Genetic Diversity in Domestic Horses: Temporal Patterns, the Influence of Assisted Reproductive Technologies, and Implications for the American Quarter Horse.

Overview

• This study examines changes in genetic diversity in domestic horses, focusing on the American Quarter Horse breed, and how assisted reproductive technologies (ART) may impact this diversity.
• The research highlights a recent decline in genetic diversity linked to ART practices, especially involving mare selection, and discusses the implications for breed health and management.

Introduction to Genetic Diversity in Domestic Horses

• Genetic diversity refers to the heritable variation found within and across horse populations.
• It is crucial for the long-term health, vitality, and ability of horses to adapt through selective breeding.
• Previous research has identified key factors shaping genetic diversity, including:
  • Effective population size – the number of individuals contributing genes to the next generation
  • Founder events – initial breeding animals that established the population
  • Artificial selection – human-driven breeding choices favoring certain traits
  • Inbreeding levels – mating among related animals that can reduce diversity
• Some horse breeds with closed studbooks (no new external genetic input) maintain moderate diversity, but others suffer rapid loss from popular sire effects (overuse of a few males) and intensive inbreeding.
• Mare lines (female lineage) still play a critical role in maintaining diversity within these populations.

The Role of Assisted Reproductive Technologies (ART)

• ART includes breeding techniques like artificial insemination, embryo transfer, ovum pickup (OPU), and in vitro embryo production (IVEP).
• These technologies allow for:
  • Greater use of top-performing stallions across many mares
  • Expanded breeding opportunities beyond natural limitations
  • Potential concentration of genetic contributions from fewer mares
• The overall impact of ART on genetic diversity in horses had been largely unknown prior to this study.

Study Focus: American Quarter Horse Breed

• The American Quarter Horse is a breed previously considered to maintain relatively high genetic diversity.
• Researchers analyzed registration-based genetic data across different cohorts, particularly comparing generations from 2012 and 2022.
• Main findings include:
  • A 1.38% decline in genetic diversity over the last generation assessed by the effective number of alleles metric.
  • Foals born via ART in 2022 were statistically less genetically diverse than those born via traditional live cover breeding (p<0.01).
  • Mares involved in ART were significantly less diverse than mares in live cover matings (p<0.01).
  • Stallion diversity did not significantly differ between ART and live cover groups.

Interpretation and Implications

• The data suggest that breeding schemes associated with ART, especially the use of less genetically diverse mares, contribute to the observed recent decline in genetic diversity.
• This is concerning because reduced genetic diversity can:
  • Increase risk of inherited disorders
  • Compromise breeding flexibility and selection potential
  • Reduce overall population health
• The study establishes important baseline metrics for monitoring how genetic diversity changes across generations and different breeding methods.
• Understanding the interaction between ART and breeding decisions is essential to:
  • Develop breeding strategies that preserve or enhance genetic variation
  • Support industry goals such as genetic improvement while maintaining breed health
  • Mitigate risks associated with popular sire effects and inbreeding facilitated by ART

Conclusion

• This research highlights the critical need to consider the genetic consequences of assisted reproductive technologies in domestic horse breeding.
• Careful management of mare genetic diversity in ART programs will be key to sustaining the health and adaptability of the American Quarter Horse and other breeds.
• Continued genetic monitoring and strategic breeding decisions are recommended to balance technological advancements with the preservation of genetic diversity.