Multi-scale coordination of planar cell polarity in planarians

Polarity is a universal design principle of biological systems that manifests at all organizational scales. Although well understood at the cellular level, the mechanisms that coordinate polarity at the tissue or organismal scale remain poorly understood. Here, we make use of the extreme body plan plasticity of planarian flatworms to probe the multi-scale coordination of polarity. Quantitative analysis of ciliary rootlet orientation in the epidermis reveals a global polarization field with head and tail as independent mediators of anteroposterior (A/P) polarization and the body margin influencing mediolateral (M/L) polarization. Mathematical modeling demonstrates that superposition of separate A/P- and M/L-fields can explain the global polarity field and we identify the core planar cell polarity (PCP) and Ft/Ds pathways as their specific mediators. Overall, our study establishes a mechanistic framework for the multi-scale coordination of planar polarity in planarians and establishes the core PCP and Ft/Ds pathways as evolutionarily conserved 2D-polarization module.