Equine sex reversal syndrome: XY, Sry positive and XX, Sry negative mares.
DAF Villagómeza, T L Learc, T Chenierb, S Leeb, J Cahilld, ER Reyesa, WA Kinga
a
Department of Biomedical Sciences, bDepartment of Population Medicine, University of
Guelph, Guelph, Ontario, (Canada); cVeterinary Science Department, M.H. Gluck Equine
Research Center, University of Kentucky, Lexington, KY (USA); dEquine Parentage and
Animal Genetics Services Centre, Massey University, Palmerston North (New Zealand)
Genetic and phenotypic heterogeneity has repeatedly been reported for the equine sex
reversal syndrome. Four new clinical cases are documented here. Normal phenotype, but
male aggressive behavior characterizes an American Quarter horse mare, which upon
surgery demonstrated two inguinal hypoplastic testes, a normal G-banded stallion
karyotype (2n 64, XY) and positive for the Sry gene. External genital ambiguity (penislike enlarged clitoris) and male aggressive behavior were common for three mares (an
Arabian, Andalusian and Tennessee Waking mare) send to the veterinary clinic, further
cytogenetic and molecular analysis for the Sry gene (PCR) defined a 64, XX, Srynegative constitution. True hermaphroditism was diagnosed by histology analysis of
gonads recovered from two of the mares (Arabian and Andalusian), which demonstrated
presence of both gonadal tissues (i.e. bilateral ovotestes). In the Tennessee Walking mare
transrectal ultrasonography failed to demonstrate the presence of gonadal structures,
mean while her high levels of testosterone (201.8pg/ml) were suggestive of testicular
tissue. Androgen insensitivity due to an X-linked mutation of the androgen receptor may
explain the presence of normal male karyotype and Sry positive for our American
Quarter horse mare case. Less is understood about sex reversal syndrome in ovotesticular
XX, Sry negative mares. Nevertheless, familial studies have proposed autosomal
inheritance of that syndrome.