Y Chromosome Ancestry in the Arabian Horse by Beth Minnich with Michael Bowling
“The Arabian horse possesses a rich history, intimately connected to the physical environment of its ancestral homeland and culture of Bedouin caretakers who nurtured its development. It is unclear when and where the horse was introduced to the area historically known as ‘Arabia’. Yet, from a regional proto-Arabian that was developed over millennia, the nomadic horse breeding tribes cultivated the foundation of the breed now known as the Arabian horse. From the mapping of the horse genome in 2007, genomic tools were developed that provide access to information offering expanded perspectives for viewing breed ancestry. These new observations provide a gateway to deeper understanding of the Arabian breed, helping to connect the threads of culture, history, and genetics. With these threads, we can weave a more detailed tapestry depicting the origins of the Arabian horse, to help guide preservation efforts of this iconic animal.” The goal of this article is to introduce equine Y chromosome research, with a focus on what has been learned (to date) about Y chromosome ancestry in the Arabian horse. Using modern scientific technology, we can gain greater understanding of the fascinating history of the Arabian horse. Keeping in mind ‘discovery’, by its nature, can yield unexpected results; some of the findings to be discussed are controversial. At the same time, good science should lead to more questions, and as we learn more about the background of the Arabian horse, further questions will arise. The additional perspective offered, alongside a basic explanation of how the conclusions have been drawn, will hopefully provide valuable context as work continues. This is important for the interpretation of data by researchers, as well as for development of narratives shared within the Arabian horse and broader equine communities.
If these words look familiar, they are the opening to “Genetic Diversity & Complex Ancestry in the Arabian Horse” (May 2021, Khamsat), which introduced findings from the scientific paper “Genome Diversity and the Origin of the Arabian Horse” (Cosgrove, et al. Scientific Reports, June 2020).1,2 Genomic studies are bringing about new ways of viewing DNA history of the Arabian horse and other breeds, helping to connect the past to the present. For a brief recap of this research, the main summary points across the study include: 1. The Arabian breed has a unique genetic profile marked by broad variation and underlying complex ancestry consistent with an ancient origin: • Globally, Arabian horses display a large degree of genetic diversity, more than many other breeds of horse.
Y Chromosome Basics3,4
• Registered Arabian horses were identified in the Middle East that carry expanded genetic and phenotypic diversity.
To establish a foundation to work from, let’s first review some Y chromosome basics. [Note: to assist with terminology, italicized terms appear in a glossary on page 18.]
• Straight Egyptians have a distinct genetic signature and less genetic variation than other Arabian bloodline groups considered.
4. Strong evidence was found for recent interbreeding of Thoroughbreds with Arabians used for flat racing.
Of a horse’s 64 chromosomes, two are sex chromosomes which are designated as X and Y. These chromosomes determine the sex of a horse, with females inheriting two copies of the X chromosome and males inheriting one X chromosome and one Y chromosome. Dams can only pass along an X chromosome to their offspring, and the Y chromosome can only be passed by the sire. But, because a stallion can contribute either an X or Y chromosome, his contribution is what determines the sex of offspring; an X chromosome from the sire produces a filly, while a Y chromosome results in a colt.
As we continue the journey to better understand how DNA can assist in unlocking the mysteries of breed history and ancestry, we will take an introductory walk through the Y chromosome component of the study. To help set the stage for digesting this material, there will also be a brief overview on inheritance and ancestry of the equine Y chromosome. Important work from the lab of Dr. Barbara Wallner, from the Institute of Animal Breeding and Genetics at the University of Veterinary Medicine, Vienna, will also be included.
Though perhaps the smallest chromosome in the mammalian genome, the Y chromosome is one of the most distinct regions. Comprising <2% of the total haploid genome, the Y chromosome is male specific, and the horse Y chromosome contains 52 genes. Some of these genes have important functions, playing a role in the development of male characteristics, spermatogenesis and male fertility. For comparison, the X chromosome comprises about 5% of the total haploid genome, is the 3rd largest chromosome, and is estimated to contain >800 genes.
2. Genomic regions were identified that may be associated with important traits of the Arabian horse, such as head shape and athletic ability. 3. Little overall genetic similarity of Arabians to Thoroughbreds was detected, including lack of evidence for Arabian stallion Y chromosome ancestry.
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