Pentadactyl pattern of the avian wing autopodium and pyramid reduction hypothesis

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Abstract
,We report,herein,that a pentadactyl,developmental,pattern,is evident,in early wing morphogenesis,of Gallus (chicken) and,Struthio (ostrich). Five avascular,zones (spatially predestined locations,of contiguous,metacarpal,and,phalangeal,aggregation),and,four interdigital,vascular,spaces are established,by the regression,patterns,of autopodial,vasculature.,Transient,vestiges,of the first and,fifth metacarpals,are confirmed,histologically,and,histochemically.,They lie within,the preaxial- most and postaxial-most avascular zones, respectively. These observations reveal conservative patterning of the avian hand and corroborate a II-III-IV metacarpal interpretation, argue for II-III- IV identity of ossified digits in birds, and favour a simple reduction rather than a homeotic shift in terms,of the,phenotype,expressed,by Hox genes,in the phylogeny,of the,avian,manus.,We suggest that gradual, bilateral reduction of phalanges and metacarpals, via apoptosis mediated by BMP, occurred,during,the evolution,of birds (Pyramid,Reduction,Hypothesis). This is congruent,with,the establishment,of a central,wing,axis that became,co-opted for coordinated,movements.,On the basis of evidence presented here, the direct avian ancestor is predicted to have been five-fingered with dominant digits (þ metacarpals) as follow: II, III, IV. J. Exp. Zool. (Mol. Dev. Evol.) 294:152–159, 2002. r 2002 Wiley-Liss, Inc. Interpretation,of the three,avian,digits surviv-
Funding Information
  • Granting Agency of Charles University in Prague, Czech Republic (136/2000/B BIO/ PřF)
  • Ministry of Education, Youth and Sport of the Czech Republic (MSM 111100005)
  • Natural Sciences and Engineering Research Council of Canada (OGP0009745)