Evolution and Development Evolution of Regeneration What explains the distribution of regeneration among organisms? - Adaptive? - Inherit to all metazoans or independently derived?

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 anterior and posterior limbs. However, the frequency of regeneration is much higher for anterior legs (83% vs 21%). From Morgan 1898 and Needham 1961

Regeneration: Inherent? 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 Inherent 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 and Evolution of Epimorphic Regeneration

Why Not Regeneration? Why don’t we observe it more? Loss of regeneration may reflect genetic changes that are associated with evolutionary changes: With respect to amniote vertebrates: Water to land transition Poikilothermy to homothermy Loss of metamorphosis Evolution of immune system

However, there is variation among amphibians Unlike salamanders, Xenopus has limited regenerative potential - 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?

100% regenerate spike after radia-ulna amputation 80% regenerate spike after humerous amputation 0% regenerate spike after complete amputation

Growth Rates Day 0 1 month 4 months No legs amputated 0.55 9.54 10.2 1 leg amputated 0.54 9.0 9.54 2 legs amputated 0.53 7.5 8.8 * Weights are in grams

The spike supports nuptial pad tissue development in males.

2 of 3 males with 1 regenerated radia-ulna spike were able to successfully amplex and mate with a female.

The results suggest that spike regeneration maybe adaptive. But why did Xenopus frogs presumably loose the ability to reform perfect limbs?