A sporadic case of the sex-reversed mare (64,XY; SRY-negative): molecular and cytogenetic studies of the Y chromosome.

The study concerns a case of a mare with a sex-reversal condition that frequently appears in horses, due to the absence of the SRY gene which is related to male characteristics. The researchers conducted molecular and cytogenetic analyses of the Y-linked genes to understand the causes and effects of this syndrome.

Background: The Sex-Reversal Syndrome

• The sex-reversal syndrome is a condition that appears frequently in horses, particularly in mares.
• Mares with this syndrome lack an SRY gene, which usually determines the male sex in mammals.
• The researchers speculate that the syndrome could result due to the transfer of the SRY gene from the Y chromosome to the X chromosome, due to abnormal meiotic exchange.

Subject and Methods of the Research

• The subject of the research is a case of a sex-reversed mare that is infertile with a 64,XY karyotype. This signifies an abnormal pairing of sex chromosomes, with the mare displaying a typically male karyotype.
• The research involved molecular analysis of the Y-linked genes in the mare. A total of 350 metaphase spreads stained by CBG banding were analysed to establish the karyotype.

Findings of the Molecular Analysis

• The molecular analysis of the genes linked to the Y chromosome revealed the absence of the SRY gene. The presence of other studied loci (ZFY, AMEL-Y, and STS-Y) was found.
• All fragments representing the X chromosome (ZFX, AMEL-X, and STS-X) were detected in the mare.
• The mare’s external genitalia were normal, but the uterus was small and the ovaries extremely small, as detected by ultrasonography.

Possible Mechanism of Sex-Reversal Syndrome Formation

• The researchers postulate the likely mechanism of the formation of the sex-reversal syndrome. They theorize that two crossing-over events might happen between the X and Y chromosomes during spermatogenesis in the father (sire).
• One of these events is suggested to happen between the ZFY and SRY loci, while the other event might occur between the SRY locus and the centromere.

Significance

• The research holds importance in understanding the sex-reversal syndrome in horses, its causes, and its impact on fertility.
• With further studies, it could open potential avenues for genetic manipulation to address such sex-reversal cases and tackle infertility issues in horses.