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Home / Equine Research Bank / Proc Biol Sci / Evidence of five digits in embryonic horses and developmenta...
Proceedings. Biological sciences2020; 287(1920); 20192756; doi: 10.1098/rspb.2019.2756

Evidence of five digits in embryonic horses and developmental stabilization of tetrapod digit number.

Abstract: Previous work comparing the developmental mechanisms involved in digit reduction in horses with other mammals reported that horses have only a 'single digit', with two flanking metapodials identified as remnants of digit II and IV. Here we show that early embryos go through a stage with five digit condensations, and that the flanking splint metapodials result from fusions of the two anterior digits I and II and the two posterior digits IV and V, in a striking parallel between ontogeny and phylogeny. Given that even this most extreme case of digit reduction exhibits primary pentadactyly, we re-examined the initial stages of digit condensation of all digit-reduced tetrapods where data are available and found that in all cases, five or four digits initiate (four with digit I missing). The persistent pentadactyl initiation in the horse and other digit-reduced modern taxa underscores a durable developmental stability at the initiation of digits. The digit evodevo model may help illuminate the biological circumstances under which organ systems become highly stabilized versus highly plastic.
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Publication Date: 2020-02-05 PubMed ID: 32019446PubMed Central: PMC7031666DOI: 10.1098/rspb.2019.2756Google Scholar: Lookup
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  • Journal Article
  • Research Support
  • U.S. Gov't
  • Non-P.H.S.

Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

The research investigates the development of digits in embryologic horses, highlighting that despite previous theories of horses having a single digit, they initially present with five digit formations which later fuse to form the existing single digit and surrounding structures.

Research Purpose and Methodology

  • The research aims to deepen the understanding of digit development in horses, challenging the traditional perception of horses having a ‘single digit’ with additional metapodials as remnants of other digits.
  • The researchers studied early horse embryos to observe how the number of digits changes throughout development.

Findings

  • Contrary to initial beliefs, the study found that early horse embryos go through a stage with five digit formations or condensations.
  • It was observed that the existing single digit and two flanking metapodials in adult horses are in fact, a result of fusions of the original five digits.
  • The anterior digits (Digits I and II) fuse to form one structure, and similarly the posterior digits (Digits IV and V) fuse to form another structure – reinforcing a framework of connectivity between ontogeny (development of an individual organism) and phylogeny (evolutionary history of a species).

Re-evaluation of Digit Development in Reduced Digit Species

  • Based on these findings, the researchers reevaluated digit formation in other tetrapods (four-legged animals) with a reduced number of digits.
  • They discovered the initial stages of digit condensation in all cases showed either five or four digits initiating; the latter case specifically missing Digit I.
  • This pentadactyl initiation is consistent across various taxas, including the horse, underlining a robust developmental stability at the start of digit formation.

Implications

  • The research establishes a strong evolutionary connect between the embryonic stages and the adult form of tetrapods, particularly horses.
  • The findings also contribute to the digit evolutionary developmental biology model, helping to evaluate why some organ systems become highly stable while others are more adaptable.

Cite This Article

APA
Kavanagh KD, Bailey CS, Sears KE. (2020). Evidence of five digits in embryonic horses and developmental stabilization of tetrapod digit number. Proc Biol Sci, 287(1920), 20192756. https://doi.org/10.1098/rspb.2019.2756

Publication

Proceedings. Biological sciences
ISSN: 1471-2954
NlmUniqueID: 101245157
Country: England
Language: English
Volume: 287
Issue: 1920
Pages: 20192756
PII: 20192756

Researcher Affiliations

Kavanagh, Kathryn D
  • Department of Biology, University of Massachusetts Dartmouth, Dartmouth, MA, USA.
Bailey, C Scott
  • Department of Clinical Sciences, College of Veterinary Medicine, North Carolina State University, Raleigh, NC 27607, USA.
Sears, Karen E
  • Department of Ecology and Evolutionary Biology, University of California Los Angeles, Los Angeles, CA, USA.

MeSH Terms

  • Animals
  • Biological Evolution
  • Body Patterning
  • Equidae
  • Extremities / embryology
  • Forelimb
  • Horses / embryology
  • Phylogeny

Conflict of Interest Statement

We declare we have no competing interests.

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Citations

This article has been cited 4 times.
  1. Smith S, Yohe LR, Solounias N. The bony cap and its distinction from the distal phalanx in humans, cats, and horses. PeerJ 2023;11:e14352.
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  2. Howenstine AO, Sadier A, Anthwal N, Lau CL, Sears KE. Non-model systems in mammalian forelimb evo-devo. Curr Opin Genet Dev 2021 Aug;69:65-71.
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  3. Luan Y, Zhong L, Li C, Yue X, Ye M, Wang J, Zhu Y, Wang Q. A dominant missense variant within LMBR1 related to equine polydactyly. Commun Biol 2024 Oct 31;7(1):1420.
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  4. Vincelette AR, Renders E, Scott KM, Falkingham PL, Janis CM. Hipparion tracks and horses' toes: the evolution of the equid single hoof. R Soc Open Sci 2023 Jun;10(6):230358.
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