Orthogonal muscle fibers have different instructive roles in planarian regeneration

The ability to regenerate missing body parts exists throughout the animal kingdom. Positional information is critical for regeneration, but how it is harbored and utilized by differentiated tissues is poorly understood. In planarians, positional information has been identified through RNA interference (RNAi) phenotypes in which the wrong tissues are regenerated. For example, Wnt pathway inhibition leads to regeneration of heads in place of tails(1–3). Characterization of such striking adult phenotypes led to identification of genes expressed in a constitutive and regional manner, associated with patterning, called position control genes (PCGs). Most PCGs are expressed within the planarian muscle(4). Despite this major positional information role for muscle in planarians, how muscle is specified and how different muscle subsets impact regeneration is unknown. Here we found distinct regulatory roles for different planarian muscle fibers during regeneration. myoD was required for formation of a specific muscle cell subset: the longitudinal fibers, oriented along the anterior-posterior (AP) axis. Loss of longitudinal fibers led to a complete regeneration failure because of defects in regeneration initiation. A different transcription factor (TF)-encoding gene, nkx1-1, was required for formation of circular fibers, oriented along the medial-lateral (ML) axis. Loss of circular fibers led to a bifurcated AP axis with fused heads forming in single anterior blastemas. Our results demonstrate distinct roles for muscle fiber types in orchestrating planarian regeneration. Whereas muscle is often viewed as a strictly contractile tissue, these findings reveal specific regulatory roles for distinct muscle classes in wound signaling and patterning to enable regeneration.