Cytogenetic, genomic, and reproductive characterization of four pura Raza Española mares carrying a 64, X, del(X),t(X; Y) complex karyotype.

Overview

• This research characterizes a rare complex chromosomal rearrangement involving sex chromosomes in four Pura Raza Española (PRE) mares, including a fertile individual who transmitted the abnormality to her offspring.
• The study combines genomic, cytogenetic, and reproductive analyses to understand how these chromosomal abnormalities affect fertility and inheritance.

Background and Importance

• Sex chromosomes (X and Y) are crucial in determining biology and reproductive function in horses.
• Abnormalities or rearrangements in sex chromosomes often disrupt fertility but may sometimes show no obvious impact, masking their presence until genetic testing is done.
• Detecting such abnormalities is important for breeding management and ensuring genetic health in horse populations.

Animals and Cases Studied

• Four PRE mares from three unrelated families were studied, all carrying the same complex karyotype abnormality described as 64, X, del(X)(p∼25), t(X;Y)(p∼25;q∼15).
• The karyotype indicates a deletion on the short arm of the X chromosome and a translocation of material from the Y chromosome onto the X chromosome.
• Among these mares, one was fertile and had a foal who inherited the same rearrangement, marking a vertical transmission of this complex karyotype.

Initial Detection of Abnormalities

• Routine Short Tandem Repeat (STR) parentage testing revealed discrepancies between physical sex characteristics and genetic markers in these mares.
• This prompted further genetic investigation to assess chromosomal composition and identify structural changes.

Genomic and Cytogenetic Analysis

• An equine SNP-array was used to analyze copy number variations and sex chromosome constitution.
• This revealed a large, heterozygous deletion of approximately 10 megabases located at the distal end of the short (p) arm of the X chromosome.
• A genomic region exclusive to the Y chromosome was also identified, suggesting translocation of Y chromosome material onto the X chromosome.
• Fluorescence in situ hybridization (FISH) confirmed the presence of Y chromosome segments on the distal p-arm of the X chromosome.

Bioinformatic and Functional Analysis

• Bioinformatic tools were applied to study CpG island abundance, indicating potential gene regulation impacts caused by the deletion and translocation.
• Genes located in the affected chromosome regions were functionally analyzed to assess potential consequences on biological processes, particularly related to reproductive health.

Reproductive Findings and Hypothesis

• Despite the chromosomal abnormality, one mare was fertile and capable of producing offspring with the rearranged chromosome.
• This suggests that the mare’s reproductive competence may rely on the presence of at least one intact X chromosome that compensates for the deletion.
• The vertical transmission of the rearrangement demonstrates that such complex chromosomal abnormalities can be inherited.

Significance and Implications

• The study highlights that horses carrying sex chromosome abnormalities might not always exhibit infertile phenotypes.
• Routine reproductive assessments may fail to detect such genetic abnormalities without targeted genetic tests.
• Genetic screening methods such as STR parentage testing and SNP arrays are valuable for early detection of hidden chromosomal abnormalities.
• Understanding genetic variations helps in breeding management, avoiding unintended transmission of potentially deleterious chromosomal conditions.
• These findings may encourage widespread cytogenetic and genomic testing in breeding horses to safeguard reproductive health and preserve genetic diversity.