Evolution of Development

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• Evolution of Development
• Chapter 25

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Evolutionary Paradox

• Phenotypic diversity
• Different genes
• Similar genes deployed and regulated differently
• Direct and indirect development
• Sea urchins
• Changes in patterns of developmental gene
  expression
• Genes have not changed, patterns of expression
  have

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Evolutionary Paradox

• Direct and indirect sea urchin development

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Evolutionary Paradox (Cont.)

• Direct and indirect sea urchin development

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Evolutionary Paradox

• 2 dozen gene families regulate animal and plant
  development
• Hox genes establish the body plan by specifying
  when and where genes are expressed
• Hox genes code for proteins that bind to
  regulatory region of other genes
• Plants shoot growth and leaf development
• Animals establish body plans

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Evolutionary Paradox

• MADS box genes code for a DNA-binding motif
• Establish the body plan of plants (flowers)
• Highly conserved
• Understanding how development evolved requires
  understanding
• Genes, gene expression, development, and
  evolution
• Alteration of the timing or position of gene
  expression

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Evolutionary Paradox

• Heterochrony alterations in timing of
  developmental events due to a genetic change
• Mutation in a gene small plant that flowers
  quickly
• Most mutations that affect developmental
  regulatory genes are lethal
• If mutation leads to increased fitness new
  phenotype will persist

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Evolutionary Paradox

• Homeosis alterations in the spatial pattern of
  gene expression
• Bithorax fruit fly homeotic mutation which
  produces two pairs of wings
• Antennapedia fruit fly leg where an antenna
  should be
• Mutations can arise spontaneously or by
  mutagenesis in the laboratory

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• Mutations in Fruit Fly Wings

Normal fruit fly with one set of wings
Mutant fruit fly with two sets of wings
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Evolutionary Paradox

• Coding sequence of a gene can contain multiple
  regions with different functions
• Regulatory region change may alter time or place
  of gene expression
• Changes in signaling pathways
• Coordinating information about next cell
• Information about external environment
• Receptor in a different cell type
• Alter targets

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Gene Mutation

• Brassica oleracea ancestral species
• Cauliflower and broccoli stop codon
• CAL and Apetala1 gene changes regular flowers
  into masses of arrested flower buds
• CAL was cloned from Brassica a stop codon, TAG,
  was found in the middle of the sequences when
  analyzed

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Gene Mutation

• Evolution of cauliflower and broccoli

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Gene Mutation

• Cichlid fish jaws demonstrate morphological
  diversity
• Different species acquire different niches based
  on feeding habits
• Rammers long snouts
• Biters intermediate snout
• Bottom feeders short snouts
• Two genes shape and size

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Gene Mutation
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Gene Mutation (Cont.)
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Same Gene New Function

• Ancestral genes co-opted for new functions
• Evolution of chordates
• Brachyury gene of ascidians (notochord, no
  vertebrate) encodes a transcription factor
• Expressed in developing notochord
• Brachyury gene, homologue in mollusk is
  associated with anterior-posterior axis
  specification

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Same Gene New Function

• Protein domain T box, transcription factor.
• Turns on or off a gene or genes

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Same Gene New Function

• Mutation in Brachyury in dogs
• Cause short tail to develop
• Humans have wild-type of the gene
• Limb formation bird wing human forearm
• Tbx5 transcription factor gene family with T
  box domain
• Tbx5 encodes a protein that turns on a gene or
  genes to make a limb

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Same Gene New Function

• Mutations in human Tbx5 cause Holt-Oram syndrome
  forelimb and heart abnormalities
• Tbx5
• Gets limb development started
• Regulates the expression of other genes
• 4-D development 3-D space and time
• Tbx5 proteins bind to different genes in birds
  and in humans

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Different Genes, Convergent Function

• Homoplastic (analogous) structures
• Same or similar functions
• Arose independently
• Phylogenies reveal convergent events
• Origin of convergence difficult to understand
• Different developmental pathways may have been
  modified

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Different Genes, Convergent Function

• Insect wing patterns demonstrate homoplastic
  convergence
• Origins of patterns recruitment of existing
  regulatory programs for new functions
• Sensory bristles and scale development initiated
  by achaete-scute transcription factor

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• Development of scales structures inverted from
  bristles
• Daughter cell neuronal connection dies
• Pigment production triggered

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Different Genes, Convergent Function

• Flower shapes have also altered in a convergent
  way
• Radially symmetrical flower two identical parts
  when cut across center -daisies, roses, tulips

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Different Genes, Convergent Function

• Bilaterally symmetrical flowers mirror-image
  halves on each side of a single central axis
  peas, snapdragons
• Bilaterally symmetrical flowers attractive to
  their pollinators
• Shape may be reason for evolutionary success

orchid
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Different Genes, Convergent Function

• Cycloidia (CYC) gene responsible for bilateral
  symmetry of snapdragon
• Snapdragons with mutations in CYC have radially
  symmetrical flowers
• Radial symmetry is the ancestral condition
• Some radially symmetrical flowers have
  bilaterally symmetrical ancestor
• Loss of CYC function radial symmetry

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Different Genes, Convergent Function

• Gain of bilateral symmetry arose independently
  among some species because of CYC gene
• Convergent evolution through mutations of the
  same gene
• Other genes also play a role in convergent
  evolution of bilateral symmetry

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Different Genes, Convergent Function
With gene expressed
Without gene expressed

• Snapdragon asymmetry

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Duplication and Divergence

• Gene duplications of paleoAP3 led to
  flowering-plant morphology
• MADS box gene duplicated
• Gave rise to PI and paleoAP3 genes
• Ancestor genes affected stamen development
  (function retained)
• paleoAP3 duplicated to produce AP3
• AP3 duplicated again gained a role in petal
  development

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Duplication and Divergence

• Petal evolution through gene duplication

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Duplication and Divergence

• Gene divergence of AP3 altered function to
  control petal development
• Mutant ap3 plants do not produce either petals or
  stamens
• PI and AP3 proteins can bind to each other
• regulate transcription of genes needed for stamen
  and petal formation

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Duplication and Divergence

• AP3 has acquired a domain necessary for petal
  development

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Functional Analysis

• Functional analysis range of experiments
  designed to test the function of a gene in
  different species
• Sequence comparison essential
• Need to distinguish paralogues from orthologues
• Single base mutation can change active gene into
  an inactive pseudogene
• Functional genomics experimenting to demonstrate
  actual function of the gene

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Case Study

• Diversity of eyes in the natural world
• Morphological evidence indicates eyes evolved at
  least twenty times
• Convergent evolution Homoplastic

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Fly and Mouse eye

• Most recent ancestor no sight
• Pax6 initiates eye development
• Triggers lens formation in insects and
  vertebrates
• Walter Gehring inserted mouse Pax6 into genome
  of a fruit fly
• Created transgenic fly
• Pax6 gene turned on by regulatory factors in the
  flys leg

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Fly and Mouse eye

• Mouse Pax6 makes an eye on the leg of a fly

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Case Study

• Pax6 and eyeless cave fish. Pax6 gene expression
  reduced. Eyes start to develop, then degenerate

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Case Study

• Pax6 expression correlates with ribbon worm
  eyespot regeneration

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Case Study

• Pax6 is not required for planaria eyespot
  regeneration

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• Hypothesis 1 Pax6 gene and its homologues
• Eye development may have a single evolutionary
  line
• Hypothesis 2 Pax6 gene could have had a
  regulatory role in the forehead of early animals
• Eye development may have occurred over and over
  and Pax6 has been independently co-opted over and
  over for eye development