title: Inhibition of Shh signalling in the chick wing gives insights into digit patterning and evolution.
abstract: In an influential model of pattern formation, a gradient of Sonic hedgehog (Shh) signalling in the chick wing bud specifies cells with three antero-posterior positional values, which give rise to three morphologically different digits by a self-organizing mechanism with Turing-like properties. However, as four of the five digits of the mouse limb are morphologically similar in terms of phalangeal pattern, it has been suggested that self-organization alone could be sufficient. Here, we show that inhibition of Shh signalling at a specific stage of chick wing development results in a pattern of four digits, three of which can have the same number of phalanges. These patterning changes are dependent on a posterior extension of the apical ectodermal ridge, and this also allows the additional digit to arise from the Shh-producing cells of the polarizing region - an ability lost in ancestral theropod dinosaurs. Our analyses reveal that, if the specification of antero-posterior positional values is curtailed, self-organization can then produce several digits with the same number of phalanges. We present a model that may give important insights into how the number of digits and phalanges has diverged during the evolution of avian and mammalian limbs.
authors.affiliation: Bateson Centre, Department of Biomedical Science, University of Sheffield, Western Bank, Sheffield S10 2TN, UK.
authors.forename: Joseph
authors.identifier: 0000-0002-5892-5159
authors.initials: J
authors.lastname: Pickering
authors.affiliation: Bateson Centre, Department of Biomedical Science, University of Sheffield, Western Bank, Sheffield S10 2TN, UK m.towers@sheffield.ac.uk.
authors.forename: Matthew
authors.identifier: 0000-0003-2189-4536
authors.initials: M
authors.lastname: Towers
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000235:genetics / Q000502:physiology; D005075:Biological Evolution; D019521:Body Patterning / Q000235:genetics / Q000502:physiology; D002642:Chick Embryo; D004622:Embryo, Mammalian / Q000378:metabolism*; D005434:Flow Cytometry; D018507:Gene Expression Regulation, Developmental / Q000235:genetics / Q000502:physiology; D053823:Hedgehog Proteins / Q000235:genetics / Q000378:metabolism*; D017403:In Situ Hybridization; D051379:Mice; D009024:Morphogenesis / Q000235:genetics / Q000502:physiology; D015398:Signal Transduction / Q000235:genetics / Q000502:physiology; D014921:Wings, Animal / Q000196:embryology / Q000378:metabolism*
chemical_list: D053823:Hedgehog Proteins
keywords: Chick development; Digits; Limb; Positional information; Self-organization; Shh