Nuclear receptor NR4A is required for patterning at the ends of the planarian anterior-posterior axis.

Positional information is fundamental to animal regeneration and tissue turnover. In planarians, muscle cells express signaling molecules to promote positional identity. At the ends of the anterior-posterior (AP) axis, positional identity is determined by anterior and posterior poles, which are putative organizers. We identified a gene, , that is required for anterior- and posterior-pole localization to axis extremes. encodes a nuclear receptor expressed predominantly in planarian muscle, including strongly at AP-axis ends and the poles. RNAi causes patterning gene expression domains to retract from head and tail tips, and ectopic anterior and posterior anatomy (e.g., eyes) to iteratively appear more internally. Our study reveals a novel patterning phenotype, in which pattern-organizing cells (poles) shift from their normal locations (axis extremes), triggering abnormal tissue pattern that fails to reach equilibrium. We propose that promotes pattern at planarian AP axis ends through restriction of patterning gene expression domains.