XY Sex Reversal in Horses: The Genes Behind the Switch

Veterinary researchers have taken a leap forward in explaining XY sex reversal

Veterinary researchers have taken a leap forward in explaining XY sex reversal, the most common equine sex chromosome abnormality. A team of scientists from the University of Kentucky's Gluck Equine Research Center and the College of Veterinary Science at Texas A&M University have been examining the Y chromosome to determine what prompts a genetic mutation that causes seemingly healthy mares to be infertile.

XY sex reversal is the development of female genitalia in a horse possessing the male Y chromosome; i.e., the horse should have displayed male genitalia. When an abnormality affects sex differentiation, the developing equine fetus "defaults" to the female developmental pathway.

"The majority of affected horses have normal female genitalia and no stallionlike behaviors," said Teri L. Lear, PhD, an immunogenetics researcher in the molecular cytogenetics laboratory at the Gluck Center. "The sex reversal is often not noted unless there is a problem with infertility."

Some "XY mares," as they are called, however, show various degrees of masculinization, visible as abnormally developed reproductive tracts.

Little is known about the underlying cause of XY sex reversal. To shed light on this condition, Lear collaborated with Terje Raudsepp, PhD, and colleagues at Texas A&M University who used state-of-the-art molecular techniques to create a map of the equine Y chromosome of XY mares.

"We looked at the Y chromosome from 18 XY mares (of various breeds) and found that there are actually two different forms of the condition," Lear explained. "The first form is a Y chromosome-linked form in which large sections of the DNA on the Y chromosome are missing. In the second form, the non-Y-linked form, the Y chromosome is the same as in normal males."

A closer look at the Y-linked form revealed that different regions of the DNA are missing, but in all mares with the Y-linked form, the gene called SRY, which is a sex-determining gene on the Y chromosome, was either partially or completely deleted.

"Horses which lack either some or all of the SRY gene will not develop into the 'right' sex, resulting in genetically male horses that look like mares," Lear continued.

Why horses possessing the entire SRY gene (the mares with non-Y linked sex reversal) become "sex reversed" is still unknown, however.

Lear said that this research provides a better understanding of a condition that translates to infertility in outwardly normal mares.

She added that despite the fact that these horses are genetically male, XY mares race against other mares, posing the question of whether they have an unfair advantage in competition.

"We do not know if the rest of the Y chromosome confers a physical advantage for racing or not," she added.

The study, "Molecular heterogeneity of XY sex reversal in horses," was published in a special edition of the journal Animal Genetics. The entire journal supplement, funded by the Dorothy Russell Havemeyer Foundation is available for free online.


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Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.