Bmp4 and Beak Morphology

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Bmp4 and Beak Morphology

• Sean Ramsden

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Two Complimentary Papers

• Bmp2 and Morphological Variation of Beaks in
  Darwins Finches
• Abzhanov et al.
• Molecular Shaping of the Beak
• Wu et al.

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Introduction

• Named after Charles Darwin and helped lead him to
  postulate the theory of evolution
• Proposed that the niche of the species determined
  its beak morphology
• Small narrow beaks for gathering nectar from
  cactus flowers
• Large, robust beaks for cracking seeds
• These two papers first to link molecular evidence
  to the morphology of beaks

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Outline of Abzhanov et al. Article

• Compared beak development in six species of
  ground finches
• Analyzed expression of growth factors expressed
  during craniofacial development
• Transfect chickens to over express Bmp4 or Noggin

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What did they find?

• Finch species with larger beaks show higher
  levels of Bmp4 earlier
• When over expressed, Bmp4 leads to larger beaks
• When Noggin, a Bmp4 antagonist, was over
  expressed, smaller beaks were observed

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Expression of Bmp 2, 4 and 7

• In situ hybridization was used to probe various
  growth factors
• Bmp 2, 4 and 7 seemed to affect beak size
• However, only Bmp4 affected beak size and
  morphology without affecting the size of the
  entire finch

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Bmp 4 Expression

• Bmp4 is first expressed in the epithelium
  maxillary and lateral prominences in early
  embryos (stage 24)
• Bmp4 expression later localized to the distal
  mesenchyme of the upper beak

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Staging System

• A staging system for finches was developed in
  order to allow comparison of expression
  experiments done in chickens with hybridization
  experiments done on collected finches

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Bmp4 Over Expression

• RCASBmp4 infectivity correlated with increased
  collagenization after mesenchymal infection

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Bmp4 or Noggin Over Expression
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Bmp4 or Noggin Over Expression

• Bmp4 over expression leads to a broader, deeper
  beak
• Noggin over expression leads to a narrower, more
  shallow beak

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Recap

• Finch species with larger beaks show higher
  levels of Bmp4 earlier
• When over expressed, Bmp4 leads to larger beaks
• When Noggin, a Bmp4 antagonist, was over
  expressed, smaller beaks were observed

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Outline of Wu et al. Article

• Compare proliferation zones of chickens and ducks
  that have distinct beak shapes
• Over express Bmp4 or Noggin to observe effect on
  beak morphology
• Ablated frontal ectodermal zone, previously shown
  to direct beak outgrowth, to observe effect on
  beak development

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What did they find?

• Proliferative zones within the frontonasal mass
  change between stages 26 and 31
• Over expression of Bmp4 resulted in significantly
  larger beaks
• Noggin over expression miniaturized beaks
• Ablation of frontonasal ectodermal zone resulted
  in beak growth arrest

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Beak Proliferative Zones Shift During Development
Chicken
Duck
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Beak Proliferative Zones Shift During Development
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Beak Proliferative Zones Shift During Development

• Three dimensional reconstruction of proliferative
  zones showing increased proliferation at the
  frontonasal mass
• Ducks show higher levels of Bmp4

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Over Expression of Bmp4 or Noggin

• RCASBmp4 transfection leads to enlarged beak
  morphology

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Over Expression of Bmp4 or Noggin

• RCASNoggin transfection leads to a small beak
  morphology

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Over Expression of Bmp4 or Noggin

• Bmp4 transduced beak prominences show enlarged
  cartilage
• Noggin transduced beak prominences show
  miniaturized cartilage

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Ablation of Frontonasal Ectoderm

• Frontonasal mass was subdivided into 3 sections,
  where section b contained the frontonasal
  ectodermal zone
• Albumin coated beads shown to have no effect on
  beak development

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Ablation of Frontonasal Ectoderm

• Ablation of FEZ gives beak morphology similar to
  RCASNoggin
• Bmp4 can rescue wild type phenotype in ablated
  chickens
• Addition of Bmp4 in non-ablated chicken gave
  RCASBmp4-like beak morphology

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Recap

• Proliferative zones within the frontonasal mass
  change between stages 26 and 31
• Over expression of Bmp4 resulted in significantly
  larger beaks
• Noggin over expression miniaturized beaks
• Ablation of frontonasal ectodermal zone resulted
  in beak growth arrest

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Conclusions

• Variation in Bmp4 expression is a principle
  factor in determining beak morphology, however it
  is not known if it acts in concert with other
  factors
• Bmp4 is sufficient to rescue a Noggin-like
  ablation phenotype

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References

• Abzhanov A, Protas M, Grant B, Grant P, Tabin C.
  Bmp4 and Morphological Variation of Beaks in
  Darwins Finches. Science. 2004. 3051462-1465
• Wu P, Jiang T, Suksaweang A, Widelitz R, Chuong
  C. Molecular Shaping of the Beak. Science. 2004.
  3051465-1466