Evolutionary loss of animal regeneration: pattern and process.

The ability to regenerate lost or damaged body parts is widespread among animals and provides obvious potential benefits. It is therefore perplexing that this ability has become greatly restricted or completely lost in many lineages. Despite growing interest in the cellular and molecular basis of regeneration, our understanding of how and why regenerative abilities are lost remains rudimentary. In an effort to develop a framework for studying losses of regeneration, here I outline an approach for rigorously identifying such losses, review broad patterns of regenerative ability across animals, describe some of the clearest examples of regeneration loss, discuss some possible scenarios by which regeneration may be lost, and review recent work in annelids that is providing new insights into loss of regenerative ability.