Gene expression and digit homology in the chicken embryo wing

Monique C M Welten, Fons J Verbeek, Annemarie H Meijer, Michael K Richardson

Research output: Contribution to journalArticle (Academic Journal)peer-review

64 Citations (Scopus)


The bird wing is of special interest to students of homology and avian evolution. Fossil and developmental data give conflicting indications of digit homology if a pentadactyl "archetype" is assumed. Morphological signs of a vestigial digit I are seen in bird embryos, but no digit-like structure develops in wild-type embryos. To examine the developmental mechanisms of digit loss, we studied the expression of the high-mobility group box containing Sox9 gene, and bone morphogenetic protein receptor 1b (bmpR-1b)-markers for precondensation and prechondrogenic cells, respectively. We find an elongated domain of Sox9 expression, but no bmpR-1b expression, anterior to digit II. We interpret this as a digit I domain that reaches precondensation, but not condensation or precartilage stages. It develops late, when the tissue in which it is lodged is being remodeled. We consider these findings in the light of previous Hoxd-11 misexpression studies. Together, they suggest that there is a digit I vestige in the wing that can be rescued and undergo development if posterior patterning cues are enhanced. We observed Sox9 expression in the elusive "element X" that is sometimes stated to represent a sixth digit. Indeed, incongruity between digit domains and identities in theropods disappears if birds and other archosaurs are considered primitively polydactyl. Our study provides the first gene expression evidence for at least five digital domains in the chick wing. The failure of the first to develop may be plausibly linked to attenuation of posterior signals.

Original languageEnglish
Pages (from-to)18-28
Number of pages11
JournalEvolution and Development
Issue number1
Early online date3 Jan 2005
Publication statusPublished - 12 Jan 2005


  • Animals
  • Birds
  • Body Patterning
  • Bone Development
  • Bone and Bones
  • Cell Differentiation
  • Chick Embryo
  • Extremities
  • Foot
  • Gene Expression Regulation, Developmental
  • High Mobility Group Proteins
  • Homeodomain Proteins
  • Models, Biological
  • Models, Theoretical
  • Phenotype
  • Phylogeny
  • Protein Structure, Tertiary
  • SOX9 Transcription Factor
  • Time Factors
  • Transcription Factors
  • Wing


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