Tuesday, October 27, 2015

[Paleontology • 2015] Deep-Time Evolution of Regeneration and Preaxial Polarity in Tetrapod Limb Development

Micromelerpeton credneri from the Early Permian era in Germany. Panels show the regeneration of the forelimb, which results in a hand with malformations
  Illustration: Kalliopi Monoyios news.brown.edu  Image: Hwa-Ja Götz/MfN [Museum für Naturkunde] Berlin
doi: 10.1038/nature15397

Among extant tetrapods, salamanders are unique in showing a reversed preaxial polarity in patterning of the skeletal elements of the limbs, and in displaying the highest capacity for regeneration, including full limb and tail regeneration. These features are particularly striking as tetrapod limb development has otherwise been shown to be a highly conserved process. It remains elusive whether the capacity to regenerate limbs in salamanders is mechanistically and evolutionarily linked to the aberrant pattern of limb development; both are features classically regarded as unique to urodeles. New molecular data suggest that salamander-specific orphan genes play a central role in limb regeneration and may also be involved in the preaxial patterning during limb development. Here we show that preaxial polarity in limb development was present in various groups of temnospondyl amphibians of the Carboniferous and Permian periods, including the dissorophoids Apateon and Micromelerpeton, as well as the stereospondylomorph Sclerocephalus. Limb regeneration has also been reported in Micromelerpeton, demonstrating that both features were already present together in antecedents of modern salamanders 290 million years ago. Furthermore, data from lepospondyl ‘microsaurs’ on the amniote stem indicate that these taxa may have shown some capacity for limb regeneration and were capable of tail regeneration, including re-patterning of the caudal vertebral column that is otherwise only seen in salamander tail regeneration. The data from fossils suggest that salamander-like regeneration is an ancient feature of tetrapods that was subsequently lost at least once in the lineage leading to amniotes. Salamanders are the only modern tetrapods that retained regenerative capacities as well as preaxial polarity in limb development.

An artist’s rendering of Micromelerpeton credneri from the Early Permian era in Germany. Panels show the regeneration of the forelimb, which results in a hand with malformations.
Illustration: Kalliopi Monoyios news.brown.edu

Well-preserved fossils, such as this Micromelerpeton credneri, allowed researchers to track the evoluton of regeneration.
Image: Hwa-Ja Götz/MfN [Museum für Naturkunde] Berlin

Nadia B. Fröbisch, Constanze Bickelmann, Jennifer C. Olori and Florian Witzmann. 2015. Deep-Time Evolution of Regeneration and Preaxial Polarity in Tetrapod Limb Development. Nature. (2015) doi: 10.1038/nature15397

Regeneration faded as most four-legged vertebrates evolved

 —Nowadays, salamanders are extraordinary among modern four-legged vertebrates: Repeatedly and throughout their lifespan, they can regenerate limbs, tails, and internal organs that were injured or lost due to amputation.

They are also special in the way their legs form during embryonic development. Generally limb development follows the same process in all four-legged vertebrates — from frogs to humans — despite the enormous variety of forms and functions that vertebrate limbs have.

“Salamanders, on the contrary, form their fingers in a reversed order compared to all other four-legged vertebrates, a phenomenon that has puzzled scientists for over a century,” said lead author Nadia Fröbisch of Museum für Naturkunde. “The question that we wanted to address was if and how this different way of developing limbs is evolutionarily linked with the high regenerative capacities.”

So the team, including co-author Florian Witzmann, visiting scientist in Brown’s Department of Ecology and Evolutionary Biology, set out to study fossils from about 300 million years ago to see how the capacity developed. The fossils used in the study, which appears in Nature, derive from the collections of a number of natural history museums including the Museum für Naturkunde Berlin.

“The amphibians fossilized under excellent conditions for preservation and are represented by a large number of individuals and developmental stages,” Witzmann said. “This extraordinary fossil record allowed for the detailed study of limb development and regeneration.”

In their studies the authors investigated different amphibian groups of the Carboniferous and Permian periods and showed that different groups were able to regenerate their legs and tails in a way that previously was exclusively known from modern salamanders.

“We were able to show salamander-like regenerative capacities in fossil groups that develop their limbs like the majority of modern four-legged vertebrates as well as in groups with the reversed pattern of limb development seen in modern salamanders,” said co-author Jennifer Olori of the State University of New York at Oswego.

That means that back then, it wasn’t just salamanders but many creatures that had the capacity to regenerate.

“The fossil record shows that the form of limb development of modern salamanders and the high regenerative capacities are not something salamander-specific, but instead were much more widespread and may even represent the primitive condition for all four-legged vertebrates,” said Fröbisch. “The high regenerative capacities were lost in the evolutionary history of the different tetrapod lineages, at least once, but likely multiple times independently, among them also the lineage leading to mammals.”

Regeneration faded as most four-legged vertebrates evolved
http://news.brown.edu/articles/2015/10/regenerate via @BrownUniversity
Capacity to regenerate body parts may be the primitive state for all four-legged vertebrates
  http://phy.so/365065883 via @physorg_com

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