How do planarians specify what to regenerate?
Planarians can regenerate entire heads, sides, and tails. Regeneration from irregularly shaped fragments involves a robust ability to generate bilateral symmetry: new tissue is produced in a regeneration blastema at the wound site and remaining body regions are rearranged to produce symmetrical and properly proportioned animals. We identified three genes that are needed for asymmetric regeneration. These genes are candidate components of a BMP signaling pathway that controls the dorsal-ventral patterning of many animal embryos: a BMP1/Tolloid-like gene (smedolloid-1), a SMAD4-like gene (smedsmad4-1), and a DPP-like gene (smeddpp-1). Intact adult animals rely upon the action of this pathway to maintain their body plans: RNAi of smedsmad4-1 in normal adults causes a slow transformation in which animals develop two ventral sides. Animal fragments with perturbed DPP signaling and left-right symmetry regenerate blastemas with midline defects. smeddpp-1 is expressed on the dorsal midline sug
