Testicular Disorder of Sex Development in a Mare With a Low Incidence of SRY‐Positive Cell Line in the Gonads

An adult mare with ambiguous external genitalia, observed at a slaughterhouse, was subjected to detailed examination. The mare exhibited fused labia and an enlarged clitoris located at the ventral commissure of the vulva. Anatomical and histopathological studies revealed the presence of two testicles, vas deferens, fallopian tubes, a small uterus with blindly ending horns and a normally developed cervix. Cytogenetic analysis, using fluorescent in situ hybridisation (FISH) of in vitro cultured lymphocytes, showed a female karyotype—64,XX. Molecular detection of X- and Y-linked genes (SRY and ZFX/ZFY) in blood cells confirmed the presence of X-linked genes only. In contrast, in hair follicles and gonadal tissue, the presence of genes originating from the Y chromosome was also detected. The use of digital droplet PCR (ddPCR) revealed the presence of a SRY-positive cell line; however, at a very low level (< 5%). Analysis of polymorphic short tandem repeats (STRs), recommended for parentage testing, did not detect chimerism, which would be indicated by the presence of three or four variants at some STR loci. In conclusion, the studied case was classified as a sex chromosome disorder of sex development (DSD) due to gonadal XX/XY mosaicism. To the best of our knowledge, this is the first reported case of such an abnormality in a DSD horse.