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Somite number and vertebrate evolution

M K Richardson1, S P Allen, G M Wright

  • 1Department of Anatomy and Developmental Biology, St George's Hospital Medical School, London, UK. m.richardson@sghms.ac.uk

Development (Cambridge, England)
|March 5, 1998
PubMed
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Vertebral segment number variation in vertebrates is linked to early somite counts, challenging the developmental hourglass model. This dissociation allows for significant evolutionary changes during conserved developmental stages.

Area of Science:

  • Evolutionary developmental biology
  • Comparative genomics
  • Vertebrate paleontology

Background:

  • Vertebrate evolution exhibits significant variation in segment number, particularly trunk vertebrae.
  • The 'developmental hourglass' model posits conserved phylotypic stages, yet exceptions exist.

Purpose of the Study:

  • To investigate the relationship between vertebral segment number and early developmental stages.
  • To test the validity of the 'developmental hourglass' model in light of segment number variation.

Main Methods:

  • Comparative analysis of somite counts at the tailbud stage across vertebrate species.
  • Examination of gene expression patterns (e.g., Bmp-7) in developing structures.
  • Correlation of developmental data with evolutionary patterns of segment number.

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Main Results:

  • Differences in vertebral number are predicted by somite counts at the tailbud stage, a supposedly conserved phylotypic stage.
  • Somitogenesis demonstrates dissociation from conserved positional fields, challenging the developmental hourglass model.
  • Other systems like limbs, feathers, and pharyngeal arches also show dissociation, complicating identification of conserved stages.

Conclusions:

  • The 'developmental hourglass' model is violated by the plasticity of somitogenesis and other developmental processes.
  • Dissociation between developmental mechanisms and conserved positional fields allows for major evolutionary changes.
  • Understanding these dissociations is key to explaining evolutionary novelty in segment number and other traits.