Evo Devo

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Evo Devo

• Based on a lecture given by
• Dr. Sean Carroll, NABT 2005

http//www-astro.physics.ox.ac.uk/hansen/evolutio
n.html
http//www.perrific.com/cds/covers/devo.jpg
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How can this happen?
Legs instead of antenna
http//www.sciencemuseum.org.uk/on-line/lifecycle/
16.asp
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Or this?

• http//webtown.typepad.com/webtown/fun/index.html

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EvoDevoEvolutionary Development
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Ed Lewis

• Edward Lewis -homeotic genes in fruit flies
• 3rd chromosome (of four)
• Set of genes sitting together in two clusters

http//www1.umn.edu/umnnews
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• First clusterBithorax- set of 3 genes control
  rear half of fly
  
• Second clusterAntennapedia-set of 5 genes
  controls front half of fly
  
• Relative order of genes corresponded to order of
  parts they affect

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Evo Devo Rule 1

• Looks are deceiving
• All animals use the same tool kit for building
  body parts
  

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The Homeobox

• Hox genes- mammalian versions of homeotic
  genes.
  
• In flies, worms, mice, humans, all homeotic genes
  contain a segment called a homeobox,
  
• made up of 180 base pairs of DNA
• codes for an important domain of a transcription
  factor (AKA DRG)
  
• Developmental regulatory gene
• regulates the activity of other genes

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Hox Genes

• on four separate chromosomes in mammals
• On a diagram, are color coded to match the parts
  of body where they are expressed
  
• these clusters probably arose by duplication
• Then diverged
• Contributes to different body plans in different
  groups of animals

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http//www.lcusd.net/lchs/mewoldsen/Homeobox.htm
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4 Secrets of Innovation

• Work with available materials (evolution doesnt
  create anything new)
  
• Redundancy of body parts (almost all animals have
  the same basic body plan
  
• Multi-functionality (parts can do different
  things depending on where they are found)
  
• Modularity of body-parts able to change
  independently from other parts

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Phylogeny of Hox genes each phylum has a unique
pattern of hox genes -- greatest amplification
in vertebrates (mouse has 39)
one Hox-like gene present in sponges (maybe)
4-7 Hox genes in jellyfish (they have no bilater
al symmetry, A/P body axis)
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Figure 24.23 Hox mutations and the origin of
vertebrates
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Evo Devo Rule 2

• Change in number, kind of repeated parts
• Branched limbs in insects
• Compare aquatic larva with adults on land
• Aquatic larva have gills to breathe
• Upper branch may have become wings
• Same genes used
• Repeated body parts become specialized

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Evolution of wings?
Trilobite
http//www.uniongas.com/
http//www.phacops.com/whatisatrilobite.html
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Gills
http//www.ces.ncsu.edu/depts/ent/
http//www.indiana.edu/bradwood
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As we all know, living insects have 3 pairs of
legs.
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However, insect ancestors all had more than 3
pairs of legs.
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We can infer that insects lost abdominal legs
during their evolution.

• If this inference is correct, investigators
  reasoned that it might be possible to recover
  this hypothesized extinct phenotype by
  manipulating genes known to be involved in
  arthropod appendage development.

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Two DRGs known to be involved in appendage
development are called Ubx and AbdA.
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Ie. Ubx sequence comparisons (a DRG)
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The expression of these genes can be blocked by
treating insect embryos with inhibitory RNA
(RNAi) which prevents the translation of specific
mRNAs (in this case Ubx and Abd-A mRNAs).
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Observe what happens in beetle larvae that have
been treated with Ubx and AbdA RNAi.
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Here is a control larva
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And here is an RNAi-treated larva
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Apparently, the normal role of Ubx and AbdA in
insects is to repress the expression of another
DRG (called distal-less) in the abdomen.Ie. UBX
represses distal-lessno UBX, no repression of
distal-lessmore segments have legs
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In the Ubx gene, this repression ability is
directly related to the acquisition of a new
segment of the coding region of Ubx found in
insects, but not in other arthropods this is
shown in the next slide.
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Ubx protein sequence comparisons
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Only insects have the ..QAQAQKAAAAAAAAAA..C-termi
nal repeat.
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Evo Devo Rule 3

• New patterns evolve when old genes learn new
  tricks
  
• Mutations in control switches, (DRGs)
• Changes how a gene is used
• Can have multiple switches controlling same gene,
  
  
• Or, different switches controlling same gene in
  different body parts

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Same gene, different locations
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How to evolve a snake
A pythons axial skeleton is almost entirely made
of thoracic vertebrae.
There is not a trace of forelimbs and only a
vestigial pelvic girdle and hind limb (arrow in
figure). Other snakes have no pelvic girdle at
all.
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Position of limbs in tetrapods

• During embryonic development, limb buds respond
  to position cues
  
• Where the HOX genes are expressed determines
  position of limbs

• Usually placed at the END of where Hox gene is
  expressed
  
• If expressed throughout the entire length, no cue
  given

Developmental basis of limblessness and axial
patterning in snakesMartin J .Cohn Cheryll
Tickle NATURE VOL 399 3 JUNE 1999
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Atavisms

• The genes themselves are still present
• Just not being expressed
• If given an exogenous signal (from outside) in
  the lab, limbs have been restored
  
• Whales, dolphins have been found with hindlimbs
• Tails a human atavism?

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Endless Forms Most Beautiful..

• Darwin closed the first edition of The Origin of
  Species with what has become perhaps the most
  widely quoted passage in all of biology
  
• There is a grandeur in this view of life, with
  its several powers, having been originally
  breathed into a few forms or into one and that
  whilst this planet has gone cycling on according
  to the fixed law of gravity, from so simple a
  beginning endless forms most beautiful and most
  wonderful have been, and are being, evolved.

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More to Explore

• http//evolution.berkeley.edu/evolibrary/article/0
  _0_0/evodevo_04