Two equine true hermaphrodites with 64,XX/64,XY and 63,XO/64,XY chimerism.

This study examines two cases of true hermaphroditism in horses, one a Welsh pony and the other a Standardbred, with unique chromosomal makeups. The researchers suggest possible causes could be double fertilization, fusion of blastocysts, or chromosomal loss during early embryonic development.

Background

• The research investigates two instances of unique chromosomal configurations in equine subjects – a Welsh pony and a Standardbred horse. These subjects exhibited characteristics of true hermaphroditism, a condition wherein an individual possesses both male and female reproductive organs.
• The normal horse karyotype, or chromosomal structure, is expected to be 64,XX for females and 64,XY for males. However, in these examined horses, the karyotypes were found to be 64,XX/64,XY and 63,XO/64,XY. This abnormal chromosomal makeup classified the subjects as “chimeric,” indicating they carry two different sets of genetic material.

Findings

• Both equine intersexes in the study exhibit true hermaphroditism with bilateral ovotestes, meaning they have both ovaries and testes. Neither horse expressed typical male behavior.
• Physical observations revealed the presence of an underdeveloped penis, bilateral seminal vesicles, and uterine tissue in each horse.
• The researchers proposed that the chimerism in these horses could have resulted from either double fertilization – where two sperm would fertilize a single egg – or fusion of blastocysts – a stage of embryonic development wherein two fertilized eggs could have potentially fused together.

Speculations and Conclusions

• In the case of the Standardbred horse with a 63,XO/64,XY karyotype, an alternative explanation was considered. Mosaicism, another form of chromosomal anomaly, could have occurred as a result of an early embryonic XY cell losing a Y chromosome during replication – a phenomenon known as ‘anaphase lag’.
• While this research offers valuable insights into the chromosomal variations that may lead to intersexuality in horses, further studies are necessary to confirm these speculations and fully unravel the genetic and developmental mechanisms that contribute to these conditions.