The planarian dorsal-ventral boundary regulates anterior-posterior axis growth and patterning

Regeneration can involve the coordination of pattern formation in an outgrowth with the spatial pattern of pre-existing tissues, such as along body axes. Planarian adult axis patterning serves as a robust context for uncovering the mechanisms of such pattern integration. We investigated how the dorsal-ventral boundary (DVB), which surrounds the animal periphery at the dorsal-ventral (DV) median plane, regulates anterior-posterior (AP) axis growth and patterning. We define a spatial DVB gene expression atlas that includes genes encoding signaling, adhesion, and transcription factors. Wnt inhibition results in anterior positional information induction and ectopic head formation that is restricted to the DVB. DVB can be transplanted, and DVB identity can be experimentally induced at ectopic locations. Ectopic DVB is competent for anterior positional identity induction following Wnt inhibition, enabling the generation of animals with ectopic heads at experimentally dictated locations. DVB removal blocks the anteriorization that normally follows Wnt inhibition and prevents anterior positional information expression during head regeneration. Anterior positional information induction at the DVB after Wnt inhibition occurs independently from anterior pole formation, which promotes head patterning in regeneration. Our findings reveal a hierarchical model of pattern integration across body axes in which DV patterning is central by producing a DVB with competence to direct formation of large AP axis regions. This mechanism enables coordination of orthogonal positional information in the context of regeneration.