A male pseudohermaphrodite horse with 63Xo?/64XX/65XXY mixoploidy.

This study investigates a case of intersexuality in a 6-month-old Arabian filly found to be a male pseudohermaphrodite. Genetic analysis reveals a peculiar chromosomal composition exhibiting mixoploidy – a rare condition defined by the presence of cells with different chromosomal numbers within the same individual.

Study Context and Objectives

• The paper adds to a limited number of studies investigating the nature of intersexuality in horses, defined as discrepancies between an individual’s physical sex traits and sexual behaviour, and their genetic sex determination. The paper aims to describe the specific scenario observed in this unique case of a male pseudohermaphrodite.

Methods and Observations

• The horse exhibited unusual sexual behaviours for a filly, including abnormal interest and interactions with mature mares in heat, and Urinates in a dorso-caudal direction owing to the hermaphroditism.
• A mid-line laparotomy revealed a small gonad in each internal inguinal ring, which appeared to be testicles with associated epididymes. There was no evidence of uterine tissue.
• Microscopic examination confirmed the gonads as testicular tissue. While there were seminiferous tubules and interstitial tissues containing Leydig cells, there was no sign of spermatogenesis.

Genetic Findings

• Chromosomal analysis across five tissue types, conducted by examining 241 metaphases, revealed the presence of cells with 64 chromosomes and an XX sex chromosome constitution. There were also cells with less than 64, and those with 65 chromosomes, presenting a rare instance of mixoploidy.
• The sex chromosome constitution across these cells varied: 5.4% of cells with 65 chromosomes had an XXY sex chromosome constitution, while 2.9% of cells with less than 64 chromosomes had a single X chromosome. Overall, the genetic sex of the horse was determined as 63XO?/64XX/65XXY.

Discussion

• The clinical, anatomical, and histological findings match those reported for five other hermaphrodite horses studied previously. They all exhibited a female phenotype with an enlarged clitoris and underdeveloped testes typically located either in the inguinal region or intra-abdominally.
• Given the horse’s mixoploidy, it closely resembles a previously reported 64XX/65XXY mixoploid. However, it is challenging to understand the importance of the 2.9% XO cells as the singular X chromosome could have been lost during metaphase flattening or due to a mitotic error in the long-term cultures.