title: Insights into bird wing evolution and digit specification from polarizing region fate maps.
abstract: The proposal that birds descended from theropod dinosaurs with digits 2, 3 and 4 was recently given support by short-term fate maps, suggesting that the chick wing polarizing region-a group that Sonic hedgehog-expressing cells-gives rise to digit 4. Here we show using long-term fate maps that Green fluorescent protein-expressing chick wing polarizing region grafts contribute only to soft tissues along the posterior margin of digit 4, supporting fossil data that birds descended from theropods that had digits 1, 2 and 3. In contrast, digit IV of the chick leg with four digits (I-IV) arises from the polarizing region. To determine how digit identity is specified over time, we inhibited Sonic hedgehog signalling. Fate maps show that polarizing region and adjacent cells are specified in parallel through a series of anterior to posterior digit fates-a process of digit specification that we suggest is involved in patterning all vertebrate limbs with more than three digits.
authors.affiliation: Department of Biology and Biochemistry, University of Bath, Claverton Down Road, Bath BA2 7AY, UK. m.towers@sheffield.ac.uk
authors.forename: Matthew
authors.initials: M
authors.lastname: Towers
authors.forename: Jason
authors.initials: J
authors.lastname: Signolet
authors.forename: Adrian
authors.initials: A
authors.lastname: Sherman
authors.forename: Helen
authors.initials: H
authors.lastname: Sang
authors.forename: Cheryll
authors.initials: C
authors.lastname: Tickle
mesh_terms: D000818:Animals; D005075:Biological Evolution; D001717:Birds / Q000033:anatomy & histology / Q000145:classification / Q000235:genetics / Q000254:growth & development*; D019521:Body Patterning; D015870:Gene Expression; D053823:Hedgehog Proteins / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction; D014921:Wings, Animal / Q000033:anatomy & histology / Q000254:growth & development* / Q000378:metabolism
chemical_list: D053823:Hedgehog Proteins