue morphogenesis requires spatial control over actomyosin contractility to drive cell shape changes 1,2 . How developmental patterning controls cell mechanics is poorly understood. In Drosophila embryos, Myosin-II is enriched at the interface between anteroposterior neighboring cells, driving planar polarized cell intercalation and extension of the embryonic axis 3-5 . G protein-coupled receptors (GPCRs) signaling controls Myosin-II activation at junctions 6,7 and Toll receptors provide a spatial bias to this process 8,9 . How Toll receptors polarize actomyosin contractility remains unknown. Here we report that cells expressing different levels of a single Toll receptor, Toll-8, activate Myosin-II at their interface. This does not require the intracellular domain of Toll-8, suggesting signaling by a proxy mechanism. We found that Toll-8 forms a molecular complex with the adhesion GPCR Cirl/Latrophilin that is required for junctional Myosin-II activation. We further show that Myosin-II is also activated at the interfaces between wild-type and cirl deficient cells. Since Toll-8 differentially recruits Cirl in trans and in cis, we propose that Toll-8 activates Myosin-II by inducing Cirl asymmetric localization at the boundary of its expression domain. Finally, we found that Toll-8 and Cirl exhibit dynamic interdependent planar polarization when neighboring cells express different levels of Toll-8. Through this feedback, Toll-8 and Cirl self-organize to drive planar polarized signaling. This work sheds new light on how embryo patterning induces cell polarization during morphogenesis.