title: Circumventing the polydactyly 'constraint': the mole's 'thumb'.
abstract: Talpid moles across all northern continents exhibit a remarkably large, sickle-like radial sesamoid bone anterior to their five digits, always coupled with a smaller tibial sesamoid bone. A possible developmental mechanism behind this phenomenon was revealed using molecular markers during limb development in the Iberian mole (Talpa occidentalis) and a shrew (Cryptotis parva), as shrews represent the closest relatives of moles but do not show these conspicuous elements. The mole's radial sesamoid develops later than true digits, as shown by Sox9, and extends into the digit area, developing in relation to an Msx2-domain at the anterior border of the digital plate. Fgf8 expression, marking the apical ectodermal ridge, is comparable in both species. Developmental peculiarities facilitated the inclusion of the mole's radial sesamoid into the digit series; talpid moles circumvent the almost universal pentadactyly constraint by recruiting wrist sesamoids into their digital region using a novel developmental pathway and timing.
authors.affiliation: Paläontologisches Institut und Museum, Universität Zürich, Zürich, Switzerland.
authors.forename: Christian
authors.initials: C
authors.lastname: Mitgutsch
authors.forename: Michael K
authors.initials: MK
authors.lastname: Richardson
authors.forename: Rafael
authors.initials: R
authors.lastname: Jiménez
authors.forename: José E
authors.initials: JE
authors.lastname: Martin
authors.forename: Peter
authors.initials: P
authors.lastname: Kondrashov
authors.forename: Merijn A G
authors.initials: MA
authors.lastname: de Bakker
authors.forename: Marcelo R
authors.initials: MR
authors.lastname: Sánchez-Villagra
mesh_terms: D000818:Animals; D005075:Biological Evolution; D002350:Carpus, Animal / Q000033:anatomy & histology / Q000196:embryology*; D055032:Electron Microscope Tomography; D005346:Fibroblast Growth Factors / Q000378:metabolism; D018507:Gene Expression Regulation, Developmental / Q000502:physiology*; D018398:Homeodomain Proteins / Q000378:metabolism; D017403:In Situ Hybridization; D008971:Moles / Q000033:anatomy & histology / Q000196:embryology*; D055758:SOX9 Transcription Factor / Q000378:metabolism; D012716:Sesamoid Bones / Q000033:anatomy & histology / Q000196:embryology*; D012788:Shrews / Q000033:anatomy & histology / Q000196:embryology*; D013030:Spain; D013045:Species Specificity
chemical_list: D018398:Homeodomain Proteins; C084119:MSX2 protein; D055758:SOX9 Transcription Factor; D005346:Fibroblast Growth Factors