The Case for Comparative Regeneration: Learning from Simpler Organisms How to Make New Parts from Old

Regeneration is a primordial attribute of all living organisms, and as a biological problem it has attracted the attention of generations of experimental biologists for almost 300 years. Yet regeneration still awaits a satisfactory mechanistic explanation. In this brief review, I will argue for the reinstatement of planarians, a classical and currently understudied experimental organism, as a viable, molecular model system in which to functionally dissect the molecular basis of animal regeneration. The developmental plasticity and phylogenetic position of planarians, coupled with the technical ability to specifically silence gene expression via RNA interference should help usher these organisms to the forefront of regeneration research. Planarians are currently in a unique position to provide us with the fundamental tools required to begin the identification and functional characterization of the genetic interactions operating behind the regenerative abilities found in the metazoans.