Evolution of Regeneration

“The regenerative process is one of the fundamental attributes of living things...” Thomas Hunt Morgan (1901), from his book Regeneration.

Evolution of Regeneration

What explains the distribution of regeneration among organisms?

- Adaptive?- Ancestral to all metazoans and repeatedly lost, or independently derived?

Evolution and Development

Regeneration is Phylogenetically Widespread

Anuran Tail Planeria

Regeneration: Adaptive?

• Seemingly, the ability to regenerate should benefit individuals of a population (i.e. is adaptive).

• Can you think of a way to test the idea that regeneration is adaptive?

Hermit Crabs regenerate their anteriorand posterior limbs. However, the frequencyof regeneration is much higher for anteriorlegs (83% vs 21%).

From Morgan 1898 and Needham 1961

Regeneration: Ancestral?

• Much of what is accomplished during regeneration is first accomplished during

embryonic development (same mechanisms are deployed).

• Can you think of a way to test the idea that regeneration is inherent?

Observations Supporting the Idea that Regeneration is Ancestral

• Phylogenetically widespread• Lost between closely related species• Aspects of regeneration are similar among

organisms in a developmental sense• Some organisms that can not regenerate

body parts, do so partially during development.

Epimorphic Regeneration: The Blastema is Very Similar Among Unrelated Taxa

Regulation andEvolution of EpimorphicRegeneration

• Loss of regeneration may reflect major evolutionary changes:

– With respect to amniote vertebrates:• Water to land transition• Poikilothermy to homothermy• Loss of metamorphosis• Evolution of immune system

Why Not Regeneration?Why don’t we observe it more?

However, there is variation among amphibians

• Can regenerate limbs as immature larvae

• Loses ability to regenerate at metamorphosis

• Adults regenerate a cartilagenous spike after limb amputation.• Is the generation of a spike an

adaptation?

Unlike salamanders, Xenopus has limited regenerative potential

100% regenerate spikeafter radia-ulna amputation

80% regenerate spikeafter humerous amputation

0% regenerate spikeafter complete amputation

Growth Rates

No legs amputated0.55 9.54 10.2

1 leg amputated 0.54 9.0 9.54

2 legs amputated 0.53 7.5 8.8

Day 0 1 month 4 months

* Weights are in grams

The spike supportsnuptial pad tissue development in males.

2 of 3 males with1 regenerated

radia-ulna spike were able to successfully

amplex and matewith a female.

• The results suggest that spike regeneration maybe adaptive.

• But why did Xenopus frogs presumably loose the ability to reform perfect limbs?