title: Gene expression and digit homology in the chicken embryo wing.
abstract: 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.
authors.affiliation: Institute for Biology, Leiden University, Kaiserstraat 63, 2311 GP Leiden, The Netherlands.
authors.forename: Monique C M
authors.initials: MC
authors.lastname: Welten
authors.forename: Fons J
authors.initials: FJ
authors.lastname: Verbeek
authors.forename: Annemarie H
authors.initials: AH
authors.lastname: Meijer
authors.forename: Michael K
authors.initials: MK
authors.lastname: Richardson
mesh_terms: D000818:Animals; D001717:Birds; D019521:Body Patterning; D001846:Bone Development; D001842:Bone and Bones / Q000378:metabolism; D002454:Cell Differentiation; D002642:Chick Embryo; D005121:Extremities / Q000196:embryology; D005528:Foot / Q000196:embryology; D018507:Gene Expression Regulation, Developmental; D006609:High Mobility Group Proteins / Q000737:chemistry / Q000378:metabolism / Q000502:physiology*; D018398:Homeodomain Proteins / Q000502:physiology; D008954:Models, Biological; D008962:Models, Theoretical; D010641:Phenotype; D010802:Phylogeny; D017434:Protein Structure, Tertiary; D055758:SOX9 Transcription Factor; D013997:Time Factors; D014157:Transcription Factors / Q000737:chemistry / Q000378:metabolism / Q000502:physiology*; D014921:Wings, Animal / Q000196:embryology
chemical_list: D006609:High Mobility Group Proteins; D018398:Homeodomain Proteins; C490939:Hoxd11 protein, mouse; D055758:SOX9 Transcription Factor; C527933:Sox9 protein, mouse; D014157:Transcription Factors