title: Evidence for an amphibian sixth digit.
abstract: BACKGROUND
  Despite the great diversity in digit morphology reflecting the adaptation of tetrapods to their lifestyle, the number of digits in extant tetrapod species is conservatively stabilized at five or less, which is known as the pentadactyl constraint.
  
  
  RESULTS
  We found that an anuran amphibian species, Xenopus tropicalis (western clawed frog), has a clawed protrusion anteroventral to digit I on the foot. To identify the nature of the anterior-most clawed protrusion, we examined its morphology, tissue composition, development, and gene expression. We demonstrated that the protrusion in the X. tropicalis hindlimb is the sixth digit, as is evident from anatomical features, development, and molecular marker expression.
  
  
  CONCLUSIONS
  Identification of the sixth digit in the X. tropicalis hindlimb strongly suggests that the prehallux in other Xenopus species with similar morphology and at the same position as the sixth digit is also a vestigial digit. We propose here that the prehallux seen in various species of amphibians generally represents a rudimentary sixth digit.
authors.affiliation: Department of Developmental Biology and Neurosciences, Graduate School of Life Sciences, Tohoku University, Sendai, 980-8578 Japan.
authors.forename: Shinichi
authors.initials: S
authors.lastname: Hayashi
authors.affiliation: Department of Developmental Biology and Neurosciences, Graduate School of Life Sciences, Tohoku University, Sendai, 980-8578 Japan.
authors.forename: Takuya
authors.initials: T
authors.lastname: Kobayashi
authors.affiliation: Department of Anatomy, The Jikei University School of Medicine, Tokyo, 105-8461 Japan.
authors.forename: Tohru
authors.initials: T
authors.lastname: Yano
authors.affiliation: Department of Developmental Biology and Neurosciences, Graduate School of Life Sciences, Tohoku University, Sendai, 980-8578 Japan.
authors.forename: Namiko
authors.initials: N
authors.lastname: Kamiyama
authors.affiliation: Department of Developmental Biology and Neurosciences, Graduate School of Life Sciences, Tohoku University, Sendai, 980-8578 Japan.
authors.forename: Shiro
authors.initials: S
authors.lastname: Egawa
authors.affiliation: Department of Developmental Biology and Neurosciences, Graduate School of Life Sciences, Tohoku University, Sendai, 980-8578 Japan ; Mammalian Genetics Laboratory, Genetic Strains Research Center, National Institute of Genetics, 1111 Yata, Mishima, Shizuoka 411-8540 Japan.
authors.forename: Ryohei
authors.initials: R
authors.lastname: Seki
authors.affiliation: Department of Developmental Biology and Neurosciences, Graduate School of Life Sciences, Tohoku University, Sendai, 980-8578 Japan.
authors.forename: Kazuki
authors.initials: K
authors.lastname: Takizawa
authors.affiliation: Department of Anatomy, The Jikei University School of Medicine, Tokyo, 105-8461 Japan.
authors.forename: Masataka
authors.initials: M
authors.lastname: Okabe
authors.affiliation: Department of Developmental Biology and Neurosciences, Graduate School of Life Sciences, Tohoku University, Sendai, 980-8578 Japan ; Department of Biochemistry and Molecular Biology, Faculty of Agriculture and Life Science, Hirosaki University, Hirosaki, 036-8561 Japan.
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Yokoyama
authors.affiliation: Department of Developmental Biology and Neurosciences, Graduate School of Life Sciences, Tohoku University, Sendai, 980-8578 Japan.
authors.forename: Koji
authors.initials: K
authors.lastname: Tamura
keywords: Amphibian; Digit; Limb; Pentadactyly; Xenopus