Evidence for an amphibian sixth digit.

Shinichi Hayashi,Ryohei Seki,Shiro Egawa,Masataka Okabe,Namiko Kamiyama,Hitoshi Yokoyama,Tohru Yano,Takuya Kobayashi,Koji Tamura,Kazuki Takizawa

doi:10.1186/s40851-015-0019-y

Abstract

IntroductionDespite 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.ResultsWe 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.ConclusionIdentification 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.Electronic supplementary materialThe online version of this article (doi:10.1186/s40851-015-0019-y) contains supplementary material, which is available to authorized users.

Highlights

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
We suggest that the prehallux present in other members of the Xenopus genus represents a rudimentary sixth digit and that the prehallux in amphibian hindlimbs generally represents a sixth digit
X. tropicalis tadpoles were reared at 25 °C in dechlorinated tap water, and the staging methods used for X. laevis according to Nieuwkoop and Faber [22] were adopted for X. tropicalis and X. borealis

Summary

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

Conclusion

Introduction

Materials and methods

Results and discussion