Evolution of Development

How is development altered to give riseto new morphological structures?

What are the evolutionarily important genes that direct development?

What is the developmental genetic basisof homologous structures?

Homeotic (Hox) Genes areConserved Among Metazoans

1. Hox genes are organized in gene complexes

2. Temporal and spatial collinearity is known forHox genes

3. Hox genes code for regulatory proteins that regulate the transcription of other genes

Hox genes specifysegmental Identity

Phylotypic Stage

Mutational analysis of Hox genes

Ubx mutant

Transformation of T3 and A1 into T2 because UBX

does not repress expressionof T2 Hox genes

T3A1

Tribolium (flour beetle) with all 8 Hox genes mutagenized

Hox gene expression is correlated with segment identity

Flower Development: ABC gene expression in Arabidopsis

Hox Genes and Morphological Evolution

• Change in gene number

• Change in spatial expression

Evolution of Regeneration

What explains the distribution of regeneration among organisms?

- Adaptive?- Inherit to all metazoans

or independentlyderived?

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: 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 andEvolution of EpimorphicRegeneration

• 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

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 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

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?