Towards a parsimonious analysis of regeneration and self-repair in animal evolution

All metazoan phyla contain species that undergo regeneration after amputation. Simple organisms as Hydra or planarians regenerate their main body axes, while salamanders and fish regenerate complex structures such as limbs, fins, tails but also heart, retina, lens. Hence, in these species the developmental programs remain accessible to reactivation throughout life, whereas in frogs they become locked after metamorphosis and in most mammals, adult regeneration is restricted to tissues as bone, liver. The consensus view is that regeneration is an ancestral character, randomly maintained or lost along evolution. To identify the possible homologies among the various regenerative processes, we selected 30 regenerative contexts where the cellular processes driving blastema formation as dedifferentiation, transdifferentiation, stem cell recruitment or proliferation of differentiated cells, were characterized. Moreover the complexity of the regenerative process and three developmental criteria including the developmental status when regeneration occurs, the presence of metamorphosis or asexual reproduction in adulthood were considered. These features were assembled to form a "regenerative code" that defines each regenerative context. These regenerative codes were then analysed under the maximum parsimony criteria to define groups of processes sharing common features. A majority-rule consensus tree shows seven unambiguous groupings that support the validity of this approach