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Evolution and Development

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					Evolution and Development

Evo-Devo
• Interested in the evolutionary forces and genetic mechanisms responsible for
  differences in embryogenesis and morphogenesis
   • Direct development vs. indirect development
       • Selection for rapid development promotes the evolution of direct
         development

Developmental pattern as an adaptation:
Direct versus indirect development

Genetics of Development
• Developmental change must have genetic basis for it to be important in
  evolutionary sense

Basic development
• Cells identified by location relative to others; this usually determines cell fate
• Relative location important for gastrulation, symmetry, differentiation of tissues
Hox cluster of homeobox genes
• A group of homeotic genes
   • Control the patterning of specific body structures
• Specify cell fates
• Found in all animal phyla
   • Controls the anterior-posterior body axis
• Organized into gene complexes or clusters
Hox cluster
•Order of genes on chromosome perfectly correlated with anterior-posterior
location of gene products in the embryo

Hox genes in Drosophila
• Occur in two clusters of genes on chromosome 3
  • Antennapedia complex
  • Bithorax complex
Visualization techniques for Hox expression

Hoxc-6 expression in chick and snake
Hox loci
• Each Hox locus contains a 180 bp sequence: the homeobox, whose amino
  acid sequence binds DNA
• This means that the protein produced will have a ‘DNA binding motif’: that is,
  the protein can regulate the transcription of other genes

Homeotic loci in Drosophila
• Genes do not specify the structure, but specify relative positions in the
  embryo, and relative time in development
• Regulate other genes (via homeobox), that then produce proteins involved in
  making the structure

Effects of homeotic mutations in Drosophila
• Antennapedia (Antp): flies missing gene products produce segment-specific
   appendages on the wrong segment, on anterior

Effects of homeotic mutations
•Ultrabithorax (Ubx) mutation
•Third thoracic segment is transformed into another second thoracic segment,
bearing wings

Evolution of Hox gene complex

It bears repeating:
• Hox genes do not encode structures, but provide positional information and
    regulate expression of other genes

The ABC model of flower development
• Same pattern in dicots and monocots
   • Patterning system is old
Developmental pathway conservation
• Compound eye of insects very different from vertebrate eye
Developmental pathway conservation
• Mutation in Eyeless (ey) gene in Drosophila causes partial or complete
  absence of eyes
• Mutation in Small eye (Sey) gene causes failure of eye development in mice
• Aniridia is homolog in humans
• Sequences are 94% identical between Drosophila and mammals
•   Eyeless (ey) gene can be activated in parts of developing fly where not
    normally expressed
     • Ectopic eyes on legs, wings, etc.
     • Can get the same result by transferring Sey gene (from mouse) to
       Drosophila!

Developmental pathway conservation

Developmental pathway conservation
• ey/Sey gene has not evolved much in 550 million years: since divergence of
  arthropods and chordates

The Distalless family genes influences outgrowths

Evolution of gene regulation
• Regulation achieved by enhancers for each gene
• One gene has a number of different enhancers
   • Regulatory modularity
   • Changes in enhancers, rather than changes in amino acid sequences, may
     be responsible for phenotypic differences

Regulation of Ubx (not new genes) creates patterns of epidermal cell hairs

A single gene controls adaptive pelvic fin development of 3-spined sticklebacks

The tb1 gene (encodes for transcription factor) underlies a major difference in
growth pattern between corn and teosinte

Co-option
• Analogous to exaptation for morphological characters
• Called recruitment or co-option in developmental biology
   • Evolution of novel functions for pre-existing genes and developmental
     pathways

Co-option
• A developmental regulatory gene may promote novel morphogenetic features
• Temporal co-option
• Spatial co-option
Temporal co-option
• Expression of a developmental regulatory protein may persist after the
  developmental stage in which it is needed
Spatial co-option
• A developmental pathway may become expressed in another region, leading
  to duplication of the structure in the new region

Co-option of developmental pathways in the evolution of novelties

Developmental genetics of heterochrony
• Heterochrony is an evolutionary change in the timing or rate of developmental
  events
   • Change in the relative timing of development of somatic features versus
     reproductive features changes features of organisms
      • Paedomorphosis (e.g. salamander)

Thyroxin triggers metamorphosis
• Thyrotropin-releasing hormone (TRH) stimulates pituitary gland to release
  thyroid-stimulating hormone (TSH), which stimulates thyroid to release
  thyroxin
• Neotony in Ambystoma mexicanum is achieved by inactivation of the TRH
  cascade

Evolution of allometry
• Allometry is the differential rate of growth of different parts of an organism
  during its development

Rapid evolution of an allometric threshold in one species of dung beetle
• Large males that reach a threshold size develop horns for male-male combat
Rapid evolution of an allometric threshold in the dung beetle
• Size at which horns develop has diverged in two introduced populations
Developmental constraints on evolution
• Absence of variation
   • For example, the lack of cells, proteins, or genes required for the
     development of a structure
• Strong correlations among characters
   • May result from interaction between tissues during development
   • May result from involvement of same genes or developmental pathways in
     more than one morphogenetic pathway
Developmental constraints in limb formation in amphibians
• Cholchicine inhibits mitosis in developing digits
• In frogs and in salamanders, different specific digits are missing
   • Due to different order of digit differentiation in these two groups
   • Last digit formed tended to be the most sensitive to cholchicine treatment
Diversity of horns in different beetle species

Gene expression in horn development of beetles

				
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