Transcript Nov26_07
Evolution of Development What is the developmental genetic basis of homologous structures? How is development altered to give rise to new morphological structures? What are the evolutionarily important genes that direct development? Homeotic (Hox) Genes are Conserved Among Metazoans 1. Hox genes are organized in gene complexes 2. Temporal and spatial collinearity is known for Hox genes 3. Hox genes code for regulatory proteins that regulate the transcription of other genes Hox genes specify segmental Identity Phylotypic Stage Mutational analysis of Hox genes Ubx mutant T3 A1 Transformation of T3 and A1 into T2 because UBX does not repress expression of T2 Hox genes 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 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?