title: Evolution of digit identity in the three-toed Italian skink Chalcides chalcides: a new case of digit identity frame shift.
abstract: Digit identity in the avian wing is a classical example of conflicting anatomical and embryological evidence regarding digit homology. Anatomical in conjunction with phylogenetic evidence supports the hypothesis that the three remaining digits in the bird wing are digits 1, 2, and 3. At the same time, various lines of embryological evidence support the notion that these digits develop in positions that normally produce digits 2, 3, and 4. In recent years, gene expression as well as experimental evidence was published that supports the hypothesis that this discrepancy arose from a digit identity shift in the evolution of the bird wing. A similar but less well-known controversy has been ongoing since the late 19th century regarding the identity of the digits of the three-toed Italian skink, Chalcides chalcides. Comparative anatomy identifies these digits as 1, 2, and 3, while embryological evidence suggests their derivation from embryological positions 2, 3, and 4. Here we re-examine this evidence and add gene expression data to determine the identity of the three digits of C. chalcides. The data confirm that the adult and the embryological evidence for digit identity are in conflict, and the expression of Hoxd11 suggests that digits 1, 2, and 3 develop in positions 2, 3, and 4. We conclude that in C. chalcides, and likely in its close relatives, a digit identity frame shift has occurred, similar to the one in avian evolution. This result suggests that changes in of digit identity might be a more frequent consequence of digit reduction than previously assumed.
authors.affiliation: Department of Ecology and Evolutionary Biology, Yale University, New Haven, CT 06520, USA.
authors.forename: Rebecca L
authors.initials: RL
authors.lastname: Young
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Caputo
authors.forename: Massimo
authors.initials: M
authors.lastname: Giovannotti
authors.forename: Tiana
authors.initials: T
authors.lastname: Kohlsdorf
authors.forename: Alexander O
authors.initials: AO
authors.lastname: Vargas
authors.forename: Gemma E
authors.initials: GE
authors.lastname: May
authors.forename: Günter P
authors.initials: GP
authors.lastname: Wagner
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001846:Bone Development; D003001:Cloning, Molecular; D019143:Evolution, Molecular; D005121:Extremities / Q000033:anatomy & histology* / Q000502:physiology*; D005260:Female; D005786:Gene Expression Regulation; D018398:Homeodomain Proteins / Q000235:genetics* / Q000378:metabolism; D017403:In Situ Hybridization; D008116:Lizards / Q000033:anatomy & histology* / Q000502:physiology*; D008957:Models, Genetic; D008969:Molecular Sequence Data; D010802:Phylogeny; D017386:Sequence Homology, Amino Acid
chemical_list: D018398:Homeodomain Proteins