2018: 64,XY (SRY-NEGATIVE) PHENOTYPIC MARE


64,XY (SRY-negative) phenotypic mare 
P.M. McCue, R.A. Ferris, J.A. Storme 

Department of Clinical Sciences, Colorado State University, Fort Collins, CO 
 

A 7-year-old American Quarter Horse mare was presented for reproductive evaluation. The 
owner’s goal was to have the mare carry her own foal to term, but the mare had never been observed to 
show behavioral estrus and had not received any medications. Physical examination revealed a 
phenotypically normal mare with a normal vulva and clitoris and a small mammary gland consistent with 
that of a maiden mare. Transrectal ultrasonographic examination showed very small gonads adjacent to 
the tip of each uterine horn. The gonadal structures were 1.5 cm in length and devoid of any visible 
follicles. Speculum examination revealed a normal vaginal vault and a normal external cervical os. 
Videoendoscopic examination revealed a normal uterine body, two uterine horns and a utero-tubular 
junction at the tip of each horn. Hormone analysis indicated low concentrations of anti-Müllerian 
hormone (AMH) (0.01 ng/ml; normal range 0.1 to 3.8 ng/ml), inhibin (0.21 ng/ml; normal range 0.1 to 
0.7 ng/ml), testosterone (27.6 pg/ml; normal range 20 to 45 pg/ml) and progesterone (0.12 ng/ml). A 
blood sample was subsequently submitted for chromosomal analysis, which indicated that the mare had a 
64, XY karyotype. Further analysis of the Y chromosome revealed that the sex-determining region of the 
Y chromosome (SRY) was absent. Specifically, PCR test results indicated that the mare was negative for 
all male-specific Y chromosome markers. Fluorescent in situ hybridization (FISH) test results confirmed 
that the small chromosome fragment was a portion of the heterochromatic part of the Y chromosome. The 
entire male specific region of Y (MSY) and the pseudoautosomal region on the Y were absent. The 
gonads were subsequently removed by standing laparoscopic surgery. Histologically the gonads were 
devoid of any normal ovarian or testicular structures. Development of a male or female reproductive tract 
is normally dictated by the genetic sex of the spermatozoon that fertilizes the oocyte. If the sperm carries 
a normal Y chromosome, the horse should have a 64, XY karyotype. The presence of an SRY region on 
the Y chromosome should drive differentiation of the gonads toward testicular development. Production 
of AMH by Sertoli cells and testosterone by Leydig cells of a normal testis would suppress development 
of the Müllerian duct and stimulate development of the Wolffian duct, respectively. Conversion of 
testosterone to dihydrotestosterone (DHT) by the enzyme 5α-reductase in peripheral tissues should lead to 
formation of a penis and scrotum. However, in the present case, absence of an SRY region on the Y 
chromosome lead to failure of gonadal development and subsequent low concentrations of the hormones 
AMH, testosterone and DHT led to formation of female internal and external genitalia. A chromosomal 
abnormality should be suspected in adult mares with bilaterally small, inactive ovaries during the 
physiologic breeding season that have not been treated with exogenous hormones or vaccinated against 
gonadotropin releasing hormone.  
 
Keywords: Equine, genetic infertility, sex-reversal syndrome, chromosomal abnormality 
 

Clinical Theriogenology • Volume 10,  Number 3 • September 2018305