Regeneration of missing body parts occurs in most animal phyla, whereas regenerative capabilities vary extensively even between closely related taxa . Much data have been gathered in recent years especially with a focus on the molecular and developmental mechanisms of regeneration and we may indeed be getting closer to a true understanding of its molecular basis .
By contrast, the evolution of regenerative capacity in animals and its ecological context has just recently shifted back into focus providing essential insights into the evolutionary history of regeneration . Thereby studies concentrated on extant animal regeneration models to investigate the distribution of regenerative capacities in a phylogenetic framework and to assess which factors may have played a role in the loss or maintenance of it, such as direct selection, pleiotropy or phylogenetic inertia .
Among tetrapods, salamanders display by far the highest regenerative capacity that includes the eyes, heart, tails and entire limbs . Therein, decades of research have been dedicated to the question of how it is possible for salamanders to repeatedly regenerate an entire limb in a matter of a few weeks and throughout their whole lifespan, while other tetrapods cannot .
The quest has undoubtedly been driven by the hope to eventually be able to induce human limbs to regenerate. Most studies investigating limb regeneration have focused on the Mexican axolotl Ambystoma mexicanum, but limb regeneration has been demonstrated in a number of additional salamander taxa, including those that undergo direct development .
One of the most striking steps in the regeneration cascade is the de-differentiation of cells that had a specific, differentiated identity prior to the injury taking place, which re-enter the cell cycle to form a growth zone, the blastema . The subsequent process of cell specification and pattern formation in the regenerating limb is not yet fully resolved. While grafting experiments and some molecular studies indicated that contrary to initial limb development, during regeneration the distal tip of the stump is specified first, followed by intercalary growth , more recent studies point towards a proximo-distal sequence of cell specification during regeneration, indicating that similar patterning modes may be used in development and regeneration . The high regenerative capabilities of salamanders have classically been regarded as exceptional among tetrapods . Among fish-like sarcopterygians (‘lobe-finned fish’), only lungfish are known to have a comparable capacity to regenerate their fore- and hind fins, including endoskeletal elements . Contrary to salamander limb regeneration, however, the morphological and molecular aspects of lungfish fin regeneration have not been addressed in detail yet, but it is known that after the initial healing of a wound a blastema forms, which is overall comparable to the blastema initiating salamander limb regeneration .
Among amphibians, frogs display some regenerative capacity and can fully regenerate their limbs until the tadpole reaches metamorphic climax and similar molecular markers controlling certain aspects of the regeneration cascade have been found in premetamorphic frogs and salamanders . As differentiation advances, the regenerative capacity of frogs gradually decreases and regenerative failure is correlated with an orderly reduction in the number of regenerated digits, inverse to the order of initial digit development until regenerative capacity is lost in the adult animal with metamorphic climax . Outside of sarcopterygians, a recent study showed that the basal actinopterygian Polypterus is capable of fully regenerating its pectoral fins at least until individuals reach reproductive age .
The question of which molecular and evolutionary differences between salamanders and other tetrapods are responsible for the high regenerative capacities of salamanders thus far remains largely unresolved. Many of the molecular mechanisms controlling regeneration of different tissues have been shown to be shared in animal regeneration . However, limb regeneration is considered one of the most complex regenerative modes, and recent studies have identified a number of specific molecular markers that seem to be unique to salamander limb regeneration .
Salamanders are the only tetrapods capable of fully regenerating their limbs throughout their entire lives. Much data on the underlying molecular mechanisms of limb regeneration have been gathered in recent years allowing for new comparative studies between salamanders and other tetrapods that lack this unique regenerative potential. By contrast, the evolution of animal regeneration just recently shifted back into focus, despite being highly relevant for research designs aiming to unravel the factors allowing for limb regeneration. We show that the 300-million-year-old temnospondyl amphibian Micromelerpeton, a distant relative of modern amphibians, was already capable of regenerating its limbs. A number of exceptionally well-preserved specimens from fossil deposits show a unique pattern and combination of abnormalities in their limbs that is distinctive of irregular regenerative activity in modern salamanders and does not occur as variants of normal limb development. This demonstrates that the capacity to regenerate limbs is not a derived feature of modern salamanders, but may be an ancient feature of non-amniote tetrapods and possibly even shared by all bony fish. The finding provides a new framework for understanding the evolution of regenerative capacity of paired appendages in vertebrates in the search for conserved versus derived molecular mechanisms of limb regeneration.
Nadia B. Fröbisch, Constanze Bickelmann, Florian Witzmann