An unusual case of disorders of sexual development in Arabian horse 2n=64, XX, SRY-negative.

“Sex reversal syndrome - SRS” is a congenital condition in which there is a complete or incomplete gonadal development but a discordance between genetic, gonadal and phenotypic sex [1,2]. The term Disorder of Sexual Development (DSD) has been proposed for all congenital abnormalities in which the chromosomal, gonadal or anatomical sex development is atypical [3]. In the horse, several types of DSDs have been described, including mosaic shapes, as follows: (i) 63, XO; (ii) 64, XX, SRY-negative; (iii) 64, XY, SRYpositive; and (iv) 64, XY, SRY-negative [4]. This work describes a DSD in a 3-year-old Arabian mare. At the clinical examination, this mare showed ambiguous external genitalia and non-typical female behaviour (nervous temperament, aggressiveness, masculine attitude). The distance between anus and vulva was shorter than normal, vulvar lips were dorsally fused except for the lower neckline showing a blind ending, from which a penis-like structure protruded. The ultrasound examination revealed the presence of cervix, uterus, hypoplastic uterine horns and small gonads with an echogenicity similar to a testicular parenchyma. Blood samples have been used for hormonal dosage and cytogenetic and genetic analyses. Blood testosterone levels ranged from 0.4 to 0.6 ng/ml. Cytogenetic analysis showed a normal female karyotype (2n=64, XX), while PCR amplification of SRY and ZFY genes revealed the absence of Y chromosome. At necroscopic examination, we found internal genitalia arising from the genital ridge in the form of masculine type structures while those deriving from the Mullerian ducts in the form of feminine type structures. In addition, we found an infundibular portion of the fallopian tube at the cranial pole of the gonads. This is a first case in equine species of DSD 2n=64, XX, SRY-, with simultaneous presence of male (hypoplastic testicles, epididymal portions and a penis-like structure) and female (cervix, horn and body of an hypoplastic uterus) genital structures. The absence of the Y chromosome suggests to hypothesize that other genes, besides those present on the Y chromosome, are involved in sexual differentiation in the equine species.